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Title: A Study of Recent Earthquakes
Author: Davison, Charles, 1858-1940
Language: English
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* * * * *
A STUDY OF
RECENT EARTHQUAKES.
BY
CHARLES DAVISON, Sc.D., F.G.S.
AUTHOR OF
"THE HEREFORD EARTHQUAKE OF DECEMBER 17TH, 1896."
WITH 80 ILLUSTRATIONS
London and Newcastle-on-Tyne:
THE WALTER SCOTT PUBLISHING CO., LTD.
1905
PREFACE.
The present volume differs from a text-book of seismology in giving
brief, though detailed, accounts of individual earthquakes rather than
a discussion of the phenomena and distribution of earthquakes in
general. At the close of his _Les Tremblements de Terre_, Professor
Fouqué has devoted a few chapters to some of the principal earthquakes
between 1854 and 1887; and there are also the well-known chapters in
Lyell's _Principles of Geology_ dealing with earthquakes of a still
earlier date. With these exceptions, there is no other work covering
the same ground; and he who wishes to study any particular earthquake
can only do so by reading long reports or series of papers written
perhaps in several different languages. The object of this volume is
to save him this trouble, and to present to him the facts that seem
most worthy of his attention.
The chapter on the Japanese earthquake is reprinted, with a few slight
additions, from a paper published in the _Geographical Journal_, and I
am indebted to the editor, not only for the necessary permission, but
also for his courtesy in furnishing me with _clichés_ of the blocks
which illustrated the original paper. The editor of _Knowledge_ has
also allowed me to use a paper which appeared four years ago as the
foundation of the ninth chapter in this book.
CHARLES DAVISON.
BIRMINGHAM,
_January, 1905._
CONTENTS.
CHAPTER I. PAGE
INTRODUCTION 1
CHAPTER II.
THE NEAPOLITAN EARTHQUAKE OF DECEMBER 16TH, 1857 7
CHAPTER III.
THE ISCHIAN EARTHQUAKES OF MARCH 4TH, 1881, AND JULY
28TH, 1883 45
CHAPTER IV.
THE ANDALUSIAN EARTHQUAKE OF DECEMBER 25TH, 1884 75
CHAPTER V.
THE CHARLESTON EARTHQUAKE OF AUGUST 31ST, 1886 102
CHAPTER VI.
THE RIVIERA EARTHQUAKE OF FEBRUARY 23RD, 1887 138
CHAPTER VII.
THE JAPANESE EARTHQUAKE OF OCTOBER 28TH, 1891 177
CHAPTER VIII.
THE HEREFORD EARTHQUAKE OF DECEMBER 17TH, 1896, AND
THE INVERNESS EARTHQUAKE OF SEPTEMBER 18TH, 1901 215
CHAPTER IX.
THE INDIAN EARTHQUAKE OF JUNE 12TH, 1897 262
CHAPTER X.
CONCLUSION 321
INDEX 349
LIST OF ILLUSTRATIONS.
FIG. PAGE
1. Diagram to illustrate Simple Harmonic Motion 4
2. Isoseismal Lines of the Neapolitan Earthquake 9
3. Diagram to illustrate Wave-path and Angle of Emergence 12
4. Diagram to illustrate Mallet's Method of determining
Position of Epicentre 14
5. Plan of Cathedral Church at Potenza 16
6. Fallen Gate-pillars near Saponara 17
7. Model to illustrate the Motion of an Earth-particle
during an Earthquake 19
8. Plan of Directions of Fall of Overturned Stone-lamps
at Tokio during the Earthquake of 1894 19
9. Meizoseismal Area of Neapolitan Earthquake 22
10. Distribution of Death-rate within Meizoseismal Area
of Neapolitan Earthquake 24
11. Diagram to illustrate Mallet's Method of determining
Depth of Seismic Focus 26
12. Vertical Section of Cathedral Church at Potenza 27
13. Diagram of Wave-paths at Seismic Vertical of Neapolitan
Earthquake 29
14. Geological Sketch-map of Ischia 47
15. Isoseismal Lines of Ischian Earthquake of 1881 51
16. Isoseismal Lines of Ischian Earthquake of 1883 59
17. Diagram of Wave-paths at Seismic Vertical of Ischian
Earthquake of 1883 62
18. Diagram showing connection between Depth of Focus
and Rate of Decline in Intensity 68
19. Isoseismal Lines of Andalusian Earthquake according
to Taramelli and Mercalli 79
20. Isoseismal Lines of Andalusian Earthquake according
to Fouqué, etc. 81
21. Magnetograph Records of Andalusian Earthquake at Lisbon 83
22. Nature of Shock of Andalusian Earthquake 88
23. Diagram to illustrate Variation in Nature of Shock 89
24. Structure of Meizoseismal Area of Andalusian Earthquake 100
25. Isoseismal Lines of Charleston Earthquake 106
26. Curve of Intensity at Charleston 110
27. Flexure of Rails at Jedburgh 113
28. Epicentral Isoseismal Lines of Charleston Earthquake
according to Sloan 118
29. Epicentral Isoseismal Lines of Charleston Earthquake
according to Dutton 119
30. Planes of Oscillation of Stopped Pendulum Clocks at
Charleston 121
31. Diagram to illustrate Dutton's Method of determining
Depth of Seismic Focus 124
32. Diagram to explain Origin of Regions of Defective
Intensity 136
33. Isoseismal Lines of Riviera Earthquake 144
34. Meizoseismal Area of Riviera Earthquake 148
35. Nature of Shock of Riviera Earthquake 152
36. Seismographic Record at Moncalieri 155
37. Distribution of Observatories at which Magnetographs
were disturbed by Riviera Earthquake 158
38. Record of Tide-gauge at Nice 163
39. Record of Tide-gauge at Genoa 164
40. Distribution of Seismic Activity in the Riviera 172
41. Isoseismal Lines of Japanese Earthquake 178
42. Structure of Meizoseismal Area of Japanese Earthquake 180
43. Plan of Directions of Fall of Overturned Bodies at
Nagoya 187
44. Map of Mean Directions of Shock and Isoseismal Lines
in Central District of Japanese Earthquake 188
45. Meizoseismal Area of Japanese Earthquake 190
46. Fault-scarp near Fujitani 191
47. Fault-scarp at Midori 192
48. Displacement of Field-divisions by the Fault near
Nishi-Katabira 193
49. Map of Swamp formed by Stoppage of River Toba by
Fault-scarp 194
50. Shifting of Trees by Fault at Uméhara 195
51. Daily Frequency of After-shocks at Gifu and Nagoya 196
52. Monthly Frequency of After-shocks at Gifu 197
53. Distribution of Preliminary Shocks in Space 202
54. Distribution of After-shocks in Space, Nov.-Dec. 1891 203
55. Distribution of After-shocks in Space, Jan.-Feb. 1892 204
56. Distribution of After-shocks in Space, March-April 1892 205
57. Distribution of After-shocks in Space, May-June 1892 206
58. Distribution of Audible After-shocks in Space, Nov.
1891-Dec. 1892 208
59. Map of Adjoining Regions in which Seismic Activity
was affected by Japanese Earthquake 210
60. Isoseismal and Isacoustic Lines of Hereford Earthquake 216
61. Nature of Shock of Hereford Earthquake 222
62. Coseismal Lines of Hereford Earthquake 228
63. Map of Minor Shocks of Hereford Earthquake 239
64. Geology of Meizoseismal Area of Hereford Earthquake 241
65. Isoseismal Lines of Inverness Earthquake 248
66. Diagram to illustrate supposed Fault-displacement
causing Inverness Earthquake 256
67. Map of Epicentres of After-shocks of Inverness
Earthquake 258
68. Isoseismal Lines of Indian Earthquake 263
69. Section of Tombs in Cemetery at Cherrapunji 270
70. Time-curve of Indian Earthquake 278
71. Seismographic Record of Indian Earthquake at Rocca
di Papa 282
72. Seismographic Record of Indian Earthquake at Edinburgh 283
73. Displacement of Alluvium at Foot of a Hill 287
74. Twisting of Monument at Chhatak 294
75. Epicentral Area of Indian Earthquake 303
76. Plan of Chedrang Fault 305
77. Re-triangulation of Khasi Hills 313
78. Diagram of Thrust-planes 318
79. Seismographic Record of Tokio Earthquake of 1894 329
80. Time-curves of Principal Epochs of Earthquake-waves of
Distant Origin 338
A STUDY OF RECENT EARTHQUAKES.
CHAPTER I.
INTRODUCTION.
I propose in this book to describe a few of the more important
earthquakes that have occurred during the last half century. In
judging of importance, the standard which I have adopted is not that
of intensity only, but rather of the scientific value of the results
that have been achieved by the study of the shocks. Even with this
reservation, the number of earthquakes that might be included is
considerable; and I have therefore selected those which seem to
illustrate best the different methods of investigation employed by
seismologists, or which are of special interest owing to the unusual
character of their phenomena or to the light cast by them on the
nature and origin of earthquakes in general.
Thus, the Neapolitan earthquake possesses interest from a historical
point of view; it is the first earthquake in the study of which modern
scientific methods were employed. The Ischian earthquakes are
described as examples of those connected with volcanic action; the
Andalusian earthquake is chiefly remarkable for the recognition of the
unfelt earth-waves; that of Charleston for the detection of the double
epicentre and the calculation of the velocity with which the
vibrations travelled. In the Riviera earthquake are combined the
principal features of the last two shocks with several phenomena of
miscellaneous interest, especially those connected with its submarine
foci. The Japanese earthquake is distinguished from others by its
extraordinary fault-scarp and the very numerous shocks that followed
it. The Hereford earthquake is a typical example of a twin earthquake,
and provided many observations on the sound phenomena; while the
Inverness earthquakes are important on account of their connection
with the growth of a well-known fault. The great Indian earthquake
owns few, if any, rivals within historical times, whether we consider
the intensity of the disturbance or the diversity and interest of the
phenomena displayed by it--the widespread changes in the earth's
crust, both superficial and deep-seated, and the tracking of the
unfelt pulsations completely round the globe.
TERMS AND DEFINITIONS.
Some terms are of such frequent use in describing earthquakes that it
will be convenient to group them here for reference, others more
rarely employed being introduced as they are required.
An earthquake is caused by a sudden displacement of the material which
composes the earth's interior. The displacement gives rise to series
of waves, which are propagated outwards in all directions, and which,
when they reach the surface, produce the sensations known to us as
those of an earthquake.
The region within which the displacement occurs is sometimes called
the _hypocentre_, but more frequently the _seismic focus_ or simply
the _focus_. The portion of the earth's surface which is vertically
above the seismic focus is called the _epicentre_. The focus and
epicentre are often spoken of for convenience as if they were points,
and they may then be regarded as the centres of the region and area in
which the intensity was greatest. This is not quite accurate, but to
attempt a more exact definition would at present be out of place.
An _isoseismal line_ is a curve which passes through all points at
which the intensity of the shock was the same. It is but rarely that
the absolute intensity at any point of an isoseismal line can be
ascertained, and only one example is given in this volume. As a rule,
the intensity of a shock is determined by reference to the degrees of
different arbitrary scales. These will be quoted when required.
In every strong earthquake there is a central district which differs
in a marked manner from that outside in the far greater strength and
complexity of the phenomena. As this district includes the epicentre,
it is sometimes referred to as the _epicentral area_, but the term
_meizoseismal area_ is more appropriate, and will be employed
accordingly.
The district over which an earthquake is perceptible to human beings
without instrumental aid is its _disturbed area_. In like manner, that
over which the earthquake-sound is heard is the _sound-area_.
A great earthquake never occurs alone. It is merely the most prominent
member of a group of shocks of greater or less intensity, and is
known as the _principal shock_ or _earthquake_, while the others are
called _minor_ or _accessory shocks_, and _fore-shocks_ or
_after-shocks_ according as they occur before or after the principal
earthquake. When the sound only is heard, without an accompanying
tremor being anywhere perceptible, it is more accurately called an
_earth-sound_, but is frequently for convenience numbered among the
minor shocks.
[Illustration: FIG. 1.--Diagram to illustrate simple harmonic
motion.]
The movement of the ground during a vibration of the simplest
character (known as simple harmonic motion) is represented in Fig. 1.
The pointer of the recording seismograph is here supposed to oscillate
along a line at right angles to AB, and the smoked paper or glass on
which the record is made to travel to the left. The distance MP of the
crest P of any wave from the line AB represents the _amplitude_ of the
vibration, the sum of the distances MP and NQ its _range_, and the
length AB the _period_ of the vibration. From the amplitude and period
we can calculate, in the case of simple harmonic motion, both the
_maximum velocity_ and _maximum acceleration_ of the vibrating
particles of the ground.[1]
A few terms describing the nature of the shock are also in common use
among Italians and Spaniards. An _undulatory_ shock consists of one or
several waves, the movement to and fro being along a nearly horizontal
line; a _subsultory_ shock of movements in a nearly vertical
direction; while a _vorticose_ shock consists of undulatory or
subsultory movements crossing one another in different directions.
ORIGIN OF EARTHQUAKES.
Earthquakes are grouped, according to their origin, into three
classes. The first consists of slight local shocks, caused by the fall
of rock in underground passages; the second of _volcanic_ earthquakes,
also local in character, but often of considerable intensity near the
centre of the disturbed area; while in the third class we have
_tectonic_ earthquakes, or those directly connected with the shaping
of the earth's crust, which vary in strength from the weakest
perceptible tremor to the most destructive and widely felt shock. Of
the earthquakes described in this volume, the Ischian earthquakes
belong to the second class, and all the others to the third.
That tectonic earthquakes are closely connected with the formation of
faults seems now established beyond doubt. They occur far from all
traces of recent volcanic action. Their isoseismal lines are elongated
in directions parallel to known faults, and this is sometimes the case
in one and the same district with faults that occur at right angles to
one another. Indeed, when several isoseismals are carefully drawn, it
is possible from their form and relative position to predict the
position of the originating fault.[2] The initial formation and
further spreading of the rent may be the cause of a few earthquakes,
but by far the larger number are due to the subsequent growth of the
fault. The relative displacement of the rocks adjoining the fault,
which may amount to thousands of feet, occasionally even to miles, is
the result, not of one great movement, but of innumerable slips taking
place in different parts of the fault and spread over vast ages of
time. With every fault-slip, intense friction is suddenly brought into
action by the rubbing of one mass of rock against the other; and,
according to the modern view, it is this friction that gives rise to
the earthquake waves.
In most earthquakes, the slip takes place at a considerable depth,
perhaps not less than one or several miles, and the vertical slip is
so small that it dies out before reaching the surface. But, in a few
violent earthquakes, such as the Japanese and Indian earthquakes
described in this volume, the slip is continued up to the surface and
is left visible there as a small cliff or fault-scarp. In these cases,
the sudden spring of the crust may increase and complicate the effects
of the vibratory shock.
FOOTNOTES:
[1] If _a_ is the amplitude of the vibration and T its period, the
maximum velocity is 2*pi*a/T and the maximum acceleration 4*pi^2a/T^2
[2] See Chapter VIII., on the Hereford and Inverness earthquakes.
CHAPTER II.
THE NEAPOLITAN EARTHQUAKE OF DECEMBER 16TH, 1857.
Half a century ago, seismology was in its infancy. On the Continent,
Alexis Perrey of Dijon was compiling his earthquake catalogues with
unfailing enthusiasm and industry. In 1846, Robert Mallet applied the
laws of wave-motion in solids, as they were then known, to the
phenomena of earthquakes; and his memoir on the Dynamics of
Earthquakes[3] may be regarded as the foundation-stone of the new
science. During the next twelve years he contributed his well-known
Reports to the British Association,[4] and prepared a series of
instructions for the observation and study of earthquake-shocks.[5]
The latter, it is worth noting, contains an outline, but hardly more
than an outline, of the methods of investigation which he developed
and employed eight years afterwards in studying the Neapolitan
earthquake.
The history of Mallet's preparation for his great work is somewhat
strange. No one else at that time possessed so full a knowledge of
earthquake phenomena. It was, however, a knowledge that had little,
if any, foundation in actual experience; for, when he was awakened by
the British earthquake of November 9th, 1852, he failed to recognise
its seismic character. Although this shock disturbed an area of about
75,000 square miles and was felt in all four parts of the kingdom, the
paucity of observations and the absence of durable records combined in
preventing the successful application of his new modes of study.[6]
Nevertheless, with confidence unshaken in their power, he awaited the
occurrence of a more violent shock, but five years had to pass before
his opportunity came towards the close of 1857.
So destructive was the Neapolitan earthquake of this year (Mallet
ranks it third among European earthquakes in extent and severity),
that nearly a week elapsed before any news of it reached the outer
world. Without further loss of time, he applied for and obtained a
grant of money from the Council of the Royal Society, and proceeded
early in the following February to what was then the kingdom of
Naples. Armed with letters of authority to different officials, he
visited the chief towns and villages in the meizoseismal area; and, in
spite of unfavourable weather and the difficulties of travelling in a
country so recently devastated, he completed his examination in little
more than two months. It was a task, surely, that would have baffled
any but the most enthusiastic investigator or one unspurred by the
feeling that he possessed the key to one of the most obscure of
Nature's problems.
Mallet's confidence in the accuracy of his methods was almost
unbounded. His great report was published four years later; but he
seems to have regarded it almost as a text-book of "observational
seismology" and the results of his Neapolitan work as mere
illustrations. His successors, however, have transposed the order of
importance, and rank his two large volumes as the model, if not the
inspirer, of many of our more recent earthquake monographs.
[Illustration: FIG. 2.--Isoseismal Lines of the Neapolitan
Earthquake of 1857. (_Mallet._)]
ISOSEISMAL LINES AND DISTURBED AREA.
The position of the meizoseismal area, to which Mallet devoted most of
his time, is indicated by the small oval area marked 1 in Fig. 2,
represented on a larger scale in Fig. 9. It is 40 miles long and 23
miles wide,[7] and contains 950 square miles. Within this area, the
loss of life was great and most of the towns were absolutely
prostrated.
The next isoseismal, No. 2, which is also shown more clearly in Fig.
9, bounds the area in which the loss of life was still great and many
persons were wounded, while large portions of the towns within it were
thrown down. Its length is 65 miles, width 47 miles, and area 2,240
square miles. The third isoseismal includes a district in which
buildings were only occasionally thrown down, though none escaped some
slight damage, and in which practically no loss of life occurred. This
curve is 103 miles long, 82 miles wide, and includes 6,615 square
miles. Lastly, the fourth isoseismal marks the boundary of the
disturbed area, which is 250 miles long, 210 miles wide, and contains
not more than 39,200 square miles; an amount that must be regarded as
strangely small, and hardly justifying Mallet's estimate of the
Neapolitan earthquake as the third among European earthquakes in
extent as well as in seventy.
DAMAGE CAUSED BY THE EARTHQUAKE.
As regards destruction to life and property, however, the Neapolitan
earthquake owns but few European rivals. Less favourable conditions
for withstanding a great shock are seldom, indeed, to be found than
those possessed by the mediæval towns and villages of the meizoseismal
area. In buildings of every class, the walls are very thick and
consist as a rule of a coarse, short-bedded, ill-laid rubble masonry,
without thorough bonding and connected by mortar of slender cohesion.
The floors are made of planks coated with a layer of concrete from six
to eight inches thick, the whole weighing from sixty to a hundred
pounds per square foot. Only a little less heavy are the roofs, which
are covered with thick tiles secured, except at the ridges, by their
own weight alone. Thus, for the most part, the walls, floors, and
roofs are extremely massive, while the connections of all to
themselves and to each other are loose and imperfect.
Again, the towns, for greater security from attacks in early times,
are generally perched upon the summits and steep flanks of hills,
especially of the lower spurs that skirt the great mountain ranges;
and the rocking of the hill-sites, in Mallet's opinion, greatly
aggravated the natural effects of the shock. The streets, moreover,
are steep and narrow, sometimes only five feet, and not often more
than fifteen feet, in width; and the houses, when shaken down, fell
against one another and upon those beneath them. As Dolomieu said of
the great earthquake in 1783, "the ground was shaken down like ashes
or sand laid upon a table."
Of the total amount of damage, not even the roughest estimate can be
made. The official returns are clearly, and no doubt purposely,
deficient, and obstacles were placed in Mallet's way when he
endeavoured to ascertain the numbers of persons killed and wounded.
Taking only the towns into account, he calculated that, out of a total
population of 207,000, the number of persons killed was 9,589, and of
wounded 1,343.[8] A few towns were marked by an excessively high
death-rate. Thus, at Montemurro, 5000 out of 7002 persons were killed
and 500 wounded; at Saponara, 2000 out of 4010 were killed; and, at
Polla, more than 2000 out of a population of less than 7000.
GENERAL OBJECTS OF INVESTIGATION.
The principal objects of Mallet's investigation were to determine the
position of the epicentre and the depth of the seismic focus. If, in
Fig. 3, F represents the seismic focus (here, for convenience,
supposed to be a point), the vertical line FE will cut the surface of
the earth in the epicentre E.[9] The dotted lines represent circles
drawn on the surface of the earth with E as centre and passing through
the places P and Q.
[Illustration: FIG. 3.--Diagram to illustrate wave-path and angle
of emergence.]
When the impulse causing the earthquake takes place at the focus, two
elastic waves spread outwards from it in all directions through the
earth's crust. The first wave which reaches a point P consists of
longitudinal vibrations, that is, the particle of rock at P moves in a
closed curve with its longer axis in the direction FP. Mallet supposes
this curve to be so elongated that it is practically a straight line
coincident in direction with FP. In the second or transversal wave,
the vibration of the particle at P takes place in a plane at right
angles to FP. These vibrations Mallet, for his main purpose, neglects.
Returning to the longitudinal wave, Mallet calls the line FP the
_wave-path_ at P. The direction EP gives the azimuth of the wave-path,
or its direction along the surface of the earth. The angle LPA, or
EPF, he defines as the _angle of emergence_ at the point P. If Q be
farther from E than P, the angle EQF is less than the angle EPF, or
the angle of emergence diminishes as the distance from the epicentre
increases. At the epicentre, the angle of emergence is a right-angle;
at a great distance from the epicentre, it is nearly zero.
Mallet argued that the direction of the wave-path FPA, or its
equivalents, the horizontal direction EPL and the angle of emergence
EPF, should be discoverable from the effects of the shock at P. The
cracks in damaged buildings, he urged, would be at right angles to the
wave-path FPA; overturned monuments or gate-pillars should fall along
the line EPL, either towards or from the epicentre according to their
conditions of support; loose or slightly attached bodies, such as the
stone balls surmounting gate-pillars, should be projected nearly in
the direction of the wave-path FPA, and their subsequent positions,
supposing the balls not to have rolled, should give the horizontal
direction EPL of the wave-path, and might, in some circumstances,
determine the angle of emergence and the velocity with which they were
projected. I shall return to details later on. For the present, it is
clear that, in the destruction wrought by the earthquake, Mallet
expected to find the materials most valuable for his purpose. Indeed,
so obvious did this mode of examination appear to him, that he could
not conceal his surprise at the blindness of his predecessors. They
seem, he says, "to have been perfectly unconscious that in the
fractured walls and overthrown objects scattered in all directions
beneath their eyes, they had the most precious data for determining
the velocities and directions of the shocks that produced them."
POSITION OF THE EPICENTRE.
_Mallet's Method of Determining the Position of the Epicentre._--In
many cases the examination of a damaged building or of an overthrown
body served more than one purpose, providing materials for
ascertaining the depth of the seismic focus as well as the position of
the epicentre. For the present, however, it will be convenient to
consider alone the method by which the latter object was to be
attained.
[Illustration: FIG. 4.--Diagram to illustrate Mallet's method of
determining position of epicentre.]
Nothing could be simpler than the principle of the method proposed.
The horizontal direction PL of the wave-path at any place P (Fig. 4),
when produced backwards, must pass through the epicentre E; and the
intersection of the directions at two places, P and Q, must therefore
give the position of the epicentre. In practice, it is of course
impossible to determine the direction with very great accuracy, and
Mallet therefore found it necessary to make several measurements in
every place, and to visit all the more important towns within and near
the meizoseismal area.
In a ruined town there are many objects from which the direction may
be ascertained, the most important of all, according to Mallet, being
fissures in walls that are fractured but not overthrown. He regarded
such fissures, indeed, as "the sheet-anchor, as respects direction of
wave-path, to the seismologist in the field," and at least three out
of every four of his determinations of the direction were made by
their means. If the buildings are detached and large, simple and
symmetrical in form, well built and not too much injured, the fissures
in the walls should, he argued, occur along lines at right angles to
the wave-path, whether that path be parallel or inclined to the
principal axis of the building. Cracks in the floors and ceilings
should also be similarly directed, and provide evidence which Mallet
regarded as only second in value to that given by the walls.
[Illustration: FIG. 5.--Plan of Cathedral Church at Potenza.
(_Mallet._)]
No building showed the different kinds of evidence on which Mallet
relied as clearly as the cathedral church at Potenza, the plan of
which is given in Fig. 5, and the vertical section along its axis in
Fig. 12. This is a modern work, nearly 200 feet long, with its axis
directed east and west. The walls are composed of fairly good rubble
masonry and brick; and the arches in the nave and transepts, the
semi-cylindrical roof and the central dome are made of brick. The
fissures represented in both diagrams were drawn to scale by the
cathedral architect before Mallet's arrival, and, as the work of an
unbiassed observer, are of special value. Most of those in the roof,
it will be seen, were transverse to the axial line of the church; but
there were others parallel to this line, one in particular running
right along the soffit of the nave and chancel. There were also
numerous small fissures in the dome, due to local structural causes
and therefore of varying direction, and a large portion of the dome
slipped westward, leaving open fissures of seven to eight inches in
width. The mean direction of the wave-path, as deduced from nine sets
of fissures, none of which differs more than four degrees from the
mean, is W. 2-1/2° S. and E. 2-1/2° N., which corresponds precisely
with the direction of throw on the displaced portion of the dome. The
great east and west fissures in the arch of the nave and chancel
Mallet attributed to a second shock, of the existence of which there
is ample evidence.
[Illustration: FIG. 6.--Fallen gate-pillars near Saponara.
(_Mallet._)]
Next to fissures, Mallet made most use of overthrown objects, such as
the two gate piers near Saponara, represented in Fig. 6. They were
made of rubble ashlar masonry, three feet square and seven feet in
height. Both were fractured clean off at the level of the ground, the
mortar being poor, and fell in directions that were accurately
parallel, indicating a wave-path towards S. 39-1/2°E. A few
observations were also made on projected stones, fissures in nearly
level ground, and the swinging of lamps and chandeliers; but their
value was small, except as corroboration of the more important
evidence afforded by fissures in the walls and roofs of buildings.
_Remarks on Mallets Method._--It would have been more difficult in
Mallet's day than it is now, to offer objections to his method of
determining the position of the epicentre. The focus, as he was well
aware, could not be a point, and, at places near the epicentre (the
very places where most of his observations were made), there must be
rapid changes of direction due to the arrival of vibrations from
different parts of the focus. He records the occurrence of the
so-called vorticose shocks at several places, though he attributes
them to another cause. Perhaps the best known example of such a shock
is that which has been so well illustrated by the late Professor
Sekiya's model of the motion of an earth-particle during the Japanese
earthquake of January 15th, 1887. The motion in this case was so
complicated that the model was, for simplicity, made in three parts,
the first of which alone is represented in Fig. 7.[10] It is clear
that in such an earthquake, Mallet's method would utterly fail in
giving definite results.
While this shock was one of great complexity, another Japanese
earthquake, that of June 20th, 1894, was unusually simple in
character. The movement at Tokio consisted of one very prominent
oscillation with a total range of 73 mm. or 2.9 inches in the
direction S. 70° W.; the vibrations which preceded and followed it
being comparatively small. Most, if not all, of the damage caused by
the earthquake must have been due to this great oscillation; and yet
the cylindrical stone-lamps so common in Japanese gardens were found
by Professor Omori to have fallen in many different directions. Taking
only those which had circular bases, twenty-nine were overthrown in
directions between north and east, sixteen between east and south,
eighty-one between south and west, and fourteen between west and
north.[11] Fig. 8 represents Professor Omori's results graphically,
the line drawn from O to any point being proportional to the number of
lamps which fell in directions between 7-1/2° on either side of the
line.
[Illustration: FIG. 7.--Model to illustrate the motion of an
earth-particle during an earthquake. (_Sekiya._)]
[Illustration: FIG. 8.--Plan of directions of fall of overturned
stone-lamps at Tokio during the earthquake of 1894.]
It will be seen from this figure that most of the stone lamps fell in
directions between west and south-west, and it is remarkable that the
mean direction of fall is S. 70° W.,[12] which is exactly the same as
that of the great oscillation. Somewhat similar results were obtained
by this able seismologist at different places affected by the great
Japanese earthquake of 1891 (Figs. 43 and 44), and the study of the
apparent directions observed during the Hereford earthquake of 1896
leads to the same conclusion.
It thus appears that an isolated observation may give a result very
different from the true direction. Indeed, if we may judge from
Professor Omori's measurements in 1894, the chance that a single
direction may be within five degrees of the mean direction is about 1
in 9. But, on the other hand, it is equally clear from these and other
observations that the mean of a large number of measurements will give
a result that agrees very closely with the true direction.
One other point may be alluded to before leaving Professor Omori's
interesting observations. It would seem, from the list that he gives,
that he exercised no selection in his measurements, but continued
measuring the direction of every fallen lamp indifferently until he
had obtained sufficient records for his purpose. Now, if the number of
fallen lamps at his disposal had been small, say 12 instead of 144,
the mean observed direction would probably have differed from the
direction given from the seismograph.[13] But, on the other hand, a
preliminary survey without any actual measurements would have revealed
at once the predominant direction of overthrow, and a fairly accurate
result might have been obtained by neglecting discordant directions
and taking the mean of those only which appeared to agree with the
mentally determined average.
This, indeed, appears to have been the course followed, more or less
unconsciously, by Mallet in his Neapolitan work. "When the observer,"
he says, "first enters upon one of those earthquake-shaken towns, he
finds himself in the midst of utter confusion. The eye is bewildered
by 'a city become an heap.' He wanders over masses of dislocated stone
and mortar, with timbers half buried, prostrate, or standing stark up
against the light, and is appalled by spectacles of desolation....
Houses seem to have been precipitated to the ground in every direction
of azimuth. There seems no governing law, nor any indication of a
prevailing direction of overturning force. It is only by first gaining
some commanding point, whence a general view over the whole field of
ruin can be had, and observing its places of greatest and least
destruction, and then by patient examination, compass in hand, of many
details of overthrow, house by house and street by street, analysing
each detail and comparing the results, as to the direction of force,
that must have produced each particular fall, with those previously
observed and compared, that we at length perceive, once for all, that
this apparent confusion is but superficial."
[Illustration: FIG. 9.--Meizoseismal area of Neapolitan
earthquake. (_Mallet._)]
_Mallet's Determination of the Epicentre._--Within the third
isoseismal line Mallet made altogether 177 measurements of the
direction of the wave-path at 78 places. These are plotted on his
great map of the earthquake; but, owing to the small scale of Fig. 9,
it is only possible to represent, by means of short lines, the mean or
most trustworthy direction at each place.[14] Producing these
directions backwards, he found that those at sixteen places passed
within five hundred yards of a point which is practically coincident
with the village of Caggiano; those at sixteen other places passed
within one geographical mile (1.153 statute miles) of this point; the
directions at sixteen more places within two and a half geographical
miles; while those at twelve places passed through points not more
than five geographical miles from Caggiano. As the direction of the
shock at places near the epicentre must have been influenced by the
mere size of the focus, this approximate coincidence is certainly
remarkable, and there can be little doubt, I think, that the
epicentre, or, at any rate, _an_ epicentre must have been situated not
far from the position assigned to it by Mallet's laborious
observations.
_Existence of Two Epicentres._--It is difficult, however, to realise
that the impulse at the focus corresponding to Mallet's epicentre was
the origin of all the destruction of life and property that occurred.
The position of the epicentre close to the north-west boundary of the
meizoseismal area, the extraordinary extension of that area towards
the south-east, and especially the great loss of life at Montemurro
and the adjoining towns, can hardly be accounted for in this manner.
Mallet himself recognised that these facts required explanation, and
he suggested that the situation and character of the towns were in
part responsible for their ruin, and the physical structure of the
country for the course of the isoseismal lines. But the comparative
escape of places much nearer Caggiano, and the wide extent of the
meizoseismal area, embracing many towns and villages of varied
character and site and many different surface-features, point
unmistakably to a different explanation.
[Illustration: FIG. 10.--Distribution of death-rate within
meizoseismal area of Neapolitan earthquake.]
One clue to the solution of the problem is afforded by the seismic
death-rate of the damaged towns. From a table given by Mallet (vol.
ii. pp. 162-163), we know the population before the earthquake of the
different communes in the province of Basilicata, and the loss of life
in each due to the shock; and from these figures we can find the
percentage of deaths at nearly every place of importance. As will be
seen from Fig. 10, it varies from seventy-one at Montemurro and fifty
at Saponara down to less than one at all the places marked to which
figures are not attached. There is thus a group of places, with its
centre near Montemurro, where the loss of life far exceeded that in
the surrounding country; and also a slightly less-marked group, with
its centre near Polla, in the north-west of the meizoseismal area;
while in the intermediate region the death-rate was invariably small.
Too much stress should not be laid upon the exact figures, for there
were no doubt local conditions that affected the death-roll. But it
seems clear that one focus was situated not far from Montemurro; while
the north-westerly group of places, combined with Mallet's
observations on the direction, point to a second focus near Polla,
about twenty-four miles to the north-west. It will be seen in a later
section that the observations on the nature of the shock also imply
the existence of a double focus.
DEPTH OF THE SEISMIC FOCUS.
_Mallet's Method of Determining the Depth of the Focus._--In
ascertaining the position of the epicentre, Mallet's work was
remarkable only for the novelty of the method employed by him; but, in
his attempt to calculate the depth of the seismic focus, he was
breaking new ground. That the depth must be comparatively small had
already been recognised, and was indeed obvious from the limited area
disturbed by nearly every earthquake. No one, however, had tried to
estimate the depth in miles; and it is impossible not to sympathise
with Mallet while he accumulated his observations with feverish
activity and subjected them to the first rough examination even if
one cannot share his confidence that he had succeeded in measuring the
depth "in miles and yards with the certainty that belongs to an
ordinary geodetic operation."
The method employed by him for the purpose is no less simple
theoretically than that used for locating the epicentre. If the
position of the latter (E) is known, one accurate measurement of the
angle of emergence EPF, at any other point P would be sufficient to
fix the depth of some point within the focus F (Fig. 11). Here, again,
Mallet relied chiefly on fissures in walls that were fractured but not
overthrown. In detail, these fissures are nearly always jagged or
serrated, for they tend to follow the lines of joints rather than
break through the solid stone, though they sometimes traverse bricks
and mortar alike. But the general course of the fissures, he urged,
would be at right angles to the wave-path, and their inclination to
the vertical should be equal to the angle of emergence.
[Illustration: FIG. 11.--Diagram to illustrate Mallet's method of
determining depth of seismic focus.]
In obtaining measurements of this angle, the buildings to be chosen
are those of large size, with few windows or other apertures, and with
walls made of brick or small short-bedded stones. The cathedral-church
at Potenza perhaps satisfies these conditions more closely than any
other structure examined by Mallet. The plan of the fissures in the
walls and roof has been given in Fig. 5, and Fig. 12 represents the
fissures In the vertical section along the axial line and looking
north, as drawn by the cathedral architect. From these fissures Mallet
calculated the mean angle of emergence at Potenza to be 23° 7'. The
distance of Potenza from Caggiano being seventeen miles, and the
height of the former being 2,580 feet, the depth of the focus
resulting from this observation alone would be 6-3/4 miles below the
level of the sea.
[Illustration: FIG. 12.--Vertical section of Cathedral Church at
Potenza. (_Mallet._)]
_Objection to Mallet's Method._--The weakest point in Mallet's method
is probably his assumption that the wave-paths are straight lines
extending outward from the focus. Even if the depth of the focus is
not more than a few miles, the waves must traverse rocks of varying
density and elasticity, and, at every bounding surface, they must
undergo refraction. If the rocks are so constituted that the velocity
of the earth-waves in them increases with the depth, then the
wave-paths must be bent continually outwards from the vertical, so
that the angle of emergence at the surface may be considerably less
than it would have been with a constant velocity throughout. In this
case, the actual depth will be greater, perhaps much greater, than the
calculated depth. For instance, if the angle of emergence at Potenza
were diminished only 5° by refraction, the calculated depth of the
focus would be too small by 1-3/4 miles.
_Mallet's Estimate of the Depth of the Focus._--Mallet measured the
angle of emergence at twenty-six places, the mean angle (_i.e._ the
mean of the greatest and least observed angles) varying from 72° at
Vietri di Potenza and 70° at Pertosa, which are about two miles from
the calculated epicentre, to 11-1/2° at Salerno, distant about 40
miles. Fig. 13 reproduces part of the diagram on which he plotted the
mean angle of emergence at different places. The horizontal line
represents the level of the sea, and the vertical line one passing
through the epicentre and focus, called by Mallet the "seismic
vertical." The lines on the left-hand side represent the commencing
wave-paths (assumed straight) to the observing stations situated to
the westward of the meridian through the epicentre, those on the
right-hand side corresponding to places to the eastward of the same
meridian. Small horizontal marks are added to indicate the depth in
miles below the level of the sea.
[Illustration: FIG. 13.--Diagram of wave-paths at seismic
vertical of Neopolitan earthquake. (_Mallet._)]
It will be seen, from this diagram, that all the wave-paths start from
the seismic vertical at depths between three and nine miles; but the
points of departure are clustered thickly within a portion, the length
of which is about 3-1/2 miles and the mean depth about 6-1/2 miles. So
great was Mallet's confidence in these calculations that he assigns
the diverging origin of the wave-paths to different points of the
focus, and thus concludes that, while the mean depth of the focus was
about 6-1/2 miles, its dimensions in a vertical direction did not
exceed 3-1/2 miles.
How far Mallet's results should be accepted as correct, it is
difficult to say in our ignorance of the constitution of the earth's
interior. There can be no doubt that the focus was of considerable
size, and that, in consequence, the wave-paths would diverge from
different points of it. But that each wave-path should actually
intersect the focus, and so enable its magnitude to be determined,
would surely involve an approach to some law connecting the direction
of a wave-path with the depth of its own origin, and no such law seems
to be ascertainable. Nor can the limitation of these apparent origins
between certain depths be held to argue that the focus, or any part of
it, was equally confined, for the wave-paths would to a great extent
be similarly refracted. I fear that the only conclusions that we can
with safety draw from Mallet's admirable work are that his figures
indicate the order of magnitude both of the vertical dimensions and of
the mean depth of the focus.
NATURE OF THE SHOCK.
It is not easy to form any precise image of the earthquake as it
appeared to the terrified witnesses within the meizoseismal area. To
minds unbalanced by the suddenness of the shock and by the crash of
falling houses, actuated too by the intense need of safety, the mere
succession of events must have presented but little interest. The
interval of two months that elapsed between the occurrence of the
earthquake and its investigation was also unfavourable to the
collection of accurate accounts from a wonder-loving people. Only one
feature, therefore, stands out clearly in the few records given by
Mallet--namely, the division of the shock into two distinct parts.
In the central district, this division is perhaps less apparent than
elsewhere. At Polla, for instance, which lies close to the north-west
epicentre, the first warning was given by a rushing sound; almost
instantly, and while it was yet heard, came a strong subsultory or
up-and-down movement, succeeded after a few seconds, but without any
interval, by an undulatory motion. At Potenza, which is not far from
the same epicentre but a few miles outside the meizoseismal area, the
separation was more pronounced. According to one observer, the first
movement was from west to east; and, within a second or two
afterwards, there was a less violent shock in a transverse direction,
followed immediately by a shaking in all directions, called by the
Italians vorticose. Naples lies sixty-nine miles from the north-west
epicentre, and here more accurate observations could be made. Dr.
Lardner, well known fifty years ago as a writer of scientific works,
describes the first movement felt there as "a short, jarring,
horizontal oscillation, that made all doors and windows rattle, and
the floors and furniture creak. This ceased, and after an interval
that seemed but a few seconds was renewed with greater violence, and,
he thought, with a distinctly undulatory movement, 'like that in the
cabin of a small vessel in a very short chopping sea.'"
In five other earthquakes studied in this volume, the separation of
the shock into two parts was a well-marked phenomenon. In the
Neapolitan earthquake, the separation was so distinct that Mallet took
some pains to account for its origin. He regarded it in every case as
due to the reflection or refraction of the earth-waves by underlying
rocks, though he does not explain why the reflected or refracted wave
should be more intense than that transmitted directly. I shall refer
to the subject in greater detail when describing the Andalusian,
Charleston, Riviera, and Hereford earthquakes. For the present, it may
be sufficient to urge that the double shock cannot have been due to
the separation of the original waves by underground reflection or
refraction, for then the second part should have been generally the
weaker; nor to the succession of longitudinal and transverse waves,
for, in that case, every earthquake-shock should be duplicated. The
only remaining supposition is that there was a second impulse
occurring either in the same or in a different focus.
Which alternative should be adopted, the evidence on the nature of the
shock is too scanty to determine. The defect is, however, supplemented
by Mallet's observations on the direction of motion; for, at many
places within and near the meizoseismal area, he met with the clearest
signs of a double direction. Sometimes this was apparent to the senses
of the observer; in other cases, damaged buildings presented two sets
of fissures. At La Sala and near Padula, the first movement was roughly
east and west, the second north and south. At Moliterno, there was
evidence of a subordinate shock at right angles to the chief one; in
the neighbourhood of Tramutola, its direction was from about E. 30° S.
In these and other cases, Mallet saw the effects of earthquake-echoes;
but the underground reflection of earth-waves would give rise to the
second part of the shock, not the first as at La Sala and Padula.
Moreover, the secondary directions, though they are seldom recorded
accurately, point nearly to an epicentre not far from Montemurro. The
observations on the nature and direction of the double shock thus
confirm the conclusion, derived from the distribution of the seismic
death-rate, that there were two detached foci, one near Polla and the
other near Montemurro.
This seems to be the best explanation of the facts recorded by Mallet.
There is, however, a possible difficulty that should not be
overlooked--namely, the apparently slight influence of the Montemurro
focus on the mean direction of the shock (Fig. 9). At a few places, of
course, the mean direction passes through both epicentres; at some
others, as we have seen, one of the two observed directions points
towards the Montemurro epicentre. It is not impossible, also, that
Mallet, after the first few days' work, may occasionally have quite
unconsciously selected and measured those fissures from the maze
presented to him which agreed most closely with his early impressions
obtained from the neighbourhood of Polla. But, for places nearer Polla
than Montemurro (and these form the majority of those visited by
Mallet), the probable explanation of the difficulty is that the
Montemurro focus was not so deep as the Polla focus. This, as will
appear more fully in the next chapter, would account for the
comparatively great intensity in the immediate neighbourhood of
Montemurro and for its rapid decline outwards; and it receives some
support from an isolated reference by Mallet to two angles of
emergence at Padula, one of 25° from the north, and the other of 8° or
10° in the perpendicular walls.
ELEMENTS OF THE WAVE-MOTION.
The elements of the wave-motion, as mentioned in the introductory
chapter, are four in number, namely, the period, amplitude, maximum
velocity, and maximum acceleration. If any two of these are known for
each vibration--and the first two are now given by every accurately
constructed seismograph--the others can be determined if the
vibrations follow the law of simple harmonic motion.[15]
_Amplitude._--To ascertain the amplitude, Mallet had to rely chiefly
on the fissures made in very inelastic walls. If the parts into which
such a wall are fractured are free to move, and yet, being inelastic,
obliged to remain in the farthest position to which they are carried
by the wave, the distance traversed by the centre of gravity of one of
the displaced parts should give a "rude approximate measure" of the
horizontal amplitude of the earth-wave. At Certosa, near Padula, he
thus found the amplitude to be about 4 inches, at Sarconi about 4-3/4
inches, and at Tramutola about 4-1/2 inches. From somewhat similar
evidence, the amplitude at Polla appears to have been about 2-1/2 or 3
inches; and, from the oscillation of a suspended clock or watch on a
rough wall, about 3-1/2 inches at La Sala and 1-3/4 inches at
Barielle. With the exception of Barielle, these places lie nearly on a
straight line passing through Mallet's epicentre, and he gives the
following table, showing an increase in amplitude with the distance
from the epicentre:--
Polla. La Sala. Certosa. Tramutola. Sarconi.
Distance in miles 4.0 13.4 19.0 23.8 30.8
Amplitude in inches 2-1/2 3-1/2 4 4-1/2 4-3/4
The existence of the Montemurro focus must, however, complicate any
relation that may connect these two quantities.
_Maximum Velocity._--The means at Mallet's disposal for determining
the maximum velocity were more numerous than those available for the
amplitude. From the dimensions of a fallen column of regular form we
should be able, he remarks, to find an inferior limit to the value of
the maximum velocity; while a superior limit at the same place may be
obtained from some other regular solid which escaped being overthrown.
If a loose body is projected by the shock at a place where the angle
of emergence is known, the horizontal and vertical distances traversed
by the centre of gravity will give the velocity of projection. Or, if
two such bodies are projected at one place, the same measures for each
will as a rule give both the angle of emergence and the velocity of
projection. A third method depends on the fissuring of walls,
supposing that we know the force per unit surface which, when suddenly
applied, is just sufficient to produce fracture. Sometimes more than
one method must be applied to the same object. The two gate-pillars
near Saponara (illustrated in Fig. 6) for example required a
horizontal velocity of 5.48 feet per second to fracture them, and an
additional velocity of 5.14 feet per second to overthrow them.
The well-known seismologist, Professor Milne, urges very forcibly that
measurements obtained from the projection or fall of columns are
unreliable, for the earlier tremors might cause the columns to rock,
and their overthrow need not therefore measure accurately the maximum
velocity of the critical vibration.[16] There can be no doubt that
Mallet was alive to this difficulty, though he may not have
appreciated it at its full value. Thus, at the Certosa de St. Lorenzo,
a monastery near Padula, a vase projected from the summit of a slender
gate-pier implied a velocity of 21-3/4 feet per second; and the excess
of about 8-1/4 feet per second above the velocity determined by other
means is attributed by him to the oscillation of the pier itself. How
far this source of error enters into other observations it is
impossible to say; but it is worth noticing how closely the velocities
obtained by different methods agree with one another. Thus, from
projection only, we have velocities of 11.5 feet per second at the
Certosa, 11.8 at Moliterno and Monticchio, 14.8 at Tramutola, and 9.8
feet per second at Sarconi; from overthrow alone, 11.0 feet per second
at Viscolione, near Saponara, and 11.6 at Barielle; from overthrow and
projection, 13.2 feet per second at Polla and 12.9 at Padula; from
fracture and overthrow, 12.3 feet per second at Potenza and 15.6 at
Saponara. The comparatively high values at Tramutola and Saponara,
Mallet imagined might be due to the oscillation of the hills on which
these towns are built. He therefore omits them in calculating the mean
maximum velocity, which he finds to be twelve feet per second, a
velocity less than that with which a man reaches the ground when he
jumps off a table.
With the same omissions, Mallet gives the following table, showing a
general decrease in the maximum velocity as the distance from his
epicentre increases:--
Polla. Padula. Certosa. Moliterno. Viscolione. Sarconi.
Distance in
miles 4.0 19.0 19.0 29.4 30.0 30.8
Max. vel. in
ft. per sec. 13.2 12.9 11.5 11.8 11.0 9.8
On the north side of the epicentre we have:--
Potenza. Monticchio. Barielle.
Distance in miles 17.3 27.1 28.2
Max. vel. in ft. per sec. 12.3 11.8 11.6
It is not impossible that the high calculated velocities at Tramutola
and Saponara were partly or entirely due to the impulse from the
Montemurro focus.
If we take 4 inches for the amplitude of the largest variation, and 12
feet per second for the maximum velocity, and assume the motion to
have been of a simple harmonic character, the period of a complete
vibration would be less than one-fifth of a second.[17] Now, we know
from seismographic records that this is roughly the period of the
small tremors that form the commencement of an earthquake-shock, while
the period of the largest vibrations may amount to as much as one or
two seconds. We may therefore conclude either that the assumption of
simple harmonic motion is incorrect, or that the maximum velocity is
too great, or more probably perhaps that the amplitude is too
small.[18]
SOUND-PHENOMENA.
Mallet was one of the first seismologists to realise the significance
of the earthquake-sound; and he attended closely to the subject,
though finding the sound even more elusive of precise observation
than the shock.
The chief result obtained by him was the comparative smallness of the
area over which the sound was heard. He estimates it at little more
than 3,300 square miles, or about one-twelfth of that over which the
shock was felt. It extends north and south from Melfi to Lagonegro,
and east and west from Monte Peloso to Duchessa and Senerchia. The
sound was thus confined to the region in which the shock attained its
most destructive character.
Towards the north and south ends of the sound-area all observers
described the sound as a low, grating, heavy, sighing rush, lasting
from twenty to sixty seconds, some adding that it was also of a
rumbling nature. Near the centre and the east and west boundaries, the
sound was distinctly more rumbling; it was shorter in duration, and
began and ended more abruptly.
The earthquake, Mallet remarks, "began everywhere with tremors; the
sounds generally arrived at the same time; the apparent direction of
movement of the tremulous oscillations appeared rapidly to change, and
still more rapidly to increase in amplitude; then the great _shove_ of
the destructive shock arrived, in some places rather before, in some a
little after, the moment of loudest sound, and it died away suddenly
(_i.e._, with extreme rapidity) into tremors again, but differing in
direction from that of the great shock itself."[19]
The earthquake-sound will be described more fully in the chapter
dealing with the Hereford earthquake of 1896, in which it will be
found that the phenomena recorded by Mallet are equally characteristic
of the slighter shocks felt in this country.
VELOCITY OF THE EARTH-WAVES.
In 1857 little was known about the velocity of earthquake-waves.
Experiments had been made by Mallet himself in 1849 in the
neighbourhood of Dublin. These gave 825 feet per second for the
velocity in dense wet sand, 1,306 feet per second in discontinuous
granite, and 1,665 feet per second in more solid granite.[20] The only
earthquake for which the velocity had been calculated was the Rhenish
earthquake of 1846, the value ascertained by Schmidt being 1,376
French feet, or 1,466 English feet, per second.
The accurate public measurement of time, which, as Mallet remarks, is
one of the surest indications of advancing civilisation, was, however,
unknown in the kingdom of Naples; and his attempt was therefore
fettered by the rarity of precise estimates of the time of occurrence.
Throughout the whole disturbed area only six good records could be
obtained, and three of these (at Vietri di Potenza, Atella, and
Naples) were derived from stopped clocks, witnesses of rather doubtful
value. At Montefermo and Barielle the time was at once read from a
watch, and at Melfi from an accurate pocket chronometer. The times
given vary from 9h. 59m. 16s. P.M. (Naples mean time) at Vietri di
Potenza to 10h. 7m. 44s. at Naples. Allowing for the supposed change
of direction by refraction at the Monte St. Angelo range on the way
to Naples, Mallet finds the mean surface velocity to be 787 feet per
second. Omitting the Naples record, and taking account of the
calculated depth of the focus, the mean velocity becomes 804 feet per
second.
MINOR SHOCKS.
A great earthquake rarely, if ever, occurs without some preparation in
the form of a marked increase of seismic activity. Perrey records
several shocks during the two years 1856-57 that were felt at places
as far apart as Naples, Melfi, and Cosenza. On December 7th, 1857, a
slight shock, with a report from beneath like the explosion of a mine,
was felt at Potenza. Then came the great earthquake on December 16th,
at about 10 P.M.
This was followed by numerous after-shocks--how numerous it is
impossible to say, for the records are of the scantiest description.
For some hours the ground within the meizoseismal area is said to have
trembled almost incessantly. At Potenza many slight shocks, both
vertical and horizontal, were felt during the night, and for a month
or more they were so frequent as to render enumeration difficult.
Mallet's last record is dated March 23rd, 1858, when four slight
shocks were felt at La Sala and Potenza, but occasional tremors were
reported to him until May 1859.
The most important of all these after-shocks was one felt about an
hour after the principal earthquake. Everywhere far less powerful, it
was yet strong enough to shake down many buildings at Polla that had
been shattered by the great shock. Towards the south at Moliterno,
and towards the north at Oliveto and Barielle, it evidently attracted
very little attention. So far as can be judged from the evidence given
by Mallet, the disturbed area seems to have been approximately of the
same form and dimensions as the meizoseismal area, and elongated in
the same direction, but concentric with the north-west focus.
On the other hand, if we may rely on too brief evidence, several
after-shocks recorded only at Montemurro, Saponara, Viggiano, or
Lagonegro, were probably connected with the south-east or Montemurro
focus.
ORIGIN OF THE EARTHQUAKE.
Mallet's theories have suffered perhaps more than any other part of
his work from the recent growth of our knowledge. From a historical
point of view, some reference to his explanation of the origin of the
Neapolitan earthquake seems desirable, and his own conscientious work
demands it. On the other hand, his conclusions are, for the present at
any rate, superseded, and it will therefore be sufficient to describe
them briefly.
Most of the wave-paths, as we have seen, pass within three miles of a
point almost coincident with the village of Caggiano. Of the
remainder, six traverse a spot about two miles farther to the
south-west, and three cross another about two miles farther to the
north-east. Neglecting other points of intersection, but taking
account of the observed emergences at Vietri di Potenza, Auletta,
Polla, etc., Mallet infers that the horizontal section of the focus
was a curve (indicated by the dotted line in Fig. 9) not less than ten
miles in length, and passing from near Balvano on the north, close to
Vietri di Potenza, Caggiano, and Pertosa, to a point about two and a
half miles west of Polla. Again, he remarks, the observed emergences
at places near the epicentre indicate that the vertical section of the
seismic focus was either more or less curved, or more probably a
surface inclined towards the south-east. He concludes, therefore, that
the seismic focus was a curved fissure, 10 miles long and 3-1/2 miles
in height, and with its centre at a depth of 6-1/2 miles below the
level of the sea.
The production of this great fissure, accompanied, perhaps by the
injection into it of steam at high pressure, was regarded by Mallet as
the cause of the principal earthquake. He imagines that the rent would
start at or near the central point of the focus and then extend
rapidly outwards in all directions. In the initial stage, vibrations
of very small amplitude would alone be transmitted, and these would
give rise to the early sounds and tremors. As the rending proceeded,
the vibrations would increase in strength up to a certain point when
they produced the shock itself. After this, they would decrease; and,
in the final stage, would give place to the small vibrations
corresponding to the sounds and tremors that marked the close of the
earthquake.
The rush of steam at high pressure into the focus Mallet does not seem
to have considered essential, though he evidently regarded it as
possible, indeed probable; and he suggests that it may have been in
part the cause of the earthquake which occurred an hour later. Though
feeling sceptical as to the existence of any general law of increase
of underground temperature, he assumes it, for the sake of
illustration, to be 1° F. for every 60 feet of descent. This would
give a temperature of 339° F. at the upper limit of the focus, 643° F.
at its central point, and 884° F. at its lower margin. If the focus
were filled with steam at each of these temperatures, the
corresponding pressures on its walls would be 8, 149, and 684
atmospheres, respectively. As the steam may be supposed to be admitted
suddenly and to be unlimited in supply, Mallet infers that it might
exist at the tension due to the highest of these temperatures, in
which case it would be capable of lifting a column of limestone 8,550
feet in height (or about one-half the depth of the upper margin of the
focus), and would exert a pressure on the walls of the focus of 4.58
tons per square inch, or of more than 640,528 millions of tons upon
its whole surface.
So many pages have already been given to this interesting earthquake
that I must sketch still more briefly my own view as to its origin.
There were, I believe, two distinct foci with their centres about
twenty-four miles apart along a north-west and south-east line, and it
was to this arrangement that the elongation of the meizoseismal area
was chiefly, though not entirely, due. The evidence is insufficient to
determine whether the earthquake was caused by fault-slipping; it is
in no way opposed to this view, but if the Neapolitan earthquake stood
alone, we should hardly be justified in drawing any further inference.
Relying, however, on knowledge obtained from the study of more recent
shocks, it seems to me probable that the two foci formed parts of one
fault with a general north-west and south-east direction. The slip
causing the first part of the double shock apparently took place
within the south-east focus, and was followed after a few seconds by
one within the north-west focus, greater in amount as well as more
deeply seated. In consequence of these displacements there were local
increases of stress, causing numerous small slips within or near both
principal foci; and, if we may judge from some slight shocks felt at
La Sala, accompanied also by other minor slips in the intermediate
region of the fault.
REFERENCE.
MALLET, R.--_The Great Neapolitan Earthquake of 1857: The First
Principles of Observational Seismology_, etc. 2 vols 1862.
FOOTNOTES:
[3] _Irish Acad. Trans._, vol. xxi., 1848, pp. 51-105 (read Feb. 9,
1846).
[4] _Brit. Assoc. Reports_, 1850, pp. 1-87; 1851, pp. 272-330; 1852,
pp. 1-176; 1853, pp. 117-212; 1854, pp. 1-326; 1858, pp. 1-136.
[5] _A Manual of Scientific Enquiry_, edited by Sir J.F.W. Herschel,
1849, pp. 196-223.
[6] _Irish Acad. Trans._, vol. xxii., 1855, pp. 397-410.
[7] The linear dimensions of the isoseismal lines are obtained by
measurements from Mallet's maps. The areas are given by him in
geographical square miles.
[8] Mallet, by some accident, omitted the losses at Polla and
neighbouring towns from this estimate. Mercalli (_Geologia d'Italia_,
pte. 3, p. 324) gives the number of killed as more than 12,300.
[9] Mallet does not make use of the term _epicentre_; he speaks of the
line FE as the _seismic vertical_. The modern and accepted terms are
used above
[10] _Japan Seismol. Soc. Trans._, vol. xi., 1887, pp. 175-177.
[11] _Ital. Seismol. Soc. Boll._, vol. ii., 1896, pp. 180-188.
[12] Professor Omori gives the mean direction as S. 71° W., but this
was obtained from observation on lamps with square, as well as with
circular bases.
[13] Twelve measurements chosen at random from Professor Omori's list
gave a mean direction of S. 78° W.
[14] When the accuracy of all the observations seemed equally
probable, he adopted the mean of the two extremes as the true
direction.
[15] If _a_ be the amplitude of a simple harmonic vibration, _T_ its
complete period, _v_ its maximum velocity, and _f_ its maximum
acceleration, we have v=2*pi*a/T and f=4*pi^2*a/T^2
[16] _Earthquakes and other Earth Movements_, pp. 81-82.
[17] Obtained from the formula: T=2*pi*a/v=2*pi*x*(1/3)/12
[18] If we take the maximum velocity to be 12 feet per second, and the
period to be one second, the amplitude would be about 11-1/2 inches.
[19] Vol. ii., p. 299. The punctuation of the original is not followed
in the above extract.
[20] _British Association Report_, 1851, pp. 272-320.
CHAPTER III.
THE ISCHIAN EARTHQUAKES OF MARCH 4TH, 1881, AND JULY 28TH, 1883.
Separated from Italy by a distance of not more than six miles, Ischia
and the intermediate island of Procida strictly form part of the
Phlegræan Fields, the well-known volcanic district to the north of
Naples. Ischia, the larger of the two islands, is six miles long from
east to west, and five miles from north to south, and contains an area
of twenty-six square miles. In 1881, the total population was 22,170,
that of Casamicciola, the largest town, being 3,963.
VOLCANIC HISTORY OF ISCHIA.
The central feature of Ischia is the great crater of Epomeo (_a_, Fig.
14). On the south side, and partly also on the east, the crater-wall
has been broken down and removed; the portion remaining is about 1-1/2
mile in diameter from east to west, and reaches a height of 2,600 feet
above the sea-level. All the upper part of the mountain is composed of
a pumiceous tufa, rich in sanidine and of a characteristic greenish
colour. At two points, to the west near Forio and to the north between
Lacco and Casamicciola, this tufa is seen reaching down to the sea;
but, in all other parts, it is covered by streams of trachitic lava,
by more recent tufas, or by a deposit of marly appearance, which is
regarded by Fuchs as resulting from the decomposition of the Epomean
tufa.
There are two distinct periods in the geological history of Ischia.
The first, a submarine period, probably began with the dawn of the
quaternary epoch, for all the marine fossils of the island belong to
existing species. About this time, Epomeo seems to have originated in
eruptions occurring in a sea at least 1,700 feet in depth--eruptions
that preceded the formation of Monte Somma and were either
contemporaneous or alternating with those that gave rise to the oldest
trachitic tufas of the Phlegræan Fields. The destruction of the south
wall may have occurred much later through some great eruptive
paroxysm, but more probably, as Professor Mercalli suggests, through
early marine erosion and subsequent subaerial denudation. To the
submarine period must also be assigned the formation of the trachitic
masses which compose Monti Trippiti, Vetta, and Garofoli (_b_, _c_,
_d_, Fig. 14), on the east side of Epomeo; and, in part only, those of
Monte Campagnano and Monte Vezza (_f_, _g_).
At or near the close of the elevation, many violent eruptions occurred
on the south-west of Epomeo, during which was formed the south-west
corner of the island, including Monte Imperatore and Capo Sant' Angelo
(_h_, _i_).
In the second or terrestrial period, when the island had practically
attained its present altitude, the eruptive activity was almost
confined to the eastern and northern flanks of Epomeo. At the
beginning Monte Lo Toppo (_j_) was formed by a lateral eruption. In
the north-west corner of the island, Monte Marecocco and Monte Zale
(_k_ and _l_) owe their origin to a gigantic flow of sanidinic
trachite, issuing probably from the depression which now exists
between them. Lastly, towards the north-east, are the recent lateral
craters of Rotaro, Montagnone, Bagno, and Cremate (_m_, _n_, _p_,
_s_), the first two being the most regular and best preserved in the
island.
[Illustration: FIG. 14.--Geological sketch-map of Ischia.
(_Mercalli._)[21]]
The earliest eruption of the historic, or rather human, period appears
to have taken place from Montagnone, and probably also at about the
same time from the secondary crater of Porto d'Ischia (_u_), about the
beginning of the eleventh century B.C. The eruptions of Marecocco and
Zale are referred to about B.C. 470; and those of Rotaro and Tabor
(_q_) to between the years 400 and 352 B.C. Another eruption is said
to have occurred in B.C. 89, but the site of it is unknown; and three
others are recorded on doubtful authority about the years A.D. 79-81,
138-161, and 284-305. The last outburst of all took place after the
series of earthquakes in 1302 from a new crater, that of Cremate
(_s_), which opened on the north-east flank of Epomeo, and from which
a stream of lava, called the Arso (_t_), flowed down rapidly and,
after a course of two miles, reached the sea.
After the first eruptions to which it owed its origin, the central
crater of Epomeo apparently remained inactive. All the later eruptions
occurred either on the external flanks of the mountain or on radial
fractures of the cone.[22] Trippiti, Lo Toppo, Montagnone and the Lago
del Bagno (_b_, _j_, _n_, _p_) lie in one line, Vetta and Cremate
(_c_, _s_) on another, and Garofoli and Vatoliere (_d_, _e_) on a
third, all passing through a point near the town of Fontana, which
occupies the centre of the old crater of Epomeo.
Professor Mercalli points out that the lateral eruptions of Epomeo
differ in one respect from those of Etna and Vesuvius. In these
volcanoes the lava ascends to a considerable height in the central
chimney, and by its own weight rends open the flanks of the cone. In
Epomeo, it appears to traverse lateral passages at some depth, perhaps
far below the level of the sea, and to rend the mountain by means of
the elastic force of the aqueous vapour, etc., which it contains. It
will be seen how important is the bearing of this difference on the
occurrence of the Ischian earthquakes.
The eruptions that have taken place during the last three thousand
years agree in several particulars. They either occurred suddenly, or,
at any rate, were not preceded by a stage of moderate Strombolian
activity; they were always accompanied by violent earthquakes; and all
succeeded intervals of long repose. As the eruption of 1302 happened
after at least a thousand years of rest, the lapse of six more
centuries does not justify us in concluding that Epomeo is at last
extinct.
We seem, on the contrary, to be drawing near another epoch of
activity. During the four and a half centuries that followed the
eruption of 1302, we have no record of Ischian earthquakes.[23] Then,
suddenly, on the night of July 28-29, 1762, Casamicciola was visited
by sixty-two shocks, some of which were very strong and damaged
buildings. On March 18th, 1796, another severe shock took place, but
destructive only in the neighbourhood of Casamicciola, where seven
persons were killed. On February 2nd, 1828, the area of damage, though
concentric with the former, enlarged its boundaries; 30 persons were
killed and 50 wounded. On March 6th, 1841, and during the night of
August 15-16, 1867, further shocks injured houses at Casamicciola, but
without causing any loss of life. Slight tremors occurred at various
dates in 1874, 1875, 1879, and 1880, leading up to the disastrous
earthquakes here described, those of March 4th, 1881, when 127 persons
were killed, and July 28th, 1883, which resulted in the death of 2,313
persons and the wounding of many others.
EARTHQUAKE OF MARCH 4TH, 1881.
The Ischian earthquakes have been fortunate in their investigators. In
the spring of 1881, Dr. H.J. Johnston-Lavis, the chronicler for many
years of Vesuvian phenomena, was residing in Naples. Impressed by a
recent perusal of Mallet's report on the Neapolitan earthquake, and
wishing to test the value of the methods explained in the last
chapter, he crossed over to Ischia on March 5th; and to his unwearied
inquiries extending over more than three weeks and lasting from
thirteen to sixteen hours a day, we are indebted for most of what we
know about the earthquake of 1881.
On March 4th, at 1.5 P.M., the great shock occurred abruptly, without
any warning tremors. Its effects were aggravated by the faulty
construction of the houses. The walls are of great thickness, loosely
put together, and connected by mortar of the poorest quality. The
chimneys and roofs also are massive, and the rafters are so slightly
inserted in the walls that they were drawn out with the rocking of the
houses. In such cases, the destruction was often so complete that no
fissures were left available for measurement.
ISOSEISMAL LINES AND DISTURBED AREA.
The isoseismal lines as drawn by Dr. Johnston-Lavis are represented by
the curves in Fig. 15. The isoseismal marked 1 bounds the area of
complete destruction; it is about 1 mile long from east to west, 2/3
of a mile broad, and contains an area of not more than half a square
mile. The next isoseismal (2) marks the area of partial, but still
serious, destruction; this is nearly 2 miles long from east to west,
1-1/4 miles broad, and 2 square miles in area. Within the isoseismal
3, buildings were more or less slightly damaged. The course of this
curve is somewhat doubtful, but, as drawn, it is about 3 miles long,
2 miles wide, and 5 square miles in area.
[Illustration: FIG. 15.--Isoseismal lines of the Ischian
earthquake of 1881. (_Johnston-Lavis._)]
Outside the last curve, the shock diminished rapidly in intensity. At
Monte Tabor and Bagno, it was very slight; in the town of Ischia, only
about half the people were conscious of any movement; and at Capella,
a small village to the south, it was not felt at all. Again, the shock
was perceptible, though only faintly, in the neighbourhood of
Campagnano, at Serrara to the south of Epomeo, and at Panza near the
south-west corner of the island. On the other hand, at Fontana, which
occupies approximately the centre of the crater of Epomeo, there were
evidences of a distinctly stronger shock. No house actually fell, and
side walls were but little injured; but the roofs, which are of great
weight, suffered considerable injury.
In the adjacent island of Procida, the shock was felt distinctly by
many people, and by some, though slightly, at Monte di Procida,
Misenum, and Bacoli, on the coast of Italy. No record whatever was
given by the seismographs in the university of Naples and the
observatory on Vesuvius. We have of course no means of estimating the
exact size of the disturbed area, but in this respect, disastrous as
the earthquake was in the neighbourhood of Casamicciola, it was
clearly inferior to all but the very weakest earthquakes felt in the
British Islands.
POSITION OF THE EPICENTRE.
In determining the position of the epicentre, Mallet's method was
closely followed. Fissures in buildings were used for the most part,
in two out of every three cases; and occasional measurements were
made from objects overthrown, projected, or shifted, and also from the
personal experiences of observers. The attempt to apply the method
was, however, fraught with difficulties. The heterogeneous structure
of the island was no doubt responsible for many divergent azimuths;
the irregularity of the buildings both in form and material and their
variety of site furnished other sources of error; even the smallness
of the area was a disadvantage in lessening the number of trustworthy
records.
Measurements were made at 55 places altogether, but in most cases they
were the results of isolated observations, not the means of several at
each place. On this account, I have not reproduced in Fig. 15 the
azimuths shown in Dr. Johnston-Lavis's map of the earthquake. A large
number of them clearly converge towards an area lying to the west of
Casamicciola; and, from their arrangement, Dr. Johnston-Lavis
concludes, though the evidence does not seem to me quite strong enough
for the purpose, that they emanated from a fracture running from a
little west of north to a little east of south.
This conclusion is, however, justified by other evidence. In the
centre of the injured district, Dr. Johnston-Lavis has traced a
meizoseismal band, in which the shock must have been nearly or quite
vertical. "The damage inflicted on buildings included within this band
was," he says, "very characteristic of the nature of the shock; the
walls having received but slight injury, whilst almost every floor and
ceiling had been totally destroyed. In fact," he adds, "many houses
would have required no other repairs than the replacing of the
divisions between the different storeys." The shaded central area in
Fig. 15 represents this band, passing in a nearly north and south
direction from a point midway between Campo and the upper part of
Lacco on the north, through the west part of Casamenella and Campo, to
a point near Frasso on the south; the length of the band being thus
about two-thirds of a mile.
If the central line of this band is produced towards the south, as
indicated by the dotted line, it grazes the west side of Fontana,
where, as we have seen, there was a second meizoseismal area, much
smaller than the other and surrounded by a district in which houses
were almost uninjured. That the shock in this town was vertical or
nearly so, is shown by the nature of the damage (p. 52) and also by
the testimony of the inhabitants. I will give Dr. Johnston-Lavis's
explanation of this detached meizoseismal area when discussing the
origin of the Ischian earthquakes; but the evidence seems to me to
favour either the existence of two distinct foci or, more probably
perhaps, the extension of the fissure to the south with an increased
impulse beneath the centre of Epomeo.
DEPTH OF THE SEISMIC FOCUS.
At nine places, Dr. Johnston-Lavis was able to make measurements of
the angle of emergence, in every case from fissures in buildings, and
therefore liable to sources of error already referred to. On the other
hand, owing to the small depth of the focus, there would probably be
less general refraction of the wave-paths than in the Neapolitan
earthquake. The depths indicated by these observations vary between
about 615 and 2,885 feet, a difference that is no greater than might
be expected, as the size of the focus was no doubt comparable with
that of the district in which observations were made. The mean depth
Dr. Johnston-Lavis finds to be about 1,700 feet, or a little less than
one-third of a mile.
NATURE OF THE SHOCK.
The limited depth of the focus is also evident from the nature of the
shock. It was only within the actual meizoseismal band that the shock
was subsultory or vertical throughout; at a short distance from the
epicentre, the movement was both subsultory and undulatory; while near
the third isoseismal, and in most of the region outside, the movement
was entirely undulatory or lateral. An observer at Perrone (which lies
1-2/3 miles east of the epicentre) gives the following account of the
shock:--"I was standing on my balcony (this faces Casamicciola)
admiring the scene ... when I felt the house rock, feeling at the same
time as if something was rolling along beneath the ground. This
movement was accompanied by a sound like this, Boob, boob -- boob --
-- boob -- -- -- boob -- -- -- -- boob. Both noise and movement seemed to
come from Casamicciola.... In a few seconds, in the distance over the
town arose a terrific cloud of white dust, so that I imagined the town
on fire.... I felt hardly any, if any, subsultory movement, but as I
leant upon the balcony rails, I was alternately pressed against them
and then drawn away."
At Fontana, however, the undulatory shock was replaced by a vertical
one. This was the universal experience, though one or two persons felt
a slight lateral movement immediately after. At Valle (near Barano)
and Piejo, both places about a mile from Fontana, the vertical
component was also perceptible.
AFTER-SHOCKS.
The after-shocks were few and of slight intensity. Dr. Johnston-Lavis
gives the following dates: March 7th, 12.5 A.M. and midday; March
11-12, 15-16, 17-18, 27 (?), April 5th and 6th, and July 18th, 8.30
P.M. The only shock of the series marked as strong occurred at
midnight on March 15-16 at Casamicciola. The last of all, that of July
18th, consisted of a rumble and slight shock, and was most perceptible
at Fango.
EARTHQUAKE OF JULY 28TH, 1883.
Undeterred by the experience of 1881 or by the warnings of
seismologists, Casamicciola was rebuilt, only to suffer more complete
disaster. On July 28th, 1883, at 9.25 P.M., occurred the most
destructive earthquake of which we have any record in Ischia. The
shock lasted about fifteen seconds, and before it was over clouds of
dust were rising above the ruins of Casamicciola, Lacco, and Forio;
1,200 houses were destroyed, 2,313 persons were killed, nearly 1,800
in Casamicciola alone, and more than 800 seriously wounded. "No better
idea," says Dr. Johnston-Lavis, "of the absolute destruction of
buildings could be conceived than what was actually realised at
Casamicciola and Campo. Looking, on the following Monday, over the
field of destruction, I could discover (with few exceptions) the
wall-stumps only remaining."
Dr. Johnston-Lavis again spent about three weeks in the island,
examining the effects of the new shock with equal zeal and wider
experience. His monograph is now our chief work of reference on
Ischian earthquakes. Inquiries were also made by several Italian
seismologists, among others by Professor M.S. de Rossi, the organiser
of earthquake-studies in the peninsula; by Professor L. Palmieri, the
founder of the Vesuvian observatory; and especially by Professor G.
Mercalli, whose valuable memoir supplements the report of Dr.
Johnston-Lavis in some important particulars.
PREPARATORY SIGNS.
The interval between July 18th, 1881, when the last shock of that year
was felt, and July 28th, 1883, was one of almost complete quiescence.
Early in March 1882, a few slight shocks were noticed at Casamicciola.
On July 24th, 1883, a watch hanging from a nail in a wall was seen to
swing at 6 A.M. and 9 A.M., and, on the same morning, at about 8.30, a
slight shock, accompanied by a rumbling sound, was felt at
Casamicciola. Again, on the 28th, about a quarter of an hour before
the great shock, one observer at Casamicciola states that an
underground noise was heard, and that some persons in consequence left
their houses.
Many assertions have been made with regard to variations witnessed a
day or two before the shock in the hot springs, such as an increase of
flow or temperature and changes in their volume and purity. Fumaroles
are alleged to have burst out with violence, and even flames to have
been seen. The statements, though widely quoted, can hardly be said
to rest on satisfactory evidence. On the other hand, Dr.
Johnston-Lavis arrived in the island within twenty-four hours after
the shock, and, before another day had elapsed, he had examined most
of the places where the phenomena were said to have occurred, but
could find no remarkable change nor any signs of such having taken
place. It is also known, as he remarks, that the temperature of the
Ischian springs and fumaroles sometimes varies considerably without
any earthquake following, that of the water of Gurgitello occasionally
changing by as much as 30° or 40°. We may therefore, I think, conclude
that, except for one or two shocks and underground noises too slight
to cause general alarm, there were no decisive heralds of the great
earthquake.
ISOSEISMAL LINES AND DISTURBED AREA.
The curves in Fig. 16 represent the isoseismal lines as drawn by Dr.
Johnston-Lavis. As in the earthquake of 1881, they bound respectively
the areas of complete destruction, partial destruction and slight
damage to buildings, the course of the outer line being to a great
extent conjectural owing to the small extent of land traversed by it.
The first isoseismal is about 2-1/2 miles long, 1-1/2 miles broad, and
3 square miles in area; the second about 4 miles long, 3-1/2 miles
broad, and 11 square miles in area; and the third about 6-1/2 miles
long, 6 miles broad, and 30 square miles in area. The curve drawn by
Professor Mercalli (Fig. 14) coincides nearly with the second of these
lines.
At Fontana, the damage exceeded that in the surrounding country,
though the difference was of course less marked than on the previous
occasion.
[Illustration: FIG. 16.--Isoseismal lines of the Ischian
earthquake of 1883.]
Outside Ischia, the shock was felt distinctly in all the island of
Procida and in Vivara; on the mainland, by some as far as Pozzuoli
and by several persons in Naples, which is twenty miles from
Casamicciola. The seismograph at the university of this city
registered two small shocks, the first at 9.10 P.M., and the second
and stronger at 9.25 P.M.; and De Rossi states that at about 9.30 P.M.
the seismographs at Ceccano, Velletri, and Rome recorded a shock
consisting of very slow undulations. There are again no materials for
estimating the size of the disturbed area, but there can be no doubt
that it was much less than that of a moderately strong British
earthquake.
POSITION OF THE EPICENTRE.
Owing to the limited size of the disturbed area, time-observations,
even had they been available, would not have sufficed to determine the
position of the epicentre, and both Dr. Johnston-Lavis and Professor
Mercalli therefore had recourse to Mallet's method, the former relying
chiefly, as before, on fissures in damaged buildings, and the latter
on the overthrow or displacement of columns and other objects.
Dr. Johnston-Lavis measured the azimuth of the wave-paths at
sixty-five places, and at about one-third of these was able to make
two or more observations. The azimuths converge towards the same
region as in 1881, but the area covered by their intersections is
larger. The meizoseismal band of maximum vertical destruction
indicated by shading in Fig. 16 is also of the same form and slightly
greater extent, reaching from the upper part of Lacco to a little
south of Frasso, and being therefore nearly a mile in length. The
centre of maximum impulse was in the same position as in 1881, or
possibly a little more to the south.
Professor Mercalli's observations were made at forty-eight places, and
in only six cases were they the same as those used by his predecessor.
He also notices that most of the azimuths converge towards
Casamenella, and intersect within an elongated area. This area runs in
the same direction as Dr. Johnston-Lavis's meizoseismal band, but is
less elongated, and situated a short distance farther to the south,
though on the whole the agreement between the two areas is remarkably
close.
There was again apparently a second epicentre at Fontana. In this
town, according to Dr. Johnston-Lavis, there were two distinct types
of damage. As in 1881, there was evidence of a vertical blow, the only
one that absolutely ruined houses; but, in addition, there was another
independent set of fissures, quite as widely distributed as the
others, though evidently caused by a less violent movement. These
indicated a wave-path with a low angle of emergence coming from
between north and north-north-west, or almost exactly in the line of
meizoseismal band. To the south of Fontana, however, there is a group
of places, including Panza, Serrara, Barano, etc., where the azimuths
diverged rather widely from the epicentre at Casamenella. These
azimuths are twelve in number, and it is worthy of notice that they
all intersected the crater of Epomeo, while half of them passed within
a few hundred yards of Fontana.
DEPTH OF THE SEISMIC FOCUS.
Measurements of the angle of emergence were made by Dr. Johnston-Lavis
at twenty-four places, and in every case from fissured walls. The
greater part of the diagram on which his results are depicted is
reproduced in Fig. 17. The horizontal line, as in Fig. 13, represents
the level of the sea, the longer vertical line one passing through the
epicentre, and the shorter another through Fontana. The short lines
on the left of the former show the incipient wave-paths to places
lying east of the epicentre; those on the right, with one exception,
represent the wave-paths to places west of the same meridian. Small
horizontal marks are inserted on the vertical lines to show the depth
in tenths of a mile below the level of the sea.
[Illustration: FIG. 17.--Diagram of wave-paths at seismic
vertical of Ischian earthquake of 1883. (_Johnston-Lavis._)]
The six angles of emergence that would give the greatest depth below
the epicentre were all measured at places in the south of the island
close to the line joining Panza and Barano, and it will be noticed
that five of these apparent depths are much greater than those
obtained from the other wave-paths. Excluding these observations, the
remaining eighteen give depths ranging from about 450 to about 3,350
feet, and a mean depth of 1,730 feet,[24] or nearly one-third of a
mile, that is, almost exactly the same as the mean depth found from
the earthquake of 1881.
The six exceptional angles of emergence come from the district of
divergent azimuths to the south of Epomeo. Three of the corresponding
azimuths pass within one-quarter of a mile from the centre of Fontana,
and none of the other three more than three-quarters of a mile from
the same point. Though disbelieving in a subsidiary focus below this
town, Dr. Johnston-Lavis has calculated its mean depth, supposing it
to exist, and found it to be about 1,560 feet below the sea level, a
result which is remarkably close to the calculated mean depth of the
focus near Casamenella.
NATURE OF THE SHOCK.
In the meizoseismal band, preliminary tremor and rumbling sound were
alike absent. So sudden, indeed, was the onset of the earthquake, that
the survivors generally found themselves beneath the ruins of their
houses before they were conscious of any shock. The destruction,
practically instantaneous, was wrought by four or five vertical blows,
so powerful that, according to some observers, Casamicciola seemed to
jump into the air. Then followed undulations, not noticed by all, that
appeared to come from every direction. The shock lasted altogether
fifteen seconds or more,[25] and was accompanied by a rumbling noise,
in the midst of which were detonations as of thunder or of great blows
given upon an empty barrel.
In the immediate neighbourhood of the meizoseismal area, at Perrone,
Pennella, and Lower Lacco, the subsultory movement was still the more
prominent; but, farther away, as at Panza, Testacchio, Barano, Ischia,
and Bagno, the subsultory motion was followed by distinctly horizontal
undulations, while outside the island of Ischia only slow undulatory
movements were perceptible.
LANDSLIPS.
The dotted areas in Fig. 16 indicate the sites of the only landslips
of importance that were precipitated by the earthquake of 1883. Two of
these occurred on the north slope of Epomeo, and the third on the
west flank of Monte Rotaro. The materials of the Epomean landslips had
evidently been separated for some time by shallow fissures from the
adjoining rock, for the surfaces of the fissures were discoloured by
fumarolic action. Immediately after the earthquake a cloud of dust was
seen to rise from the spots; the masses, already detached laterally,
were merely set in motion by the shock; and they continued to slide
down during the following days either through the action of the
after-shocks or of the heavy rains that followed.
All over the island, however, fissures and minor landslips occurred.
At two places on the north coast the steep cliffs of incoherent tufa
were so much damaged that, according to Dr. Johnston-Lavis, "large
quantities of their materials were thrown into the sea. The water then
sorted out the pieces of pumice, which in many cases were of very
large size, and were seen floating about in the neighbourhood for some
days," giving rise to the supposition that a submarine eruption had
taken place to the north of the island.
AFTER-SHOCKS.
The after-shocks in 1883 were much more numerous than in 1881. Between
9.25 P.M. on July 28th and noon on August 3rd, twenty-one slight
shocks were recorded at Casamicciola. At 2.15 P.M. on August 3rd, a
violent shock occurred that caused further damage at Forio, and even
at places so far from the epicentre as Fiaiano, Barano, and Fontana,
and increased the displacements of the landslips on Epomeo. This
shock was also registered at the observatory on Vesuvius.
After this the shocks became less frequent and slighter, twelve being
felt at Casamicciola during the remainder of the year, and six in the
first half of 1884. Several shocks and rumbling noises were also
observed in other parts of the island. Among them may be mentioned
noises heard at Fontana on August 12th and 15th, and a slight shock at
the same place on August 17th; also on September 4th, at 10.30 and
10.40 A.M., slight shocks at Barano, Serrara, and Forio. On March
27th, 1884, at 2.7 P.M., another strong shock occurred; strongest at
Serrara, where the shock was subsultory and accompanied by noise; and
less strong, though still subsultory, at Ciglio, Panza, Forio,
Fiaiano, and Casamicciola, and very slight at Ischia. The series seems
to have ended during the following summer, with a slight shock at
Casamicciola on July 21st, and a stronger one on July 23rd, felt from
Casamicciola on the north to Serrara on the south.
Most of the after-shocks must have originated in the neighbourhood of
Casamicciola, but it is worthy of notice that more than one centre was
in action. Several were recorded at Ischia only. Others, as mentioned
above, affected chiefly the south part of the island, and especially
the small towns of Serrara and Fontana.
CHARACTERISTICS OF ISCHIAN EARTHQUAKES.
After the eruption of 1302, there succeeded a period of comparative
repose in Ischia. The revival of activity dates from 1762, and, since
that year, there have been four great earthquakes, namely, those of
1796, 1828, 1881, and 1883. In every respect but that of increasing
intensity, these earthquakes were apparently identical; each, as
Professor Mercalli says, was merely a replica on a different scale of
those that preceded it. The principal features in which they resemble
one another, and differ from the average tectonic earthquake, are the
coincidence of the epicentres, the small depth of the foci, and the
sudden onset of the principal shock.
1. _Coincidence of Epicentres._--In Fig. 14, which is copied from
Professor Mercalli's map, are shown the areas in which buildings were
seriously damaged by these four earthquakes. The curves for 1796,
1828, and 1881 are approximately concentric. In 1796, the shock was
disastrous only to the west of Casamicciola; in 1828, according to
Covelli, "the ground most injured was not precisely the region of
Casamicciola, but that which lies between the district called Fango
and that known as Casamenella, situated to the west of Casamicciola,
and a short distance from it."[26] The epicentres may have varied
slightly in size, but, in position, it is clear that all four were
nearly or quite coincident. The meizoseismal bands in 1881 and 1883
were also similar in form and elongated in the same direction.
In the last two earthquakes there was, as we have seen, very distinct
evidence of a secondary meizoseismal area surrounding Fontana, and it
is remarkable that this was also noticeable in the earthquake of 1828.
"Besides the centre of vibration in the district of Fango," says
Covelli, "another less powerful centre showed itself in the locality
of Fontana; this made itself felt more heavily than in surrounding
localities; as if another centre of movement had taken place from that
part, independent of the former."
2. _Small Depth of the Foci._--Mallet's method, as noted above, cannot
be trusted to yield accurate estimates of the focal depth, or to
indicate more than its order of magnitude. But it is remarkable that
the depths calculated by Dr. Johnston-Lavis for the last two
earthquakes are both only a little less than a third of a mile, and it
is probable that the actual depth did not differ very greatly from
this amount. The nature of the shock, vertical or nearly so close to
the epicentre and horizontal at a short distance from it, is merely
personal testimony of the same character as fissures in masonry, and
of course points to the same result.
[Illustration: FIG. 18.--Diagram showing connection between depth
of focus and rate of decline in intensity.]
But the most conclusive evidence on which we have to rely is the
extraordinary intensity of the shock at the centre of a very small
distributed area. In Great Britain, an earthquake felt over a district
of equal size would hardly at the centre exceed the trembling produced
in a station platform by a passing train. The curves in Fig. 18 show
how the rate of decline in intensity depends on the depth of the
focus. They are drawn on the supposition that the intensity at any
point on the surface varies inversely as the square of its distance
from the focus; the curves _a_, _b_, _c_ corresponding to foci
situated at depths of one-third of a mile, one mile, and two miles
respectively, and the figures below the horizontal line denoting the
distance in miles from the epicentre. Thus, the rapid decline of
intensity from the epicentre outwards shows that, in each of the four
great Ischian earthquakes, the depths of the focus must have been very
small.
3. _Suddenness of the Shocks._--In 1796, we have no record of
preparatory shocks, but the evidence is scanty; in 1828 and 1881, none
are mentioned; in 1883, one or two tremors and underground noises,
possibly of seismic origin, gave warning to a few. Fore-shocks, for
all practical purposes, were conspicuous by their absence.
Still more remarkable is the sudden advent of the great shocks. There
were no preliminary tremors or rumbling sound, no animals showed signs
of uneasiness and no birds fluttered screaming from trees or ground.
The shock of 1828, says Covelli, "was announced by three powerful
blows coming almost vertically, from below upwards;" and the same
words apply equally well to the earthquakes of 1881 and 1883. The
destruction of houses in every case was practically instantaneous, and
coincident with the first vibration.
In all respects, tectonic earthquakes differ widely from the Ischian
shocks. The epicentres of successive earthquakes are rarely
coincident, but show a distinct tendency to migration along certain
lines; the decline in intensity outwards from the epicentre is nearly
always very gradual, and therefore indicative of a comparatively
deep-seated focus; they are almost invariably preceded either by a
series of slight shocks and rumbling sounds, or, in an unstable
district, by a marked increase in their frequency. Distinctions, so
great as these are, evidently remove the Ischian shocks from the
category of tectonic earthquakes.
ORIGIN OF THE ISCHIAN EARTHQUAKES.
On the other hand, the Ischian earthquakes possess several features
which connect them closely with true volcanic earthquakes.
1. They originate beneath the northern slope of Epomeo--a volcano that
we have no reason to consider absolutely extinct, but rather as one
subject to eruptions at long intervals of time--in a region as yet
unoccupied by parasitic craters, but having the same relation to the
central cone of Epomeo as those in which the recent craters of Monte
Rotaro, Montagnone and Cremate are situated.
2. In both the earthquakes of 1881 and 1883, the epicentre is an
elongated band, the axis of which, if produced, would pass through the
centre of the old crater of Epomeo. Along the line of this band, occur
the fumaroles of Monte Cito and Ignazio Verde and the thermal springs
of the Rita and Capitello. These facts, as Professor Mercalli
suggests, lead us to believe that the foci of the earthquakes coincide
with a radial fracture of the volcano, the course of which, as traced
by him, is represented by the continuous line in Fig. 14.[27]
3. Except in their relations with actual eruptions, the Ischian
earthquakes resemble closely the true volcanic earthquakes which from
time to time shake the flanks of Etna. These are marked by great
intensity of the shock at the centre of a comparatively small
disturbed area, epicentres often elongated radially to the cone,
frequent repetition with similar characters in the same districts; and
as a rule they precede by a short interval, but sometimes accompany or
follow, volcanic eruptions.[28]
Two other phenomena may be referred to as probably indicating some
connection between Ischian earthquakes and the structure and history
of Epomeo.
We have seen that, in the three earthquakes of 1828, 1881, and 1883,
there is distinct evidence of a second meizoseismal area at Fontana,
within which the shock was mainly subsultory. Dr. Johnston-Lavis,
though recognising the possibility of the existence of two epicentres,
prefers another explanation.[29] But the wide extension of the
southern boundary of the area of destruction in 1883, and the
limitation of several of the after-shocks to the south of the island,
seem to me to favour the existence of a second focus beneath the
crater of Epomeo, though, it may be, not entirely detached from the
chief focus beneath Casamenella.
Again, as Professor Mercalli remarks, all historic eruptions on the
flanks of Epomeo were accompanied by very violent earthquakes; while,
previously to 1302, only one disastrous earthquake, so far as known,
occurred in the island without being attended by an eruption. It
should be noticed also that the principal shocks during the recent
revival of activity (_i.e._, since 1762) show a continual increase in
intensity, whether this be measured by the damage to buildings, the
loss of life, or the extent of the area of destruction (Fig. 14).
It therefore seems legitimate to conclude that, in the recent Ischian
earthquakes, we have merely so many unsuccessful attempts to force a
new volcanic eruption. The passages once existing through Epomeo and
its parasitic craters having become blocked, the highly heated magma
beneath is compelled to find a new outlet. Its tension slowly
increasing, the crust above is at last rent, or an incipient rent is
enlarged, the fluid rock is injected almost instantaneously with great
force into the open fissure, and its sudden arrest by the containing
walls is the ultimate cause of an earthquake. With the expansion of
the magma, its tension is at once correspondingly reduced, and some
time must elapse before it can again reach the critical point at which
a further rupture, resulting in a second shock, takes place.[30]
Thus, with each great Ischian earthquake, we are, I believe, advancing
a step nearer the time, which may be close at hand or may be very
remote, when the fracture will at last reach the surface, and above
the site of Casamenella a new parasitic cone will rise, from which, as
from Cremate in 1302, a stream of lava may flow down towards the sea.
REFERENCES.
1. BALDACCI, L.--"Alcune osservazioni sul terremoto avvenuto all'
Isola d'Ischia il 28 luglio 1883." _Ital. Com. Geol. Boll._,
vol. xiv., 1883, pp. 157-166.
2. DAUBRÉE, A.--"Rapport sur le tremblement de terre ressenti à
Ischia le 28 juillet, 1883; causes probables des tremblements
de terre." Paris, _Acad. Sci._, _Compt. Rend._, vol. xcvii.,
1883, pp. 768-778.
3. DU BOIS, F.--"The Earthquakes of Ischia." _Japan Seism. Soc.
Trans._, vol. vii., pt i., 1883-84, pp. 16-42.
4. ---- "Farther Notes on the Earthquakes of Ischia." _Ibid._, vol.
viii., 1885, pp. 95-99.
5. JOHNSTON-LAVIS, H.J.--_Monograph of the Earthquakes of Ischia_
(1885).
6. MERCALLI, G.--_Vulcani e fenomeni vulcanici in Italia_ (vol. iii.
of _Geologia d'Italia_, by G. Negri, A. Stoppani, and G.
Mercalli), 1883, pp. 46-50, 331-332.
7. ---- _L'Isola d'Ischia ed il terremoto del 28 luglio 1883_
(Milano, 1884).
8. PALMIERI, L., E A. OGLIALORO.--"Sul terremoto dell' Isola
d'Ischia della sera del 28 luglio 1883." Napoli, _R. Accad.
Atti_, vol. i., 1884, pp. 1-28.
9. ROSSI, M.S. DE.--"Il terremoto di Casamicciola del 4 marzo 1881."
_Bull. del Vulc. Ital._, anno viii., 1881, pp. 5-12. (In the
same volume are brief notices by different writers on pp. 22,
38-42, 52-53, 67-68, 70-74.)
10. ---- "Raccolta di fatti, relazioni, bibliografie sul terremoto
di Casamicciola del 28 luglio 1883, con brevi osservazioni."
_Bull. del Vulc. Ital._, anno xi., 1884, pp. 65-172.
11. ---- "Intorno all' odierna fase dei terremoti in Italia e
segnatamente sul terremoto in Casamicciola del 4 Marzo 1881."
_Ital. Soc. Geogr. Boll._, 1881.
12. SERPIERI, A.--"Sul terremoto d'Ischia il 28 luglio 1883."
_Scritti di Sismologia_, Pte. ii., pp. 207-216.
13. ---- "Sul terremoto dell' Isola d'Ischia il 28 luglio 1883."
_Ibid._, pp. 217-232.
FOOTNOTES:
[21] The shaded areas indicate the principal trachytic masses, the
broken lines represent the boundaries of the craters that are still
recognisable, and the dotted lines the boundaries of the areas within
which buildings were damaged by the earthquakes of 1796, 1828, 1881,
and 1883 (according to Mercalli). The continuous curved line shows the
position of the radial fracture with which the earthquakes were
probably connected. The trachytic masses and craters are denoted by
the following tables:--
_a._ Epomeo.
_b._ Trippiti.
_c._ Vetta.
_d._ Garofoli.
_e._ Vatoliere.
_f._ Campagnano.
_g._ Vezza.
_h._ Imperatore.
_i._ C. St. Angelo.
_j._ Lo Toppo.
_k._ Marecocco.
_l._ Zale.
_m._ Rotaro.
_n._ Montagnone.
_p._ Bagno.
_q._ Tabor.
_r._ P. Castiglione.
_s._ Cremate.
_t._ Arso.
_u._ Porto d'Ischia.
[22] It is possible that Monte Campagnano may form an exception to
this statement.
[23] Shocks were felt in the island in 1559 and 1659, but one at least
was of external origin.
[24] Prof. Mercalli, from the five estimates of the angle of emergence
which he considered most reliable, found the mean depth to be about
3,280 feet.
[25] Professor de Rossi estimated the mean duration as not much
exceeding ten seconds. Dr. Johnston-Lavis, on the other hand,
considers the general estimate of fifteen seconds as far too low. In
one case, at Casamicciola, he ranks it as high as thirty-one seconds.
[26] Quoted from the useful translation of Covelli's memoir given by
Dr. Johnston-Lavis.
[27] Baldacci supposes that the thermal springs and fumaroles of
Forio, Stennecchia, Montecito, Casamicciola, and Castiglione lie along
a tangential fracture starting from Forio and passing by Casamicciola
to near Punta di Castiglione. Mercalli, however, argues forcibly
against this inference.
[28] Professor Mercalli adds, as a fourth point of contact between
Ischian earthquakes and volcanic phenomena, the changes in the
fumaroles and hot springs which preceded or accompanied or followed
the earthquakes of 1828, 1881, and 1883.
[29] "Fontana," he says, "occupies the centre of the great crater of
Epomeo..., and therefore lies immediately over the ancient chimney,
which in all probability is filled by an old plug of consolidated
trachyte, which must descend to the igneous reservoir. Any mass of
igneous matter, that might determine the further rupture of a
collateral fissure, would result in the conduction of any changes of
pressure or vibrations, along the column of highly elastic trachyte;
whilst the same earth-waves would be annulled or absorbed by the
inelastic tufas surrounding it, so that the blow would be struck
perpendicularly to the surface, and in a small area with well defined
limits. The undulatory sensations, after the principal local shock,
were those that arrived from the great centre of impulse beneath
Casamenella."
[30] The above paragraph is a summary of the reasoning stated with
admirable clearness by Dr. Johnston-Lavis. It should be mentioned that
the late Professor Palmieri, relying on the extremely limited
disturbed area, dissented from this view; but his difficulty is met by
supposing the focus to be small as well as shallow, a supposition that
is supported by the shortness of the meizoseismal band, as well as by
the elongation of the isoseismal lines in the direction perpendicular
to this band.
CHAPTER IV.
THE ANDALUSIAN EARTHQUAKE OF DECEMBER 25TH, 1884.
In most countries the principal seismic districts are of limited
extent. Thus, in central Japan, the east coast is frequently visited
by earthquakes, while the west coast is relatively undisturbed. Of the
earthquakes felt in the kingdom of Greece during the years 1893-98, 63
per cent. were observed in Zante, and were for the most part confined
to that island. In the interior of the Iberian peninsula--in Leon and
in New and Old Castile--destructive earthquakes are practically
unknown; while the littoral regions of central and southern Portugal,
Andalusia, and Catalonia are noted for their disastrous shocks.
During the eighteenth century seismic activity was chiefly
concentrated in Portugal, and culminated in the great Lisbon
earthquake of 1755. In the following century the seat of disturbance
was transferred from the west to the south of the peninsula; Portugal
remained throughout in comparative repose, while Almeria experienced
destructive shocks in 1804, 1860, and 1863, and Murcia in 1828-29 and
1864, leading up to the Andalusian earthquakes of 1884-85, described
in the present chapter.
The preparation for the principal earthquake of December 25th, 1884,
was unusually indistinct. For a day or two before, shocks were felt
here and there in Andalusia, but so weak were they that they passed
almost unperceived. During the night of December 24-25, one slight
shock was noticed at Colmeñar (Fig. 19) and another at Zafarraya. On
the 25th, a faint movement of the ground was noticed at Malaga, and a
few weak tremors at Periana; and shortly after came the great shock at
about 8.50 P.M. mean time of Malaga, or about 9.8 P.M. Greenwich mean
time.
This earthquake was investigated by no fewer than three official
committees. The first in the field was nominated by the Spanish
Government on January 7th, 1885, and consisted of four members, the
President being Señor M.F. de Castro, the director of the Geological
Survey of Spain. The report of this commission was presented to the
Minister of Agriculture, etc., on March 12th. Early in February a
French Commission, appointed by the Academy of Sciences, proceeded to
the scene of the disaster. With Professor F. Fouqué as chief, and MM.
Lévy, Bertrand, Barrois, Offret, Kilian, Bergeron, and Bréon as
members, this committee resolved itself after a time into one for
studying the geology of the central area; and, of their voluminous
report of more than 700 quarto pages (published in 1889), only 55 are
immediately concerned with the earthquake. At the beginning of April,
Professors Taramelli and Mercalli, sent by the Italian Government,
arrived in Andalusia; and their memoir, read a few months later before
the Reale Accademia dei Lincei, forms by far the most valuable
contribution to our knowledge of the earthquake.
DAMAGE CAUSED BY THE EARTHQUAKE.
The meizoseismal area (see Figs. 19 and 20) lies in a mountainous
district, almost equidistant from the cities of Malaga and Granada. In
this area, which contains nearly 900 square miles, the shock was
disastrous to all but well-built houses. Whole villages were
overthrown. In the surrounding zone many buildings escaped serious
damage, and only a few were completely destroyed. It is estimated by
the Spanish Commission that, in the province of Granada, 3,342 houses
were totally, and 2,138 partially, ruined; in the province of Malaga,
1,057 houses were totally, and 4,178 partially, ruined; while in the
two provinces together 6,463 houses were damaged; making a total of
17,178 buildings more or less seriously injured.
As usual in the South of Europe, bad construction and narrow streets
were largely responsible for the loss of property, houses that were
regularly built and made of good materials being only slightly
injured. But, in this case, the great slope of the ground, the bad
quality of the foundations, and the nature of the underlying rocks
were contributing factors. Many buildings also had been damaged by
previous shocks, and their ruin was only completed by the earthquake
of 1884.
The total loss of life is variously estimated. According to the
Spanish Commission, 690 persons were killed and 1,426 wounded in the
province of Granada, while 55 were killed and 59 wounded in that of
Malaga, making a total of 745 persons killed and 1,485 wounded. The
Italian seismologists, having additional materials at their disposal,
raise the total figures to 750 persons killed and 1,554 severely
wounded. Careful inquiries were also made on this subject by the
conductors of the newspaper _El Defensor de Granada_. In Granada
alone, they reckon that 828 persons were killed and 1,164 wounded.
From the table given in the Italian report, it appears that 330
persons were killed at Alhama, 118 at Arenas del Rey, 102 at
Albuñuelas, 77 at Ventas de Zafarraya, and 40 at Periana; the
percentage of mortality being 9 at Arenas del Rey, about the same at
Ventas de Zafarraya, and 3 or 4 at Alhama, Albuñuelas and Periana.
Comparing these latter figures with the death rates of 71 per cent. at
Montemurro, caused by the Neapolitan earthquake, and of about 45 per
cent. at Casamicciola, by the Ischian earthquake of 1883, it will be
seen that the loss of life during the Andalusian earthquake was
comparatively small--an exemption which is attributed by the Italian
commissioners to the absence of inhabited places from the immediate
neighbourhood of the epicentre, and to the fact that the destructive
vibrations occurred towards the end of the shock, thus allowing
opportunity for escape.
ISOSEISMAL LINES AND DISTURBED AREA.
Fig. 19 shows the principal isoseismal lines as drawn by the Italian
commissioners. The meizoseismal area, which included all places at
which the shock was disastrous, is bounded by an ellipse (marked 1 on
the map) 40 miles long from east to west, 28 miles wide, and about 886
square miles in area. The next isoseismal (2) includes the places in
which some buildings were ruined, but not as a rule completely, and in
which there was no loss of life. Its bounding line is also elliptical,
the longer axis being about 71 miles long and running nearly east and
west. Towards the south this zone is interrupted by the sea. It will
be noticed that these isoseismals are not concentric, the second
extending much farther to the west and south-west than in the
opposite direction. A third isoseismal (not shown in the map) encloses
the district in which the shock was "very strong," or just capable of
producing cracks in the walls of houses. It is similar in form to the
second isoseismal, reaching as far as Estepone to the south-west,
Osuna, Cordova, and Seville to the west, Jaen to the north, while
towards the east it stops short of Almeria.
[Illustration: FIG. 19.--Isoseismal lines of Andalusian
earthquake. (_Taramelli and Mercalli._)]
The French Commission have also published a map of the earthquake,
and, though the work of an experienced seismologist like Professor
Mercalli is probably more trustworthy, it is interesting to compare
his isoseismal lines with those obtained by his French colleagues,
which are reproduced in Fig. 20. The curves in this figure are drawn
so as to include the places that were, respectively, ruined, seriously
damaged, and slightly damaged, by the shock. They should therefore
correspond with the lines in Fig. 19. It will be seen that they differ
considerably in form, but at the same time they present certain points
of agreement, such as the east and west elongation of the meizoseismal
area, and the great extension of the two outer isoseismals towards the
west and south-west The greatest difference is to be found in the
eastern portion of the third isoseismal, which, according to the
Italians, extends beyond the limits included in Fig. 20, and,
according to the French, is bayed back by the great masses of the
Sierra Nevada.
Outside Andalusia the earthquake was sensibly felt to the north as far
as Madrid and Segovia, to the west at Huelva, Cárceres and Lisbon, and
to the east at Valencia and Murcia. Towards the south, the greater
part of the disturbed area was cut off by the Mediterranean, and there
are no records forthcoming from the opposite coast of Africa. The
total area disturbed by the earthquake is roughly estimated by the
French Commission at about 154,000 square miles, and by the Italian
Commission at about 174,000 square miles; but, as the shock was
strong enough to stop clocks and ring bells at Madrid, it is evident
that even the greater of these values is too small.
[Illustration: FIG. 20.--Isoseismal lines of Andalusian
earthquake. (_Fouqué, etc._)]
THE UNFELT EARTHQUAKE.
Far beyond the limits of the disturbed area, however, the long slow
waves sped over the surface, disturbing magnetographs and other
delicate instruments. More than a century before, the great Lisbon
earthquake of 1755 had caused oscillations in Scottish lakes, and on
other occasions the effects of remote earthquakes had been witnessed
at isolated places. But, in 1884, the concurrent registration of the
Andalusian earth-waves at distant observatories attracted general
attention, and in part suggested the world-wide network of
seismological stations, the foundation of which was laid before
another decade had passed.
In Italy, probable records of the earthquake were obtained at two
observatories, but, owing to the approximate times given, their
connection with it is not established. At Velletri, near Rome,
Professor Galli's seismodynamograph registered a very slight movement
at 10 P.M., and at Rome itself Professor de Rossi found a tromometer
making unusual oscillations at 10.15 P.M.[31]
The most interesting records, however, are those furnished by the
magnetographs at Lisbon, Parc Saint-Maur (near Paris), Greenwich, and
Wilhelmshaven. At Lisbon, the records are extremely clear. The curves
of the declination, horizontal force and vertical force magnets, as
seen in Fig. 21, are abruptly broken at 8.33 P.M. (Lisbon time, or 9h.
9m. 45s., G.M.T.). The disturbances, which are greatest on the
declination curve and least on the vertical force curve, lasted in all
three for about 12 minutes, and are quite distinct from the ordinary
magnetic perturbations. At Parc Saint-Maur, the magnetographs seem to
be ill-adapted to act as seismographs, for only a slight mark was
discovered on a re-examination of the curves, beginning at 9.24 P.M.
(Paris time, or 9h. 14m. 39s., G.M.T.) At Greenwich, Mr. W. Ellis
writes, there was "a small simultaneous disturbance of the declination
and horizontal force magnets, occurring at 9h. 15m.... Both magnets
were at this time set into slight vibration, the extent of vibration
in the case of declination being about 2' of arc, and in horizontal
force equivalent to .001 of the whole horizontal force nearly." Of the
three instruments at Wilhelmshaven, only one showed any movement at
the time of the earthquake. The declination magnet was undisturbed,
the horizontal force curve was accidentally interrupted, but the
vertical force curve indicated a very perceptible shock. Beginning at
9.52 P.M. (Wilhelmshaven mean time, or 9h. 29m. 29s., G.M.T.), the
curve was broken for four minutes, for the rapid swinging of the
needle could not be registered until the motion became fainter.
Further disturbances also occurred at 9.59, 10, 10.2, and 10.5
P.M.[32]
[Illustration: FIG. 21.--Magnetograph records of Andalusian
earthquake at Lisbon. (_Fouqué, etc._)]
POSITION OF THE EPICENTRE.
The innermost isoseismal being too large, and the time-records too
inaccurate, to give the position of the epicentre, both Commissions
resorted to observations of the direction, Professor Fouqué and his
colleagues depending chiefly on the oscillation of hanging lamps, and
Professors Taramelli and Mercalli on the fall or displacement of
statues and other objects, and all avoiding as far as possible the
evidence of fissures in buildings.
The Italian observers point out that, among the divergent directions
visible at any place, there is generally one more distinctly marked
than the others, and this, they consider, corresponds to the movement
coming almost directly from the centre of disturbance. Plotting these
directions (36 in number), they find that they converge as a rule
within the triangle formed by joining Ventas de Zafarraya, Alhama, and
Jatar, while a large number of them traverse the elliptical area,
whose boundary is represented by the dotted line in Fig. 19. This area
is about 9 miles long and 2-1/2 miles wide, its longer axis runs
nearly east and west, and its centre coincides with the western focus
of the ellipse which forms the boundary of the meizoseismal area. It
lies, moreover, close to Ventas de Zafarraya and Arenas del Rey, the
two places where the seismic death-rate was highest, while its major
axis almost coincides with the line joining them.
The evidence of hanging lamps collected by the French Commission was
more consistent than that of the fallen objects. At every place, the
plane in which the lamps oscillated was nearly constant, the
deviations being generally attributable to irregularities in the mode
of suspension. The azimuths again intersect within an elliptical area,
which, according to the Commission, differs little from the central
region of the earthquake (Fig. 20). It Is clear, however, from the map
accompanying the French report, that the majority converge towards a
narrow band extending east and west from near Arenas del Rey to near
Ventas de Zafarraya, and therefore agreeing closely with the
epicentral area as determined by Professors Taramelli and
Mercalli.[33]
DEPTH OF THE SEISMIC FOCUS.
If the depth of the seismic focus amounts to several miles, one of the
most serious objections to Mallet's method lies in the varying
refractive power of the different strata traversed by the earth-waves
(p. 28). At present we have no way of meeting this objection, and all
calculations of the depth of the focus are therefore more or less
doubtful. A difficulty in practice has also been urged, depending on
the widely differing inclinations of the fractures at any place; but
the Italian observers found that the errors from this source were
greatly reduced by avoiding all fissures in poorly-built houses, or
which start from windows or other apertures, and selecting only those
which occur in homogeneous walls directed towards the epicentre. The
best angles of emergence thus measured by them are thirteen in number,
all made at places lying within 5 and 23 miles from the centre of the
epicentral area, and, with two exceptions, inside the meizoseismal
zone (Fig. 19). The depths corresponding to the different wave-paths
vary from 5.3 to 23.0 miles, the mean depth of the focus given by all
thirteen observations being 7.6 miles.
The only estimate made by the French Commission--and it is one that
they rightly regarded with considerable doubt--was based on a method
devised by Falb. As the sound generally precedes the shock, Falb
assumes that it travels with a greater velocity. If the velocities of
both series of waves are known, and if they start at the same instant
and from the same region, the interval that elapses between the
arrivals of the sound and shock should give the distance traversed by
them and consequently the depth of the focus. It is unnecessary to
mention more than two of the serious objections to this method. The
duration of the preliminary sound should increase rapidly with the
distance from the focus, and of this there is not the slightest
evidence. Moreover, the sound-vibrations that are first heard do not
necessarily come from the same part of the focus as those which cause
the shock, but, as will be seen in Chapter VIII., probably from its
nearer lateral margin. The French Commission, finding the average
duration of the fore-sound near the epicentre to be 5 seconds,
estimate the depth of the focus at about 7 miles--a result which
agrees remarkably with that obtained from the angles of emergence, but
which is not, on that account, entitled to credit.
NATURE OF THE SHOCK.
In the nature of the shock, there was a singular uniformity throughout
the whole disturbed area, the chief variation noticed being evidently
dependent on the observer's distance from the epicentre.
For instance, in the meizoseismal area (Fig. 19), at Ventas de
Zafarraya, a loud sound like thunder was first heard, and before it
ceased there came a violent subsultory movement preceded by a very
brief oscillation, then a pause of one or two seconds, and lastly a
more intense and longer series of undulations, the whole movement
lasting 12 seconds. At Cacin, three phases were distinguished, the
first a slight undulatory movement coincident with the sound, followed
immediately by the subsultory motion, a pause, and stronger
undulations, the total duration being 15 seconds. The variations
noticeable in this zone seem to have been apparent only, sensitive
observers perceiving a tremulous motion before the vertical
vibrations, and in the pause between them and the concluding
undulations. In both phases, the intensity increased to a maximum and
then gradually decreased. The movement at Ventas de Zafarraya and
Cacin is represented by Professors Taramelli and Mercalli by the
curves _a_ and _b_ in Fig. 22.
In the second zone (Fig. 19), the same two phases were universally
observed, but the subsultory movement was less pronounced or the
movement was partly subsultory and partly undulatory, and occasionally
both phases are described as undulatory. The motion near Malaga is
represented by the curve _c_ in Fig. 22.
[Illustration: FIG. 22.--Nature of shock of Andalusian
earthquake. (_Taramelli and Mercalli._)]
Outside the ruinous zone, the first phase rapidly lost what remained
of its subsultory form, and the pause between the two parts was
noticeably longer than near the epicentre. Thus, at Seville and
Cordova, two shocks were felt, separated by an interval of some
seconds; the second according to some observers at Seville,
terminating with vertical tremors. At Madrid, also, the two parts were
perceived, the interval between them being 3 or 4 seconds in length;
but, as a rule, outside Andalusia, only a single undulatory shock was
felt, without any preliminary sound.
That the changes observed in the shock were merely an effect of less
or greater distance, will be obvious from Fig. 23, in which the
intensity at any moment is that represented by the distance of the
corresponding point on the curve from the different base-lines, the
base-line _a_ corresponding to a place near the epicentre, and _b_,
_c_, _d_, etc., to places at gradually increasing distances. Thus, at
a place corresponding to the base-line _b_, the intensity of the
tremors during the intervening pause (represented by the short line
PN) was so slight that they frequently escaped notice, while the
preliminary tremors observed by some near the epicentre were
altogether imperceptible. At the places corresponding to the
base-lines _c_, _d_, _e_, _f_, the duration of the whole shock and of
each part gradually diminished, while the interval between the two
parts increased owing to the gradual extinction of the final
vibrations of the first part and of the initial vibrations of the
second. At the farthest of these places (_f_) the first part was so
weak that it sometimes passed unobserved. Lastly, at a place
corresponding to the base-line _g_, the first part was imperceptible
to all observers, and the shock consisted of a single series of
horizontal undulations.
[Illustration: FIG. 23.--Diagram to illustrate variation in
nature of shock of Andalusian earthquake.]
_Origin of the Double Shock._--If the double shock were observed at
only a few places, we should naturally look for some local explanation
of the peculiarity. The second shock, for instance, might be a
subterranean echo, the earth-waves being reflected at the bounding
surface of two different kinds of rock. In the case of the Andalusian
earthquake, such an explanation is precluded by the almost universal
observation of the double shock, the greater intensity of the second
part, and the longer period of its vibrations.
The Italian observers, who paid considerable attention to the double
shock, give a more general explanation. They regard the two parts of
the shock as corresponding in the main to longitudinal and transversal
waves starting simultaneously from the same focus (see p. 13). The
former vibrations would be vertical at the epicentre and would
gradually become horizontal in spreading outwards; the latter would be
horizontal at the epicentre and at a distance from it (_e.g._ at
Seville) nearly vertical. Also, as the longitudinal waves travel more
rapidly than others, the interval between the two parts of the shock
would increase with the distance from the origin. Owing again, to the
large size of the focus, the first part of the shock would at no place
be instantaneous, and its later vibrations might coalesce with the
earlier transverse vibrations, so that, within and near the
meizoseismal area, the second part of the shock might be stronger than
the first. A similar result might be produced in the same district if
the transverse vibrations coincided with reflected longitudinal
vibrations, and Professors Taramelli and Mercalli think that such
reflection would occur from the old crystalline rocks of the Sierra de
Almijara and possibly also from the calcareous and crystalline rocks
to the south-west of Cartama.
Satisfactory as it seems to be in some respects, this explanation is
open to serious objections, of which I will mention only two. The
first is that, though the pause between the two parts of the shock
does increase with the distance, it does not increase rapidly enough;
at Seville, it should be two or three minutes, instead of "some
seconds" in length. A more fatal objection, however, is that, if the
explanation were correct, every earthquake-shock should consist of two
parts, and this is only the case with a small minority.
On the other hand, if the velocities of the waves composing each part
were the same, the slight increase in the length of the interval is
readily accounted for, as we have seen, by the gradual extinction of
its weak terminal vibrations. But in any case, the long interval that
elapsed between the beginnings of the two parts at a place so near the
epicentre as Ventas de Zafarraya, shows that each part was due to a
distinct impulse; and, judging from the directions of the respective
movements, it would seem that the focus of the first impulse was
situated at a greater depth than the focus of the second. Whether the
epicentres corresponding to the two foci were coincident or more or
less separate is not clear from the nature of the shock; but it is
probable that they were nearly or quite detached, and that a second
epicentre was situated near the eastern focus of the ellipse bounding
the meizoseismal area.
SOUND-PHENOMENA.
In the Neapolitan earthquake, the sound was only heard in a district
of about 3,300 square miles immediately surrounding the epicentres,
while the whole area disturbed by the shock was not less than 39,000
square miles. A similar limitation was noticed in the Andalusian
earthquake. According to the Spanish Commission, the sound was heard
at only one place (Cordova) outside the provinces of Granada and
Malaga; and its audibility was a rule confined to the area within
which buildings were damaged by the shock. It was compared at
different places to the noise of a passing train or a carriage heavily
laden running on a paved road, of distant thunder, a great storm, or
the discharge of heavy guns.
At every place where the sound was heard, it distinctly preceded the
shock, frequently allowing time for escape from houses that were
afterwards ruined. Its duration within the meizoseismal area was on an
average about five or six seconds, rarely perhaps did it exceed ten
seconds. At some places in the same area, it overlapped the beginning
of the shock, but generally it was separated from the latter by a very
short interval, estimated at a second. From this precedence of the
sound, the Italian Commission conclude that the sound-waves travelled
more rapidly than those which formed the shock, an inference that
depends on the assumption that both waves started simultaneously from
within precisely the same focal limits. A different explanation, not
based on these assumptions, will be considered more fully in Chapter
VIII, dealing with the recent earthquakes of Hereford and Inverness.
VELOCITY OF THE EARTH-WAVES.
If, in a highly-civilised country, the time-records of an earthquake
vary within wide limits, it is not surprising that those given for the
Andalusian earthquake should be wholly untrustworthy. Even the clocks
in public buildings and railway stations differed by as much as 25
minutes in their indications. An interesting observation is, however,
described in the French report and is worth repeating, though it does
not lead to any accurate result. At the time of the principal shock,
two telegraph-clerks were in communication, one at Malaga and the
other at Velez-Malaga. The latter, surprised by the shock, suddenly
stopped his message; and, about six seconds later, the arrival of the
earth-waves at Malaga explained the interruption to his colleague. As,
according to the French report, Velez-Malaga is 9 kms. (or about 5-1/2
miles) nearer than Malaga to the mean epicentral point, it follows
that the velocity of the earth-waves must have been about 1.5 kms., or
nearly a mile, per second.[34]
The only observations of any real value in determining the velocity
are those given by the stopped clock at the observatory of San
Fernando (Cadiz) and by the magnetographs at Lisbon, Parc Saint-Maur,
Greenwich, and Wilhelmshaven. Taking the times at Cadiz, Lisbon,
Greenwich, and Wilhelmshaven at 9.18, 9.19, 9.25, and 9.29 P.M.
respectively (Paris mean time) and the mean epicentral point as
coinciding with Alhama, the French Commission estimates roughly the
mean surface-velocity between Cadiz and Lisbon at 3.6 kms. per second,
between Cadiz and Greenwich at 4.5 kms. per second, between Cadiz and
Wilhelmshaven at 3.1 kms. per second, and between Greenwich and
Wilhelmshaven at 1.6 kms. per second. Dr. Agamennone, however,
notices that the distances from Alhama are not correctly measured, and
substitutes for the above figures 4.83, 3.43, 2.82, and 1.75 kms. per
second respectively.
These results apparently show a decrease in the velocity with the
outward spread of the earth-waves, but, as Dr. Agamennone again points
out, a comparatively small error in the time at Cadiz would neutralise
the apparent decrease. It is not to be supposed that the astronomical
clock at this observatory was wrong by more than a second or two, but
the behaviour of clocks during an earthquake is so irregular--some
stopping at once, others staggering on for some seconds before
arrest--that the Cadiz time may differ from the true time by several
seconds.
Besides this possible error, there is also considerable uncertainty in
the records from the magnetic observatories, owing to the slow rate at
which the photographic paper travels. At Parc Saint-Maur this rate is
only 10 mm. per hour, and at the other observatories about 15 mm. per
hour. Allowing, therefore, for an error of half-a-minute in the
time-record at Cadiz, of one minute in those of Lisbon, Greenwich, and
Wilhelmshaven, and of two minutes in that at Parc Saint-Maur, and
taking the mean epicentral point as determined by the Italian
observers, Dr. Agamennone, applying the method of least squares, finds
the probable value of the velocity of propagation to be 3.15 kms. (or
nearly 2 miles) per second, with a possible error of .19 kms. per
second. This result agrees closely with the value found for the long
slow undulations of more recent earthquakes.
MISCELLANEOUS PHENOMENA.
_Connection between Geological Structure and the Intensity of the
Shock._--While a great part of the injury to buildings must be
attributed to their faulty construction, the connection between the
nature of the underlying rock and the amount of damage was very
clearly marked. Other conditions being the same, houses built on
alluvial ground suffered most of all; and the destruction was also
great in those standing on soft sedimentary rocks such as clays and
friable limestones. On the other hand, when compact limestones or
ancient schists formed the foundation-rock, the amount of damage was
conspicuously less than in other cases.
The members of both the French and the Italian Commissions agree in
ascribing the peculiar form and relative positions of the isoseismal
lines to geological conditions. To the east of the epicentre, the
schists and crystalline limestones form a deep, uniform, and compact
mass; while, to the west, the old crystalline rocks are covered by
jurassic, cretaceous, and eocene formations, constituting a less
homogeneous and less elastic mass, in which the intensity of the shock
would fade off much more rapidly, with the result that the epicentre
occupies the western focus of the elliptical boundary of the
meizoseismal area (Fig. 19).[35]
That mountain-ranges have an important influence on the form of
isoseismal lines is evident from both maps (Figs. 19 and 20), but
especially from that published by the French Commission (Fig. 20).
The resistance offered by the Sierra Nevada to the propagation of the
earth-waves is shown in the former map by the approximation of the
first and second isoseismals at the east end, and in the latter by the
great bay in the third isoseismal line. Whichever interpretation of
the evidence is the more accurate, the action of the mountainous mass
is clearly to lessen rapidly the intensity of the shock--an effect
which is probably due to the abrupt changes in the direction and
nature of the strata encountered normally by the earth-waves. On the
opposite side of the epicentre, the waves meet the Sierra de Ronda
obliquely. In traversing this range, the shock lost a great part of
its strength, while it continued to be felt severely along its eastern
foot, thus giving rise to the south-westerly extension of the third
isoseismal in Fig. 20, and, though to a less extent, that of the
second in Fig. 19.
_Fissures, Landslips, etc._--The earthquake resulted in many
superficial changes, such as fissures, landslips, and derangement of
the underground water-system--all changes of the same order as the
destruction of buildings--but, so far as known, in no fault-scarps or
other external evidence of deep-seated movements.
Some of the fissures were of great length. One of the most remarkable
occurred at Guevejar, a village built on the south-west slope of the
Sierra de Cogollos. It was in the form of a horse-shoe, and was about
two miles long, from ten to fifty feet wide, and of great depth. In
its neighbourhood, innumerable small cracks appeared, some
perpendicular and others parallel to the great fissure. The ground
within, a bed of clay resting on limestone, also slid down towards the
river. Houses near the centre of the fissured tract were shifted as
much as thirty yards within the first month, and others near its
extremity about ten feet; while the accumulation of the material at
the south end of the fissure resulted in the formation of a small
lake, of about 250 to 350 square yards in area and about 30 feet deep.
All streams within the fissured zone disappeared, and the spring,
which provided the drinking-water of the village, ceased to flow.
The underground water-system was generally affected throughout the
central area. In some places, mineral springs disappeared; in others,
new springs broke out or old ones flowed more abundantly. At Alhama,
the increased flow was accompanied by a permanent rise in temperature
from 47° to 50° C., and by a marked change in character.
AFTER-SHOCKS.
Frequent after-shocks are a characteristic of the earthquakes of
Southern Spain. After the Cordova earthquake of 1170, they continued
for at least three years. The Murcian earthquake of 1828 was followed
by 300 minor shocks during the next twenty-four hours, and for more
than a year slight tremors were often felt. For some time after the
great earthquake of 1884, the movements of the ground were extremely
numerous in the immediate neighbourhood of the epicentre, farther away
they were rarer and of less intensity, and outside the area of damaged
buildings they were nearly absent.
Thus, during the night of December 25-26, 110 after-shocks were
counted at Jatar, from 14 to 17 at Alcaucin, Ventas de Huelma, Motril,
Cacin, Durcal, Malaga, etc.; about 11 at La Mala and Albuñuelas; 9 at
Velez-Malaga and Lenteje; and from 5 to 7 at Frigiliana, Riogordo, and
Cartama. The strongest of these shocks occurred at 2.20 A.M., and,
though none was violent, several helped to complete the ruin of many
houses that had been damaged by the principal shock.
From this time, after-shocks occurred almost daily until the end of
May, after which they became much less frequent. According to the list
given in the Italian report, which closes at the end of January 1886,
237 shocks were felt, 23 up to the end of December, 30 in January
1885, 25 in February, 27 in March, 46 in April, and 43 in May. In June
1885, only three are recorded, and the average number during each of
the following seven months lies between five and six. This list,
however, does not include the very weak shocks,[36] for nearly all
those contained in it were felt as far as Malaga or its neighbourhood.
The shocks varied considerably in intensity as well as in frequency,
five of them being much more violent than the rest. One that occurred
on December 30th was felt strongly in all the damaged area, two others
on January 3rd and 5th caused fresh injury to buildings, a fourth, on
February 27th, disturbed an area bounded roughly by the second
isoseismal of the principal earthquake (Fig. 19), while the fifth and
strongest, that of April 11th, was felt over a large part of the zone
beyond.
At places within and near the meizoseismal area, earth-sounds were
sometimes heard without any sensible shock; occasionally, also,
tremors were felt with no attendant sound; but, as a rule, the shocks
were accompanied by sound, and in every such case, as in the principal
earthquake, the sound preceded the shock, or at most was partly
contemporaneous with it.
Several of the after-shocks resembled the principal earthquake in
their division into two parts separated by an interval of rest or
weaker movement from half a second to a second in length, though the
whole duration of the shock itself in no case exceeded five or six
seconds. Occasionally, the likeness was still closer, in the
succession of sound, subsultory motion and concluding horizontal
undulations.
GEOLOGY OF THE MEIZOSEISMAL AREA AND ORIGIN OF THE EARTHQUAKES.
The meizoseismal area and surrounding zones lie in the midst of the
mountainous region that separates the basin of the Guadalquiver from
that of the Mediterranean, the essential structure of which, according
to the geologists of the French Commission, is outlined in Fig. 24. In
this sketch-map, the lightly-shaded bands correspond to an upper
series of crystalline schists, and the cross-shaded bands to the lower
series of mica-schists and dolomites that form the anticlinal folds of
the Sierra de Ronda, the Sierra de Mijas, and the Sierra Tejeda.
In addition to the faulting and intense folding in the direction of
their strikes, these rocks are also intersected by three nearly
parallel transverse faults of post-Triassic age, which, aided by
subsequent denudation, have cut up the whole range into a number of
distinct sierras. They are represented by the broken lines in Fig. 24.
[Illustration: FIG. 24.--Structure of meizoseismal area of
Andalusian earthquake. (_Fouqué, etc._)]
One of these faults, that which passes near Motril, traverses the
meizoseismal area, whose boundary, as laid down by the French
Commission, is indicated by the dotted line on the sketch-map.[37] In
the neighbourhood of Zafarraya, the fault intersects the broken
anticlinal fold of the Sierra Tejeda, and the epicentre is thus
situated in one of the most disturbed tracts of the whole region. The
evidence, both seismic and geological, is insufficient to support any
precise view as to the origin of the earthquake, but there can be
little doubt that it was closely connected with movements along one or
more of the system of faults that intersect not far from Zafarraya.
REFERENCES.
1. AGAMENNONE, G.--"Alcune considerazioni sui different metodi fino
ad oggi adoperati nel calcolare la velocità di propagazione
del terremoto andaluso del 25 dicembre 1884." Roma, _R.
Accad. Lincei, Rend._, vol. iii., 1894, pp. 303-310.
2. ---- "Velocità superficiale di propagazione delle onde sismiche
in occasione della grande scossa di terremoto dell' Andalusia
del 25 dicembre 1884." _Ibid._, vol. iii., 1894, pp. 317-325.
3. CASTRO, M.F. de.--_Terremotos de Andalucía: Informe de la
comision nombrada para su estudio dando cuenta del estado de
los trabajos en 7 de marzo de 1885._ (Madrid, 1885; 107 pp.)
4. FOUQUÉ, F., etc.--"Mission d'Andalousie: Études relatives au
tremblement de terre du 25 décembre 1884, et à la constitution
géologique du sol ébranlé par les secousses." Paris, _Acad.
Sci. Mém._, vol. xxx., pp. 1-772.
5. MACPHERSON, J.--"Tremblements de terre en Espagne." Paris, _Acad.
Sci., Compt. Rend._, vol. c., 1885, pp. 397-399.
6. NOGUÉS, A.F.--"Phénomènes géologiques produits par les
tremblements de terre de l'Andalousie, du 25 décembre 1884 au
16 janvier 1885." _Ibid._, pp. 253-256.
7. ROSSI, M.S. de.--"Gli odierni terremoti di Spagna ed il loro eco
in Italia." _Bull. Vulc. Ital._, anno xii., 1885, pp. 17-31.
8. TARAMELLI, T., and G. MERCALLI.--"I terremoti Andalusi cominciati
il 25 dicembre 1884." Roma, _R. Accad. Lincei, Mem._, vol:
iii., 1885, pp. 116-222.
9. Paris, _Acad. Sci., Compt. Rend._, vol. c., 1885, pp. 24-27,
136-138, 196-197, 256-257, 598-601, 1113-1120, 1436 (the last
three by F. Fouqué).
FOOTNOTES:
[31] These times correspond to about 9.10 and 9.25 P.M., Greenwich
mean time. The earthquake stopped a clock at the Royal Observatory of
San Fernando (Cadiz), at 8h. 43m. 54.5s. mean local time,
corresponding to 9h. 8m. 44s., G.M.T.
[32] The earthquake is also said to have been registered at the
observatory of Moncalieri, near Turin, but I have not been able to
ascertain the time of occurrence. A movement felt at about 10.20 P.M.
at Ramsbury, in Wiltshire, was attributed to the earthquake, though
the time is about an hour too late. On December 26th, an astronomical
clock was stopped at Brussels and its pillar displaced; and, on the
evening of the same day, the large telescope at the observatory was
also found to have been shifted. These effects, it is suggested, were
caused by the Andalusian earthquake, but the connection between them
seems to me very doubtful.
[33] The French observers have also applied a method depending on the
time of occurrence of the shock. Joining places where the recorded
times were the same, they notice that the perpendicular bisectors of
these lines intersect within an area which agrees practically with
that determined by the azimuths. The inaccuracy of the time-records
must, however, lessen the significance of this result.
[34] Dr. Agamennone points out that, according to the Italian report,
the difference in distance is 22 kms. (or 13-3/4 miles), leading to a
velocity of about 3.6 kms., or 2.3 miles per second.
[35] It should be remembered that it is not improbable that there were
two detached epicentres, coinciding roughly with the two foci of this
curve.
[36] Only eight are recorded during the night of December 25-26. On
several occasions during April and May 1885, groups of slight shocks
were felt; but as their individual times are not given, they are
regarded as equivalent to one shock each in the above totals.
[37] The boundary, as drawn in this figure, differs slightly from that
given in Fig. 20.
CHAPTER V.
THE CHARLESTON EARTHQUAKE OF AUGUST 31ST, 1886.
The Charleston earthquake stands alone among the great earthquakes
described in this volume, and indeed among nearly all great
earthquakes, in visiting a region where seismic disturbances were
almost unknown. Calabria and Ischia, the Riviera and Andalusia, Assam
and the provinces of Mino and Owari in Japan, are all regions where
earthquake-shocks are more or less frequent and occasionally of
destructive violence. But, from the foundation of Charleston in 1680
until 1886, that is, for more than two centuries, it is probably not
too much to say that few counties in Great Britain were so free from
earthquakes as the State of South Carolina.[38]
The practical isolation of the earthquake of 1886 left its trace on
the character of the investigation. Not only were the observers
untrained, but the investigators themselves were unprepared. For
instance, the scale of intensity used in drawing the isoseismal lines
was not adopted until after the first letters of inquiry were issued.
On the other hand, nothing could exceed the energy and ability with
which the epicentral tracts were examined by Mr. Earle Sloan and the
collection of time-records made by Mr. Everett Hayden. To them, and to
Major C.E. Dutton, whose valuable monograph supersedes all other
accounts, we are indebted for the two chief additions to our knowledge
resulting from the study of the Charleston earthquake. These are the
determination of the double epicentre, and the measurement of the
velocity with which the earth-waves travelled.
DAMAGE CAUSED BY THE EARTHQUAKE.
The land-area disturbed by the earthquake and the isoseismal lines are
shown in Fig. 25, the small black oval area (which Includes
Charleston) being that within which the greatest damage to buildings
occurred. The chief part of the epicentre, however, lies from 12 to 15
miles to the west and north-west of Charleston, in a forest-clad
district, containing only two villages and various scattered houses.
The city of Charleston, whose population between 1880 and 1891
increased from fifty to fifty-five thousand, is built on a peninsula
between the Cooper River on the east and the Ashley River on the
south-west. Originally, this was an irregular tract of comparatively
high and dry land, intersected by numerous creeks, which, as the city
grew, were filled up to the general level of the higher ground. It is
on this "made land" as a rule that the more serious damage to
buildings occurred.
At 9.51 P.M. (standard time of the 75th meridian), the great
earthquake occurred, and, one minute later, there was left hardly a
building in the city that was not injured more or less seriously. "The
destruction," as Major Dutton remarks, "was not of that sweeping and
unmitigated order which has befallen other cities, and in which every
structure built of material other than wood has been levelled
completely to the earth in a chaos of broken rubble, beams, tiles, and
planking, or left in a condition practically no better." The number of
houses entirely demolished was not great, but several hundred lost a
large part of their walls, and many were condemned as unsafe and
afterwards pulled down. A board of inspectors, appointed to
investigate the condition of the houses, reported that not one hundred
out of fourteen thousand chimneys examined by them escaped damage, and
that 95 per cent. of those injured were broken off at the roof. The
total cost of the necessary repairs, it was estimated, would amount to
about one million pounds.
According to the official records, 27 persons were killed in
Charleston during the earthquake, but, by cold, exposure, etc., this
number was brought up to not less than 83. The number of persons
wounded was never ascertained.
ISOSEISMAL LINES AND DISTURBED AREA.
In drawing the isoseismal lines (represented by the continuous curves
in Fig. 25), Major Dutton made use of the well-known Rossi-Forel scale
of seismic intensity, a translation of which is given below.[39] The
curves range from the highest degree, 10, corresponding to disastrous
effects on buildings, down to the lowest but one, 2, which would be
applied to a shock felt only by a small number of persons at rest. It
is evident, I think, that these lines cannot be regarded as drawn with
great accuracy. The number of records (nearly 4000, from about 1,600
places), great as it is, is hardly sufficient for the purpose; and
many were collected from newspapers. The circulars of inquiry also
contained no distinct questions corresponding to the different degrees
of the scale employed, and therefore it is not always certain that the
intensity recorded was the maximum observed. But, if the curves might
have varied in detail with a larger and more accurate series of
observations, they must represent in their main features the
distribution of seismic intensity throughout the disturbed area. One
point of importance is the partial earthquake-shadow in the region of
the Appalachian Mountains shown by the southward incurving of the
isoseismals 4, 5, and 6, and especially by the first two of these
lines. Another is the close grouping of the isoseismals in the State
of Mississippi, illustrating a rapid fading of intensity as the
earth-waves crossed the unconsolidated materials of the Mississippi
delta.
[Illustration: FIG. 25.--Isoseismal lines of Charleston
earthquake. (_Dutton, etc._)]
Owing to the short distance between the epicentre and the sea-coast,
it is impossible to make more than a rough estimate of the extent of
the disturbed area. Even when the boundary lies on land, it traverses
some districts which are thinly populated and others where the
inhabitants are unobservant, and unlikely to notice the slow
oscillations which were alone perceptible at great distances. The
shock was, however, felt at Boston (800 miles from the epicentre), La
Crosse on the upper Mississippi (950 miles to the north-west), at
several places in Cuba (between 700 and 710 miles), and in Bermuda
(950 miles). To the south, the limits are unknown, there being no
report from Yucatan, the nearest point of which is distant about 930
miles. If we assume the disturbed area to have a mean radius of 950
miles, then it must have covered no less than 2,800,000 square miles.
And, that this estimate is not excessive, will be evident from the
fact that the land-area disturbed (including parts of the great lakes
and inlets in the sea-coast) amounted to about 920,000 square miles.
PREPARATION FOR THE EARTHQUAKE.
The preparation for the earthquake seems to have begun about three
months before. During June, and even earlier, slight but undoubted
tremors are said to have been felt in Charleston, but no record of
them was kept until about 8 A.M. on August 27th, when a decided
earthquake occurred at Summerville, a village twenty-two miles to the
north-west. The shock and sound were simultaneous, the shock a single
jolt or heavy jar, the sound loud and sudden; they were such as might
have been caused by the firing of a heavy cannon or the explosion of a
boiler or blast of gunpowder. At 4.45 A.M. on August 28th, the shock
and sound were repeated, only more strongly, the former being
distinctly felt as far as Charleston. During that day and the next,
there were several other shocks at Summerville, and then rest and
quiet succeeded until the evening of August 31st.
NATURE OF THE SHOCK.
At 9.51 P.M. (to take one of the best descriptions), the attention of
an observer in Charleston was "vaguely attracted by a sound that
seemed to come from the office below, and was supposed for a moment to
be caused by the rapid rolling of a heavy body, as an iron safe or a
heavily-laden truck, over the floor. Accompanying the sound there was
a perceptible tremor of the building, not more marked, however, than
would be caused by the passage of a car or dray along the street. For
perhaps two or three seconds the occurrence excited no surprise or
comment. Then by swift degrees, or all at once--it is difficult to say
which--the sound deepened in volume, the tremor became more decided,
the ear caught the rattle of window-sashes, gas-fixtures, and other
movable objects; the men in the office ... glanced hurriedly at each
other and sprang to their feet.... And then all was bewilderment and
confusion.
"The long roll deepened and spread into an awful roar, that seemed to
pervade at once the troubled earth and the still air above and around.
The tremor was now a rude, rapid quiver, that agitated the whole
lofty, strong-walled building as though it were being shaken--shaken
by the hand of an immeasurable power, with intent to tear its joints
asunder and scatter its stones and bricks abroad....
"There was no intermission in the vibration.... From the first to the
last it was a continuous jar, adding force with every moment, and, as
it approached and reached the climax of its manifestation, it seemed
for a few terrible seconds that no work of human hands could possibly
survive the shocks. The floors were heaving under-foot, the
surrounding walls and partitions visibly swayed to and fro, the crash
of falling masses of stone and brick and mortar was heard overhead and
without....
"For a second or two it seemed that the worst had passed, and that the
violent motion was subsiding. It increased again and became as severe
as before. None expected to escape. A sudden rush was simultaneously
made to endeavor to attain the open-air and fly to a place of safety;
but, before the door was reached all stopped short, as by a common
impulse, feeling that hope was vain--that it was only a question of
death within the building or without, of being buried beneath the
sinking roof or crushed by the falling walls. The uproar slowly died
away in seeming distance. The earth was still, and oh! the blessed
relief of that stillness."
If somewhat sensational in form, this report gives an extremely vivid
and generally accurate account of the great shock. Other observers in
Charleston concur in dividing the movement into five phases. The
preliminary tremors and murmuring sound lasted about twelve seconds,
and, although they increased in strength, they were succeeded somewhat
suddenly by the violent oscillations of the second phase, followed by
a third phase of much less intensity and a fourth of stronger
oscillations, these three phases lasting altogether about fifty
seconds. The fifth phase, in which the tremors died out rather
rapidly, continued about eight seconds; so that the total duration of
the earthquake was not less than seventy seconds. The variation of the
intensity with the time is represented roughly by the curve in Fig.
26.
[Illustration: FIG. 26.--Curve of intensity at Charleston.
(_Dutton._)]
At Charleston, there were thus two decided maxima of intensity, nearly
equal in strength, though the first seems to have been slightly more
powerful than the second. As in the Andalusian earthquake, the
intervening tremors were imperceptible at a distance from the
epicentre, and the earthquake appeared in the form of two distinct
shocks, separated by an interval the average duration of which was
estimated at slightly less than half a minute. At most places, the
first shock is described as the stronger, but the difference in
intensity of the two parts could not have been great, for both were
noticed at several places more than 600 miles from the epicentre.
_Visible Earth-Waves._--Many persons in the meizoseismal area assert
that they saw waves moving along the surface of the ground. At
Charleston, according to an observer who was facing a street-lamp at
the time, "the progress of the waves as they passed the house, going
towards the south-east, was plainly observed, although they travelled
with incomparable swiftness. The shadow of each moving ridge cast from
the gas-light was distinctly seen. The waves were not in long rollers,
but had rather the appearance of 'ground-swells' in deep water," the
height of which from crest to trough he estimated at not less than two
feet. In the words of another observer, "The vibrations increased
rapidly and the ground began to undulate like the sea. The street was
well lighted, having three gas-lamps within a distance of 200 feet,
and I could see the earth waves as they passed as distinctly as I have
a thousand times seen the waves roll along Sullivan's Island beach.
The first wave came from the south-west, and as I attempted to make my
way ... I was borne irresistibly across from the south side to the
north side of the street. The waves seemed then to come from both the
south-west and north-west, and crossed the street diagonally,
intersecting each other, and lifting me up and letting me down as if I
were standing on a chop sea. I could see perfectly, and made careful
observations, and I estimate that the waves were at least two feet in
height."
THE DOUBLE EPICENTRE.
For seismological purposes, it is unfortunate that the epicentral
district should be one containing so few buildings and other objects
that could preserve the effects of the shock. It is for the most part
a barren, forest-clad region, in places swampy, with occasional
scattered houses. But it is crossed by three lines of railway
diverging from Charleston, and the damage which they suffered
supplements to some extent the defects arising from the scarcity of
buildings. These railway lines are the South Carolina, the
North-Eastern, and the Charleston and Savannah, denoted by the letters
A, B, and C, respectively, in Figs. 28 and 29.[40] It will be
convenient to follow Major Dutton, and trace the variation of
intensity exhibited along each line.
For six miles along the South Carolina Railway (A) the damage to the
line, though indicative of a strong shock, was of little consequence.
In the first half of this distance no repairs were required, but at
3-2/3 miles the rails were bent and the joints between them opened; at
5 miles, the fish-plates were torn from their fastenings and the
joints between the rails opened seven inches; and at nearly 6 miles
the joints were again opened, and the road-bed depressed six inches.
After this point, deflections of the line and elevations and
depressions of the road-bed were no longer rare. Near the 9-mile
point, the intensity of the shock seemed to increase most rapidly;
lateral displacements of the line became more frequent as well as
greater in amount. The distortions of the lines were probably greatest
between 10 and 11 miles; here they were often displaced laterally,
sometimes depressed or elevated, and occasionally twisted into
S-shaped curves, while many hundred yards of the track were shoved
bodily towards the south-east. "The buckling always took place when
this lateral shoving encountered a rigid obstacle, usually a long
rigid trestle. At the north-western end of the trestle the
accumulation of rails resulted in a sharp kink. Corresponding
extensions of the track by the opening of the joints and shearing of
the fish-plate bolts occurred some distance to the north-westward." At
11-1/2 miles, the lines were again stretched and the joints opened by
about seven inches; but, from this point for more than four miles, the
sharp kinks revealing a sliding of the track were entirely absent,
though there were still long slight flexures in the lines and changes
of level in the road-bed. The railway in this section traverses a
district which is partly a swamp and partly a rice-field; and thus it
may be, as Major Dutton suggests, that the ground was less fitted to
preserve the effects of the shock.[41] At about 18 miles, the line
reaches higher and firmer ground; and, from here to Summerville
(21-2/3 miles), there were many sinuous flexures. For six miles
farther, violent distortions of the rails ceased to occur, the rate of
decrease in intensity being most marked near the 23-mile point. The
last flexure occurred at Jedburgh (27-1/2 miles) at the south end of a
long, heavy trestle (Fig. 27).
[Illustration: FIG. 27.--Flexure of rails at Jedburgh.
(_Dutton._)]
There is thus a certain symmetry in the damage to this line with
respect to a point about 15 or 16 miles from the Charleston terminus.
The changes of intensity are most rapid at distances of about 9 and 23
miles from the terminus. Also, on the south-east side of the 16-mile
point, the longitudinal displacements of the line are always to the
south-east; on the other side, always to the north-west. Major Dutton
therefore infers that the epicentre must be on a line drawn nearly
through the 16-mile point at right angles to the railway.
Somewhat similar changes were noted along the North-Eastern Railway
(B), the Charleston terminus of which is about three-quarters of a
mile to the south-east of that of the South Carolina Railway. Slight
flexures in the line occurred at distances of 1-1/2 and 4 miles from
the terminus, and at about 6 miles the road-bed was depressed, in one
part by as much as 22 inches. At about 6-1/3 miles, the joints between
the rails were opened 14 inches, and there were slight sinuous
flexures in the line near the 7-mile and 8-mile points. The
indications of great intensity then rapidly increased, the rate of
change being greatest near the 9-mile point. Here, there was a long
lateral flexure with a shift of 4 inches eastward. Half-a-mile
farther, the fish-plates were broken and the rails parted 8-1/2
inches. A little beyond the 10-mile point, an embankment 15 feet high
was pushed 4-1/2 feet eastward along a chord of 150 feet. At the
12-mile point and beyond, fish-plates were broken, lines were bent and
the joints opened; the road-bed was cut by a series of cracks, one of
which was 21 inches wide, while the beginning of a long trestle was
shifted 8-1/3 feet to the west. From 12-1/2 to 14-1/2 miles, several
buildings were damaged or destroyed by a movement which was clearly
more vertical than horizontal. Near the 16-mile point, the ground was
fissured and thrown into ridges, the rails being similarly bent in a
vertical plane. Soon after this, the line reaches a broad, sandy
tract, and, though the thickness of the sand is probably not much more
than 40 feet in any place, the disturbances diminish almost at once,
and, for a distance of more than two miles, there was little damage
done to the line. At Mount Holly Station (18 miles), the intensity was
so slight that the houses suffered no injury more serious than the
loss of chimneys. Half-a-mile farther, the ground becomes less sandy,
and the effects of the shock more distinct. The lines were bent in
places for about a quarter of a mile, after which they again pass into
the sandy area with a decrease of damage, the last flexure being near
the 21-mile point. The rate of change of intensity in this part of the
line appears to have been greatest at a distance of about 19-1/2 miles
from the terminus, but the exact distance is obviously somewhat
uncertain.
There is again a rough symmetry in the damage to the line, the central
point being about 14 miles from the Charleston terminus. A line drawn
through this point at right angles to the North-Eastern Railway (or
rather to that part of it between the 9-mile and 19-1/2-mile points)
should pass through the epicentre. It meets the corresponding line for
the South Carolina Railway in a point which is indicated in Figs. 27
and 28 by a small circle (W). Houses and other buildings are rare in
the surrounding district; but, as the intensity of the shock
diminished outwards in all directions, this point must mark
approximately the position of the epicentre. As it is close to the
Woodstock Station on the South Carolina Railway, it is called by Major
Dutton the Woodstock epicentre.
The Charleston and Savannah Railway (C) uses the same lines as the
North-Eastern for the first seven miles from Charleston, and then
turns off in a south-westerly direction. For 4-1/2 miles from the
junction the signs of disturbance were few and unimportant. The
railway then crosses the Ashley River, the banks of which slid towards
one another and jammed the drawbridge; but for four miles farther
there was no serious damage done to the lines. At about 16-1/2 miles
the effects of the shock became rapidly more apparent. For nearly
1-1/2 mile the entire railroad was deflected into an irregular curve,
the displacement being greatest at the bridge, where it crosses the
Stono River. Here, it was as much as 37 inches to the south. After
Rantowles Station (18 miles), there were many displacements, both
lateral and vertical. At 18-1/2 miles, a long southward deflection
began, the amount of which reached 25 inches at the 19-mile point, 50
inches half-a-mile farther on, and was still greater at 20-2/3 miles.
For two miles more, sinuous flexures were continuous, but, at the
22-2/3-mile point, they rapidly disappeared, the railroad passing on
to higher and firmer ground. Between 25 and 27 miles, there were
occasional slight flexures in the line or depressions of the railroad;
but, after the 27-1/4-mile point, they seldom occur, and, when they
do, are of little consequence.
Some of the effects described in the last paragraph may, as Major
Dutton suggests, be due to the varying nature of the surface-rocks. It
is important to notice, however, that disturbances of the lines were
exceedingly rare in the section that lies nearest to the Woodstock
epicentre, and that they increase in violence for some distance from
that region, the maximum intensity being reached a mile or two to the
west of Rantowles Station. This points clearly to the existence of a
second focus. Unfortunately, there are very few houses or other
objects in the neighbourhood, and the position of the corresponding
epicentre cannot be determined accurately. Major Dutton places it in
the position indicated by a small circle (R), and calls it the
Rantowles epicentre from its vicinity to the station of that name.
If the meizoseismal area had been a thickly populated one, the
evidence of ruined and damaged houses would have provided materials
for the construction of isoseismal lines surrounding the two
epicentres. It is difficult, as it is, to gauge the equality of the
effects on objects so different as railway-lines and buildings; and
the isoseismals shown in Figs. 28 and 29 can therefore lay no claim to
accuracy.
Fig. 28 shows the epicentral isoseismals as they are drawn by Mr.
Earle Sloan. They do not correspond to the degrees of any definite
scale of seismic intensity; but they may be taken as representing the
impressions of a very careful observer, who traversed the district
immediately after the occurrence of the earthquake, and who, when
drawing these lines, was biassed by no preconceived theory.
Major Dutton, relying chiefly on Mr. Sloan's written notes, interprets
the evidence differently, and obtains the series of curves shown in
Fig. 29. In this case, also, the isoseismals correspond to no
expressed standard of intensity. They are intended merely to represent
the forms of the curves, and, by their less or greater distance apart,
the more or less rapid rate at which the intensity varied.
The chief difference between the two maps concerns the form of the
Woodstock isoseismals. Major Dutton draws them approximately
circular, leaving the map blank towards the north, where hardly any
evidence was forthcoming. Mr. Sloan attributes the scantiness of
effects here to a diminution of intensity, and makes the lines curve
in towards the epicentre. They certainly must do so in crossing the
North-Eastern Railway; and the somewhat southerly trend of Mr. Sloan's
curves to the east of this railway seems to me to furnish the better
representation of the distinctly greater intensity in that region.
[Illustration: FIG. 28.--Epicentral isoseismal lines of
Charleston earthquake according to Mr. Sloan. (_Dutton._)]
[Illustration: FIG. 29.--Epicentral isoseismal lines of
Charleston earthquake according to Major Dutton. (_Dutton._)]
More important, however, than this divergence of opinion is the
agreement in one respect between the two sets of curves. Both show a
marked expansion around the points known as the Woodstock and
Rantowles epicentres, especially about the former, and a contraction
in the intermediate region. The evidence of these isoseismals
therefore confirms that of the damaged railway lines, and establishes
Major Dutton's inference that there were two distinct foci, the
epicentres of which were about thirteen miles apart.
ORIGIN OF THE DOUBLE SHOCK.
In the last chapter, it was shown that the double shock of the
Andalusian earthquake could be due only to two distinct impulses
taking place either within the same focus or, more probably, in two
detached foci. Similar reasoning applies to the Charleston earthquake.
The double maximum or double shock was observed in no less than
fourteen States. Moreover, the interval between the two maxima at
Charleston appears from Fig. 26 to have been about 34 seconds in
length. Thus, the duplication of the shock cannot have been a merely
local phenomenon, nor can it have resulted from the separation into
two parts of the earth-waves proceeding from a single disturbance.
Each maximum must therefore be connected with a distinct impulse.
Combining this inference with Major Dutton's discovery of the double
focus, no doubt can remain as to the origin of the repeated shock. It
is clear, also, that the impulse at the Woodstock focus was the
stronger of the two; for the isoseismals spread out more widely round
the corresponding epicentre, and there was no rapid decline of
intensity from that point, such as might be associated with a weaker
disturbance within a shallow focus.
[Illustration: FIG. 30.--Planes of oscillation of stopped
pendulum clocks at Charleston.]
Again, since the earlier part of the shock is almost uniformly
described as the stronger, it follows that the Woodstock focus was the
first in action. A curious fact recorded by Major Dutton supports this
inference. In Charleston, four clocks were stopped by the shock, the
errors of which at the time were certainly less than eight or nine
seconds. The planes in which their pendulums oscillated are shown by
the lines lettered A, B, C, and D in Fig. 30, the broken lines W and R
representing respectively the directions from Charleston of the
Woodstock and Rantowles epicentres. Clock A stopped at 9h. 51m. 0s., B
at 9h. 51m. 15s., C at 9h. 51m. 16s., and D (which had been reset to
the second earlier in the day) at 9h. 51m. 48s. Now, if the plane of
oscillation coincided nearly with the direction of the shock, the only
effect would be a temporary change in the period of oscillation; but
if it was at right angles to the direction of the shock, the clock
might be stopped by the point of the pendulum catching behind the
graduated arc in front of which it oscillated. The planes of the first
three clocks, it will be seen, were approximately at right angles to
the direction of the Woodstock epicentre, and B and C were indeed
stopped in the manner just described; while the plane of shock D was
nearly perpendicular to the direction of the Rantowles epicentre. As
the pendulums of B and C might make a few staggering oscillations
before their final arrest, Major Dutton assigns 9h. 51m. 12s. as the
epoch of the first maximum at Charleston; and, as the interval between
the two maxima was about 34 seconds, this would give about 9h. 51m.
46s. for the epoch of the second maximum--a time which agrees very
closely with that given by clock D. Thus, clocks A, B, and C must have
been stopped by the Woodstock vibrations, and clock D about
half-a-minute later by those coming from the Rantowles focus.
DEPTH OF THE SEISMIC FOCI.
Two methods of estimating the depth of the seismic focus have been
described in the preceding pages--namely, Mallet's, depending on the
angle of emergence, and Falb's, based on the interval between the
initial epochs of the sound and shock. To these, Major Dutton adds a
third method, in which he relies on the rate at which the intensity of
the shock varies with the distance from the epicentre.
_Dutton's Method of determining the Depth of the Focus._--If the
seismic focus is either a point or a sphere, and the initial impulse
equal in all directions, and if the intensity of the shock diminishes
inversely as the square of the distance from the focus, then the
continuous curve in Fig. 31 will represent the variation of intensity
along a line passing through the epicentre E. The form of the curve on
these assumptions does not depend in any way on the initial intensity
of the impulse; it is governed solely by the depth of the focus. The
deeper the focus, the flatter becomes the curve, as we have seen in
discussing the Ischian earthquakes (p. 68). In all directions from the
epicentre, the intensity at first diminishes slowly; but the rate of
change of intensity with the distance soon becomes more rapid, until
it is a maximum at the points C, C; after which it again diminishes
and dies out very slowly when the distance becomes great. It will be
evident from Fig. 18 that the deeper the focus the greater also is the
distance EC of the points where the intensity of the shock changes
most rapidly. It may be easily shown, indeed, that this distance
always bears to the depth of the focus the constant ratio of 1 to
sqrt(3), or about 1 to 1.73.[42]
Now, if a series of isoseismals could be drawn corresponding to
intensities which differ by constant amounts, we should have a series
of circles like those surrounding the Woodstock epicentre in Fig. 29,
the distance between successive lines at first decreasing gradually
until it is a minimum at the dotted circle and afterwards gradually
increasing. This dotted circle is obviously that which passes through
all points where the intensity of the shock changes most rapidly.
Major Dutton calls it the _index-circle_ and, when its radius is
known, the depth of the focus is at once obtained by multiplying the
radius by 1.73.
In 1858, Mallet proposed a method which bears some resemblance to the
above,[43] but depending only on the intensity of the longitudinal
waves. Major Dutton claims for his method that the effects of the
longitudinal and transverse waves are not separated, that it takes
account of the "total energy irrespective of direction or kind of
vibration."
[Illustration: FIG. 31.--Diagram to illustrate Dutton's method of
determining depth of seismic focus.]
_Objections to Dutton's Method._--I have described this method
somewhat fully, though it seems to me open to more serious objections
than Mallet's first method which it is intended to replace.
We have, in the first place, no reason for supposing that the focus is
either a point or a sphere, or that the initial impulse is uniform in
all directions. If the earthquake were caused by fault-slipping, both
assumptions would be untrue, and it is for those who employ the method
to prove their validity.
But of greater consequence is the fact that, if the method were
correct, all earthquakes originating at the same depth must have
index-circles of equal radii. If the depth of the focus were, say, ten
miles, then the index-circle must have a radius of about six miles,
whether the initial disturbance be of extreme violence or so weak that
it is not felt at the surface at all, much less so far as six miles
from the epicentre. The law of the inverse square is of course only
true for a perfectly elastic and continuous medium, and the real curve
of intensity is not that of the continuous line in Fig. 31, but
something of the form represented by the dotted line. In this case,
the rate of change of intensity is greatest at some point C', nearer
than C to the epicentre, and the application of Major Dutton's rule
would give a point F', nearer the surface than F, for the focus. Thus,
assuming that the method can be applied in practice--and the test
involved is one so delicate that it would be difficult to apply except
with refined measurements--then all that we can assert is that the
calculated depth is certainly less than the true depth.
_Dutton's Estimate of the Depth of the Seismic Foci._--In applying the
method, the chief difficulty is to obtain a series of isoseismal lines
corresponding to equidistant degrees of intensity. As already pointed
out, those given in Fig. 29 are merely diagrammatic; but the
index-circle of the Woodstock focus, represented by the dotted line,
is made to pass through the places where the rate of change of
intensity was found to be greatest. The radius of this circle being
very nearly seven miles, it follows that the resulting depth of the
Woodstock focal point would be about twelve miles. Major Dutton
regards this estimate as probably correct within two miles.
In the neighbourhood of the Rantowles epicentre, the isoseismals in
both Figs. 28 and 29 are elongated in form. The _index-circuit_, as it
would be called in such a case, cannot be drawn completely, but its
radius parallel to the shorter axis of the curves is about 4-1/2
miles, and the resulting depth of the Rantowles focal point would be
nearly eight miles.
VELOCITY OF THE EARTH-WAVES.
The recognition of the double epicentre is, from a geological point of
view, the most important fact established by the investigation of the
Charleston earthquake. But of equal interest, from a physical point of
view, is the estimate of the velocity of the earth-waves, which is
probably more accurate than that determined for any previous shock.
Owing to the existence of the standard time system in the United
States, the exact time is transmitted once a day to every town and
village within reach of a telegraph line; and the effect of small
errors in the observations is considerably lessened by the great
distance traversed by the earth-waves, sixty good reports coming from
places more than 500 miles from the epicentre, and ten from places
more than 800 miles distant.
The total number of time-records collected is 316, but of these 130
had to be rejected, either because they were obviously too early or
too late, or because they were only given to the nearest
five-minutes' interval. There remain 186 observations which are
divided by Major Dutton into four classes according to their probable
value.
In an earthquake of such great duration (about 70 seconds at
Charleston), it is necessary in the first place to select some special
phase of the movement as that to which the records mainly refer, and
then to determine as accurately as possible the time of occurrence of
this phase at the origin.
There can be little doubt as to which phase should be chosen. The
shock began with a series of tremors, which passed somewhat abruptly
into the oscillations that formed the first and stronger maximum.
These were clearly felt all over the disturbed area, and, as the
beginning of the first maximum at places near the epicentre and the
beginning of the shock at distant stations were probably due to the
same vibrations, this particular phase may be fairly selected as that
to which the time-measurements refer.
The time of this phase at the origin can only be ascertained from the
time at which it reached Charleston, and our knowledge of this depends
chiefly on the evidence of stopped clocks. How unreliable this may be
is well known. Clocks may indeed be stopped at almost any phase of the
movement; and, whenever stopped clocks can be compared with really
good personal observations, they almost invariably show a later time.
At Charleston three good clocks were stopped by the vibrations from
the Woodstock focus, two of them being in close agreement (p. 121);
and, allowing for a few oscillations before their final arrest, Major
Dutton places the time of arrival of the selected phase at Charleston
at 9h. 51m. 12s. P.M. The evidence of these clocks is also supported
by that of other observations, which show that 9.51 was certainly the
nearest minute to the time of arrival, and favour a somewhat later
instant much more strongly than one a little earlier.
Now, the distance of Charleston from the Woodstock epicentre is
sixteen miles, and from the corresponding focus (with the calculated
value of its depth) twenty miles. A first estimate of the velocity
gives a value of a little more than three miles a second, and the time
at the Woodstock focus may therefore be taken as 9h. 51m. 6s. with a
probable error of a few seconds.[44]
Proceeding to the observations at a distance, we find them, even after
all rejections, to be very different in value. They were therefore
divided into groups consisting of observations which are as nearly as
possible homogeneous.
The first group contains five records from places between 452 and 645
miles from the Woodstock epicentre. They give the time to within 15
seconds, obtained from an accurately kept clock, or from a clock or
watch that was compared with such within a few hours of the
earthquake. The resulting velocity is 3.236 plus or minus .105 miles
(or 5205 plus or minus 168 meters) per second.[45]
In the second group there are eleven observations (between distances
of 438 and 770 miles) which satisfy the same conditions as those in
the first group, except that the time is only given to the nearest
minute or half-minute. The velocity obtained from them is 3.226 plus
or minus .147 miles (or 5192 plus or minus 236 metres) per second.
The third group included all but the above records and those obtained
from stopped clocks. They are 125 in number (between distances of 80
and 924 miles), but it is uncertain whether they correspond to the
selected phase of the movement, and the errors of the clocks and
watches used were unknown. They give a mean velocity of 3.013 plus or
minus .027 miles (or 4848 plus or minus 43 metres) per second.
In the fourth group, we have the evidence of 45 stopped clocks (at
places between 20 and 855 miles), which apparently give a velocity of
2.638 plus or minus .105 miles (or 4245 plus or minus .168 metres) per
second. At six places, however, the times indicated by stopped clocks
can be compared with good personal observations; and these show that
the time of traverse from the origin obtained from the former is on an
average 1.28 times the time of traverse obtained from the latter. If a
similar correction be made for all the stopped clocks, the corrected
velocity of the earth-waves would be from 3.17 to 3.23 miles (or 5100
to 5200 metres) per second.
In obtaining the mean value of the velocity from all the observations,
those of the fourth group are omitted, and the weights of the first
three groups are taken inversely as the squares of the probable
errors--that is, as 2: 1: 4. The resulting mean velocity is 3.221 plus
or minus .050 miles (or 5184 plus or minus 80 metres) per second; and,
though it does not follow that all other estimates are erroneous (for
the velocity may vary with the strength of the earthquake and with
other conditions), it is probable that this result is more nearly
accurate than any other previously obtained.
MISCELLANEOUS PHENOMENA.
_Fissures and Sand-Craters._--In point of size, there was nothing
remarkable about the fissures in the ground produced by the Charleston
earthquake. The largest were only a few hundred yards long, and,
except near the river-banks, they rarely exceeded an inch in width.
They seem, however, to have been unusually abundant; for, within an
area of nearly 600 square miles surrounding the two epicentres, they
were almost universal, and over a much wider area they still occurred
in great numbers, though with somewhat less continuity.
From many of these fissures water was ejected, carrying with it large
quantities of sand and silt, and so abundantly that every stream-bed,
even though generally dry in summer, was flooded. By the passage of
the water, some part of the fissures was often enlarged into a round
hole of considerable size, ending in a craterlet at the surface.
Certain belts within the fissured area contained large numbers of
these craterlets, of all sizes up to twenty feet or more in diameter.
One near Ten-Mile Hill was twenty-one feet across. In this district,
they were apparently larger and more numerous than elsewhere; many
acres of ground being covered with sand, which, close to the orifices,
was two feet or more in depth.
Here and there, the water was ejected with considerable violence, as
was manifest from the heights to which it spurted. The testimony of
witnesses on this point is of course doubtful, for the earthquake
occurred after nightfall, but in a few places the branches and leaves
of trees overhanging the orifices were smirched with sand and mud up
to a height of fifteen or twenty feet.
_Effects on Human Beings._--It is interesting to notice the behaviour
of different races under the influence of a violent earthquake, and
perhaps no greater contrast could be observed than between the
calmness exhibited by the Japanese in the presence of disaster and the
wild fear merging into helpless panic that characterised the
residents, and especially the negroes, of Charleston. "As we dashed
down the stairway," says a writer already quoted (p. 108), "and out
into the street, from every quarter arose the shrieks, the cries of
pain and fear, the prayers and wailings of terrified women and
children, commingled with the hoarse shouts of excited men.... On
every side were hurrying forms of men and women, bareheaded, partially
dressed, some almost nude, and all nearly crazed with fear and
excitement.... A few steps away, under the gas-lamp, a woman lies
prone and motionless on the pavement, with upturned face and
outstretched limbs, and the crowd which has now gathered in the street
passes her by, none pausing to see whether she is alive or dead ... no
one knows which way to turn, or where to offer aid; many voices are
speaking at once, but few heed what is said."
Between the selfish rush for safety here described and the calm
interest of the most distant observers, Major Dutton records nearly
every possible variety of mental effects, the actions resulting from
which may be roughly classified as follows:
A. No persons leave their rooms.
B. Some persons leave their houses.
C. Most persons run into the streets, which are full of excited
people.
D. People rush wildly for open spaces and remain all night
out-of-doors.
In the map of the isoseismal lines (Fig. 25), the dotted curves bound
the areas in which the effects corresponding to the three highest
degrees of the above scale were observed. The curve for the first
degree (A) coincides of course with the isoseismal line of intensity
2.
It will be seen that there is a certain rough correspondence between
these curves and the isoseismal lines. The curve D and the isoseismal
8 are close together; in other words, people thought it wiser to camp
out-of-doors for the night if the shock was strong enough to damage
buildings slightly. The curve C and the isoseismal 6 are similarly
connected; that is, if the movement made pictures swing, etc., people
rushed into the streets. On the whole, the curve B and the isoseismal
3 roughly coincide, or, if the shock was not strong enough to make
doors and windows rattle, some persons left their houses and public
meetings were dispersed.
_Feeling of Nausea._--A feeling of nausea was experienced by many
persons at the time of the earthquake, somewhat rarely it appears in
the neighbourhood of the epicentre and even outside the isoseismal 7,
but more frequently beyond these limits, and perceptible as far as the
broken line in Fig. 25. The most distant places at which it was
noticed are Blue Mountain Creek (New York) and Dubuque (Iowa), which
are respectively 823 and 886 miles from Charleston.
AFTER-SHOCKS.
As Summerville lies six miles to the north-west of the Woodstock
epicentre and Charleston 16 miles to the south-east, it is probable
that many of the after-shocks were unfelt and a still greater number
unrecorded. In Charleston, seven shocks, all much slighter than the
principal shock, were felt during the night of August 31--September 1,
and thirty before the end of September. Of these, the shock of
September 3rd, at 11 P.M., was the strongest, but those which occurred
on September 1st, 2nd, 21st, and 27th were also described as severe,
and the remainder as moderate or slight. For weeks after the great
shock, curious sensations were distinctly perceptible during the still
hours of the night "as though the crust of the earth were resting on a
gelatinous mass in constant motion." The last shock felt in Charleston
seems to have been one recorded on March 18th, 1887.
At Summerville, many shocks occurred that were scarcely perceptible in
Charleston, and those noticed in both places were usually stronger,
and the motion more nearly vertical, at Summerville. "The peculiar
characteristic of all of them was the deep, solemn, powerful boom,
like the report of a heavy cannon, usually accompanied by a quick,
short jar. Sometimes it was prolonged into a heavy roar or rumble, as
if many reports were delivered in a volley. The number of them was
never recorded." On September 3rd, Mr. W.J. McGee, of the United
States Geological Survey, arrived at Summerville. During the evening
of that day, detonations were heard at intervals, averaging perhaps
half-an-hour, accompanied occasionally by very slight spasmodic
tremors of an instant's duration. They were much like peals of thunder
at a distance of half-a-mile or more, though rather more muffled. "It
was my impression," Mr. McGee remarks, "that the sound was sometimes
about as grave as the ear can perceive, resembling somewhat the
tremulous roar sometimes accompanying combustion in locomotives."
These sounds continued, but with diminishing frequency, throughout the
remainder of the year and as late as July 1st, 1887.
ORIGIN OF THE EARTHQUAKE.
Major Dutton's valuable monograph is a record of the
earthquake-phenomena. He offers no theory as to the cause of the
shock, and is therefore in no way responsible for the account given in
the remaining part of this chapter.
That there were two seismic foci he has shown, I think, conclusively;
and my object is now to trace out briefly the probable nature of the
movements that produced the double shock.
Referring to Figs. 28 and 29, it will be seen that, according to both
Mr. Sloan and Major Dutton, the isoseismals surrounding the Rantowles
epicentre are distorted along a line which runs from a few degrees
east of north to a few degrees west of south. Their oval form is in
all probability connected with a focus elongated in about the same
direction. Near the Woodstock epicentre, the isoseismals are drawn
differently in the two maps, and in neither case do they offer any
sure guide as to the form of the seismic focus. If, however, we follow
Mr. Sloan's interpretation of the evidence, and suppose the earthquake
to have been fault-formed, then it is probable that in this region the
fault bends round slightly towards the east.
The only other evidence on this point is that afforded by the regions
of defective intensity, real or apparent, along the three
railway-lines diverging from Charleston. One of these occurred near
Mount Holly Station on the North-Eastern Railway (B, Figs. 28 and 29),
another for four miles starting from the 11-1/2-mile point on the
South Carolina Railway (A), and a third along the Charleston and
Savannah Railway (C) over a distance of four miles from the Ashley
River. In the first two cases, Major Dutton suggests that the less
amount of damage was due to the nature of the soil traversed by the
railway; but it is on the softer ground that the effects of an
earthquake-shock are generally the more disastrous. On the whole, it
seems to me probable that the three tracts referred to are really
regions of less intensity, and it is worthy of notice that they lie
along a nearly straight line.
To show the bearing of these remarks, let CD (Fig. 32) represent the
section of a fault and EF that of the surface of the earth, and
suppose the rock-mass A to slip slightly and suddenly downwards. Then
the particles of A at the surface of the fault will, by impulsive
friction, be drawn sharply upwards, and those of B correspondingly
downwards; so that the earth-waves in the two rock-masses will start
in opposite phases of vibration. Along the line of fault, every
particle of rock, being urged upwards and downwards almost equally,
will remain practically at rest. Thus, regions of defective intensity
may arise from partial interference by the spreading of either
earth-wave in the adjoining rock-mass.
[Illustration: FIG. 32.--Diagram to explain origin of regions of
defective intensity.]
If this be the correct explanation, the path of the originating fault
may be taken as that indicated by the broken line in Fig. 28, a line
which is nearly parallel to the chief branches of the isoseismal
curves.[46] As both epicentres lie on the west side of this line, the
fault must hade or slope in this direction. The distortion of the
Woodstock isoseismals towards the north-west confirms the latter
inference, for the intensity of the shock is greater on the side
towards which the fault hades.
From the comparative absence of earthquakes in South Carolina, we may
infer that the fault is one subject to displacements at wide intervals
of time. The gradually increasing stress along its surface was
relieved at one or two points in or near the Woodstock focus on August
27th and 28th, and perhaps during the preceding months. But the first
great slip took place suddenly in that focus, and spread gradually
southwards--for there was no interruption in the movement--until about
half-a-minute later it reached the Rantowles focus, where the second
great slip occurred. Eight or ten minutes afterwards there was another
slip--in what part of the fault is uncertain--and this was followed at
irregular intervals by many small movements gradually diminishing in
frequency and in focal area. Within a year from the first disturbance,
the fault-system attained once more its usual condition of rest.
REFERENCES.
1. DUTTON, C.E.--"The Charleston Earthquake of August 31st, 1886."
_Amer. Geol. Survey, Ninth Annual Report_, pp. 209-528.
2. _Nature_, vol. xxxv., 1887, pp. 31-33; vol. lxiii., 1901, pp.
165-166.
FOOTNOTES:
[38] The authorities for this statement are Mallet's Catalogue of
Recorded Earthquakes (_Brit. Assoc. Rep._, 1852, pp. 1-176; 1853, pp.
117-212; 1854, pp. 1-326), which closes with the year 1842, and Fuchs'
_Statistik der Erdbeben von 1865-1885_. According to Mallet, there was
an earthquake in S. Carolina in November 1776, and the New Madrid
earthquake of December 16th, 1811, was felt at Charleston. Fuchs
records two earthquakes at Charleston on May 12th, 1870, and December
12th, 1876; and two in S. Carolina on December 12th and 13th, 1879.
[39] 1. Recorded by a single seismograph, or by some seismographs of
the same pattern, but not by several seismographs of different kinds,
the shock felt by an experienced observer.
2. Recorded by seismographs of different kinds; felt by a small number
of persons at rest.
3. Felt by several persons at rest; strong enough for the duration or
direction to be appreciable.
4. Felt by several persons in motion; disturbance of movable objects,
doors, windows; creaking of floors.
5. Felt generally by every one; disturbance of furniture and beds;
ringing of some bells.
6. General awaking of those asleep; general ringing of bells;
oscillation of chandeliers, stopping of clocks; visible disturbance of
trees and shrubs; some startled persons leave their dwellings.
7. Overthrow of movable objects, fall of plaster, ringing of church
bells, general panic, without damage to buildings.
8. Fall of chimneys, cracks in the walls of buildings.
9. Partial or total destruction of some buildings.
10. Great disasters, ruins, disturbance of strata, fissures in the
earth's crust, rock-falls from mountains.
[40] In order to simplify these figures, the rivers, most of the
inlets, and other details are omitted. Small figures are added along
the railway lines to denote the distance in miles from the stations in
Charleston.
[41] If this were so, the decrease in intensity would be only
apparent; but it may have been real, and a possible explanation on
this supposition is given later on (p. 135).
[42] If _c_ be the depth of the focus, _a_ the intensity at unit
distance from the focus, and _y_ the intensity on the surface at
distance _x_ from the epicentre, then
y=a/(c^2+x^2)
The inclination of the curve at any point is given by
dy/dx=-2*a*x/(c^2+x^2)^2,
and this is a maximum when
d^2y/dx^2 or (3*x^2-c^2)/(c^2+x^2)^3
is zero, which is satisfied when c=x*sqrt(3)
[43] _British Association Report_, 1858, pp. 101-103.
[44] The above time would have to be increased by one second if the
depth of the focus were very small, and diminished by one second if it
were as great as 23 miles; the difference in either case being less
than the probable error.
[45] The method employed is as follows: Let t_0 be the computed time
(9h. 51m. 6s.) at the focus, _x_ seconds the error in this estimate,
_t_ the reported time at a given place, _D_ its distance from the
focus in miles, and _y_ the number of seconds required to travel one
mile; then, assuming that _y_ does not vary with the distance, we have
x+Dy=t+t_0. An equation of this form is obtained from each
observation, and the method of least squares is then applied to
determine the most probable values of _x_ and _y_.
[46] This seems to me the more probable course. It is possible,
however, that the fault-line may pass from Mount Holly Station to the
east of the Woodstock epicentre as shown in Fig. 28, and then to the
west of the Rantowles epicentre, the fault changing its direction of
hade in the intermediate district.
CHAPTER VI.
THE RIVIERA EARTHQUAKE OF FEBRUARY 23RD, 1887.
Few earthquakes have aroused a more widespread interest than those
which struck the thronged cities of the Riviera on February 23rd,
1887. The first and greatest of the shocks occurred at about 6.20
A.M., the second nine minutes later, and the third, intermediate in
strength, at about 8.51 A.M.[47] All three shocks were of destructive
violence, the damage wrought by them extending along the coast and for
a short distance inland from Nice to beyond Savona. Most of the injury
to property and nearly all the loss of life were, however,
concentrated on the eastern side of the frontier; and it therefore
fell to the lot of the Italian Government to provide for the
scientific investigation of the earthquakes, as well as to meet the
wants of those deprived of home and support. Professors Taramelli and
Mercalli, who two years before had studied the earthquakes in
Andalusia, were again nominated, the former to examine the geology of
the central regions, and the latter to report on the seismic
phenomena. Their joint memoir forms one of the most complete accounts
that we possess of any earthquake, and is the chief authority for the
description given in this chapter. Another valuable monograph is that
prepared by Professor A. Issel, of Genoa, who received an independent
appointment from the same Ministry. A third official commission was
also sent to estimate the amount of damage caused by the earthquakes
in the Italian towns and villages. In France, the destruction of
property was much less serious, and attention was confined chiefly to
the records of the shock provided by magnetographs and other
instruments in distant observatories. In Switzerland, the effects
remarked were merely those due to the evanescent vibrations of a
remote earthquake; but many interesting records were collected by the
permanent seismological commission established in that country.
DAMAGE CAUSED BY THE EARTHQUAKES.
Owing to variations in the nature, foundation, and site of buildings,
there is always great diversity in the destructive effects of an
earthquake. In one and the same town, most of the houses may be razed
to the ground, while in their midst may be found some that are
shattered but still standing, and others perhaps that are practically
unharmed. The stronger after-shocks often complete the ruin of the
partially damaged houses; though in such cases the real loss is as a
rule comparatively small.
The close succession of the two strong after-shocks of February 23rd
made it impossible as a rule to separate their effects from those due
to the first shock; but it has been roughly estimated that about
one-quarter of the total damage was caused by the two after-shocks
together. To them also must be referred in part the comparatively
small number of wounded, many persons buried beneath the ruins having
no doubt perished from subsequent falls before they could be
extricated.
Taking all three shocks together, the total loss to property,
according to Professor Mercalli, must be valued at about 22 million
francs in Italy alone. For the province of the Alpes Maritimes in
France, full details are wanting, but the loss there cannot fall far
short of three million francs. The total amount of damage must
therefore be placed at about a million pounds. From the figures given
by the official commissions, it appears that the earthquakes were most
disastrous at Diano Marina and Diano Castello; while other places,
such as Oneglia, Bussana, Baiardo, Pompeiana, and Vallecrosia,
suffered only a little less severely. At Mentone about 155 houses, and
at Nice about 61 houses, were rendered uninhabitable, and many others
were badly injured.
In Italy, 633 persons were killed, 432 seriously wounded, and 104
slightly wounded; in France, 7 persons were killed and 30 seriously
wounded, the number of persons slightly wounded being unknown. The
majority of the deaths occurred in two or three places. Thus, at Diano
Marina, 190 persons were killed and 102 wounded; at Baiardo, 220 were
killed and 60 wounded; at Bussana, there were 53 killed and 27
wounded. The death-rates were, however, comparatively small, amounting
for the above places to not more than 8-1/2, 14, and 6-1/2 per cent.,
respectively; figures which only slightly exceed those obtained for
places in the meizoseismal area of the Andalusian earthquake.
Though the damage can hardly be regarded as excessive, it was
nevertheless largely due to the peculiar architecture prevalent in the
Riviera. Arches in the walls are common even in the upper storeys,
and, in Oneglia and Diano Marina, if not also in other places, the
floors are nearly always brick arches abutting against the walls and
without other lateral support. Professor Mercalli believes that, in
private houses, more than 90 per cent. of the dead bodies were crushed
beneath these fallen arches. The height of the buildings is also great
in proportion to the foundation and to the thickness of the walls; and
the main walls are interrupted by numerous apertures, from the corners
of which nearly all the fissures sprang. In some of the coast towns,
the houses are built of rounded stones gathered from the beach, or of
rubble with stones of all shapes and sizes, bound by cement of the
poorest quality. Lastly, as much of the damage due to previous
earthquakes had been badly repaired, it is evident that the
destructiveness of the Riviera earthquakes must to a great extent be
referred to preventable causes.
The occurrence of the principal shock shortly after six on the morning
of Ash Wednesday must also have increased the death-rate; for many
persons, after a night of amusement, had lain down for a short time
and were sleeping heavily; while others had already risen and were
collected in the churches; the circumstances in either case rendering
escape more difficult.
Taking account, however, of this accidental increase in the number of
victims, Professor Mercalli considers that the earthquake of 1887 was
the most disastrous of all those which have visited either the
Riviera or northern Italy in the last three centuries; though, during
the nineteenth century, there were three Italian earthquakes of far
greater destructive power, but all confined to the southern part of
the peninsula--namely, the Neapolitan earthquakes of 1805 and 1857,
and the Ischian earthquake of 1883.
PREPARATION FOR THE EARTHQUAKES.
It is difficult, as usual, to specify the exact moment when the first
earthquake of the 1887 series took place; but it is evident that the
preparation for the great shock was very brief. At Oneglia, it is
alleged that faint shocks and sounds were observed many times, chiefly
at night, during the month preceding February 23rd; though they were
not at the time supposed to be of seismic origin. A slight shock is
also reported from Diano at about midnight on February 21-22.
The first undoubted shock occurred on February 22nd, at about 8.30
P.M., or ten hours before the principal earthquake. Though very
slight, it was felt throughout the Riviera and in part of Piedmont.
Another shock, also weak, took place at about 11 P.M.; and a third,
sensible only in the eastern part of the Ligurian Apennines, on
February 23rd, at about 1 A.M.; at which time the tide-gauge at Genoa
recorded some abnormal oscillations. An hour later, a more important,
though by no means a strong, shock occurred; this was perceptible all
over the Riviera, in Piedmont, and in Corsica; in other words, it
disturbed a region agreeing closely with the central area of the
disastrous shock. At about 5 A.M., a fifth shock, somewhat weaker
than the preceding, was felt over the same area, concurrently, or
nearly so, with another abnormal oscillation of the tide-gauge at
Genoa; while a sixth shock was noticed at several places a few minutes
before the great shock.
During the night of February 22-23, nervous persons in many towns and
villages were agitated without apparent reason. Birds and animals,
more sensitive than human beings to faint tremors, were more
distinctly affected, especially for some minutes before the
earthquake. Horses refused food, were restless or tried to escape from
their stables, dogs howled, birds flew about and uttered cries of
alarm. As these symptoms were noticed at more than one hundred and
thirty places within the Italian part of the central area, there can
be little doubt that they were caused by microseismic movements for
the most part insensible to man.
ISOSEISMAL LINES AND DISTURBED AREA.
The only complete map of the isoseismal lines is that drawn by
Professor Mercalli.[48] In this map, reproduced in Fig. 33, the
continuous curves represent the principal isoseismal lines; the dotted
curves define the disturbed areas of two of the stronger after-shocks.
The meizoseismal area, bounded by the curve marked 1 in Fig. 33, is
also shown on a larger scale in Fig. 34. At the places denoted by
small circles in the latter figure, the principal shock was
"disastrous," some of the houses in each being either totally or
partially ruined. At those marked by a small cross, the shock was
"almost ruinous"; in other words, numerous houses were damaged, but in
no case was the injury of a serious character. The meizoseismal area
is thus a narrow band, skirting the Riviera coast from Mentone to
Albissola, a distance of 106 miles, and extending inland for not more
than from nine to twelve miles. The greatest intensity, corresponding
to the ruin of many houses with considerable loss of life, was
reached at only a few places between Bussano and Diano Marina, all
lying within a littoral band about twenty miles in length and three to
three and a half miles in width. If, however, the epicentre had lain
on land, the area would have been much greater, Professor Mercalli
estimates about four times greater, than its actual amount.
[Illustration: FIG. 33.--Isoseismal lines of the Riviera
earthquake. (_Mercalli._)]
The curve marked 2 (Fig. 33) bounds the "almost ruinous" zone; its
expansion towards the north and contraction towards the west,
north-west, and east, being its most noteworthy features. The next
zone, that of slight damage, is contained between the isoseismals 2
and 3, the latter curve probably grazing the north end of Corsica.
Beyond this lies the "strong" zone, in which the shock was generally
felt without causing any damage to buildings. Its boundary (marked 4)
passes near Marseilles, Como, and Parma, and includes nearly the whole
of Corsica; towards the north-west, in the valley of Aosta, it curves
in towards the isoseismal 3.
In the outermost zone of all the shock was "slight," and towards the
margin was only just perceptible. The boundary, which of course
defines that of the disturbed area, reaches as far north as Basle and
Dijon, to Perpignan on the west, Trento, Venice, and Pordenone on the
east, and to the south as far as Tivoli (near Rome) and the northern
end of Sardinia. In eastern Switzerland, it shows a marked curve
inwards; possibly, as Professor Mercalli suggests, from the vibrations
having to cross the northern Apennines in a direction nearly at right
angles to their axis. Except for this bay, however, the curve differs
little from a circle, the centre of which lies in the sea, a little to
the south of Oneglia, close to the position assigned by other evidence
to the epicentre. The radius of this circle being about 264 miles, it
follows that the disturbed area must have contained about 219,000
square miles--by no means a large amount for so strong an earthquake.
POSITION OF THE EPICENTRES.
It is evident, from the form of the meizoseismal area shown in Fig.
33, that a mere fringe of it lies upon land, and that the epicentre
must be situated some distance out at sea. Other facts may be
mentioned which point to the same conclusion. There were, for
instance, no purely vertical movements observed, even in the districts
where the damage done by the shock was greatest. Nor were any large
landslips to be seen in those areas; there were no lasting changes in
the underground water-system; and in general, as Professor Mercalli
remarks, all the superficial distortions of the ground which are so
characteristic of the epicentral area of a great earthquake were
conspicuous by their absence. There is evidence, again, of some
disturbance of the sea-bed in the death and flight of fishes from
great depths and in the seismic sea-waves recorded by the tide-gauges
at Genoa and Nice. These phenomena will be described in a later
section, but reference should be made here to an interesting
observation at Oneglia on the occurrence of some of the stronger
after-shocks. Persons on the coast, it is said, saw the sea curling
and moving, and immediately afterwards the shock was felt.
In determining the position of the epicentre, Professor Mercalli had
recourse as usual to observations on the direction of the shock,
especially those derived from the oscillation of lamps or other
suspended objects, the projection or fall of bodies free to move,
fractures, etc., in damaged houses, and the stopping of pendulum
clocks. Such observations were made at 120 places--72 in the western
Riviera and the Alpes Maritimes, and 48 at Piedmont, Lombardy, and
Tuscany.
At many of these places the movement was extremely complicated. In
nearly all parts of the area most strongly shaken, for instance, the
direction of the shock changed more than once; and it was therefore
necessary to select whenever possible the principal direction of the
shock at each place. In some towns, such as Oneglia, Mentone, Antibes,
Cuneo, etc., the shock had two dominant directions, and these appeared
to be sensibly at right angles to one another; an inclination which,
as Professor Mercalli suggests, may be due in part to the
approximation of the real directions to those of the principal walls
of the houses in which the observations were made.
Most of the lines of direction, when plotted on the map, converge
towards an area lying between the meridians of Oneglia and San Remo,
and between nine and fifteen miles from the coast. For places near the
epicentre, the most trustworthy, in Mercalli's opinion, are those made
at Oneglia, Mentone, Taggia, Bordighera, Castel Vittorio, Nice, and
Genoa; and the points in which these lines Intersect one another are
Indicated by small crosses on the map of the meizoseismal area (Fig.
34). All of them lie at sea at distances between six and fifteen miles
to the south of Oneglia. The most probable position of the principal
epicentre is that marked by the small circle A, which is situated
about fifteen miles south of Oneglia.
[Illustration: FIG. 34.--Meizoseismal area of the Riviera
earthquake. (_Taramelli and Mercalli._)]
There are, however, several lines of direction which can have no
connection with this epicentre. Besides the east and west lines at
Nice, Mentone, and Antibes, there are others at the same places which
run north and south or nearly so. Professor Mercalli believes that
they were due to vibrations coming from a second focus lying to the
south of Nice, and there are also several lines of direction at more
distant places which converge towards the neighbourhood of the
corresponding epicentre.
This conclusion receives unexpected support from some of the best
time-records. At the railway-stations of Loano and Pietra Ligure, the
times of occurrence were given as 6h. 20m. 5s. and 6h. 20m.
respectively--estimates which are probably accurate to within a few
seconds; for, at the moment of the shock, the officer who brought the
exact time along the railway-line from Genoa was at Loana, and had
just passed through Pietra Ligure. On the other hand, the estimates
for Mentone and Nice--namely, 6h. 18m. 35s. and 6h. 19m. 43s., if not
equally exact, cannot err by many seconds, certainly not by so much as
one minute. Since the distances of Loana and Pietra Ligure from the
principal epicentre are 31 and 32 miles, and those of Mentone and Nice
28 and 37 miles, it is therefore clear that the vibrations which
arrived first at Nice and Mentone must have come from a local focus,
where the impulse preceded that at the principal focus by several
seconds.
DEPTH OF THE PRINCIPAL FOCUS.
Inaccurate as are all the methods of determining the depth of focus,
it seems probable, as Professor Issel argues, that the principal
Riviera focus was situated at a considerable distance from the
surface. In no part of the meizoseismal area was the shock a really
violent one; yet its intensity must have faded very slowly outwards,
for it was strong enough to stop clocks at places in Switzerland and
elsewhere not less than 250 miles from the origin.
Professor Mercalli regards Mallet's method with greater favour than
most seismologists. He points to the gradual increase in the angle of
emergence from the outer zones disturbed by the Riviera earthquake
towards the meizoseismal area, where several good observations were
made from fissures in walls parallel to the dominant direction of the
shock. The angles of emergence which he considers as most trustworthy
are those of 35° at Taggia, 40° at Oneglia, and about 30° at
Bordighera. The corresponding depths for the focus are 10.4, 10.4, and
11.6 miles, giving an average of about 10-3/4 miles.
There are no similar observations forthcoming for the depth of the
secondary focus near Nice and Mentone; but Professor Mercalli observes
that it must have been shallower than the other, for the vertical
component of the vibrations from this focus was much less sensible
than that of the motion coming from the principal focus.
NATURE OF THE SHOCK.
_The Double Shock._--In the valuable collection of records made by
Professors Taramelli and Mercalli there appears at first sight to be
the utmost diversity in the evidence with regard to the nature of the
shock. Thus, in the province of P. Maurizio alone, the shock was
described as subsultory first and then undulatory or vorticose at 25
places, undulatory and then subsultory at 22, undulatory and then
subsultory and again undulatory or vorticose at 13, and subsultory
first, then undulatory, and finally subsultory and vorticose at two
places. It is clear that the shock was of considerable duration, not
less than half-a-minute as a rule, and that there were several phases
in the movement; and it would seem that one or more of these phases
may have passed unnoticed owing to the alarm occasioned by the shock,
and to the fact that most of the observers were asleep when the
earthquake began. Defects of memory must also have an influence not to
be neglected, for, even with the simple shocks felt in the British
Isles, persons in the same or neighbouring places differ greatly in
their testimony.
But, if we confine ourselves to the accounts of careful persons alone,
the discrepancies to a large extent disappear. Indeed, all over the
ruinous area (Fig. 33) the shock maintained a nearly uniform
character. At Oneglia, for instance, there were two well-marked
phases, the first of which began with a brief subsultory movement,
followed by more horizontal undulations of longer period; a pause,
lasting but for an instant, was succeeded by vibrations which, though
not vertical, were highly inclined to the horizon; they continued
throughout the second phase, but, towards the end, new undulations
were superposed, and these, coming from different directions, resulted
in an apparently vorticose movement. Professor Mercalli represents the
motion diagrammatically by the curve _a_ in Fig. 35. At Diano Marina,
as will be seen from the curve _b_, the shock again consisted of two
phases, each beginning with a few subsultory vibrations and ending
with horizontal undulations of much longer period. In the first phase,
the undulations were marked by a dominant direction, but, towards the
close of the second phase, there was no determinate direction, and the
impression was again that of a vorticose shock. At Savona, the
movement, which is represented by the curve _c_, must have lasted from
twenty-five to thirty seconds. It also consisted of two phases, with
subsultory vibrations and undulations in the same order; and it was
noticed that the second part of the shock was much stronger than the
first. According to some observers, the concluding movements were
vorticose.
[Illustration: FIG. 35.--Nature of shock of Riviera earthquake.
(_Taramelli and Mercalli._)]
In the zone surrounding the ruinous area, the vertical component of
the motion was observed to diminish with the intensity; but, in other
respects as well as in duration, the shock retained the same general
form. At Genoa, Turin, Acqui, Alessandria, Antibes, and other places,
two distinct phases were perceived, occasionally separated by a brief
pause, the first being invariably the weaker. At some places, the
observers speak of two shocks at about 6.20 A.M., separated by an
interval of a few seconds; and this division was noticeable as far as
Salò on the shore of Lake Garda and Vicenza in Venetia. Only in
Switzerland and other districts near the boundary of the disturbed
area did the weaker part of the shock become insensible, the other
consisting of horizontal oscillations, remarkable for their slowness
and regularity, and lasting for as much as twenty or thirty seconds.
We may thus conclude, with Professor Mercalli, that the earthquake
resulted from the almost immediate succession of two distinct shocks,
in each of which the nearly vertical vibrations were more marked at
the beginning, while the slower undulations predominated towards the
close, those of the second phase generally becoming vorticose through
the superposition of movements coming from different directions. The
second part of the shock in all of the more carefully written accounts
is described as the stronger, especially as regards the subsultory
vibrations in the meizoseismal area; except in the immediate
neighbourhood of Nice, where the second phase was generally regarded
as the weaker, or at any rate as not stronger than the first.
_Origin of the Double Shock._--These observations show, not only that
the principal earthquake consisted of two distinct shocks, but also
that the shocks originated in different foci. For, if the vibrations
of both had started from one focus, the second shock would have been
everywhere the stronger; instead of which there was a small area near
Nice where the intensity of the first was the greater. This points
clearly to the existence of another focus situated not far from Nice;
and it is evident that the greater intensity of the first part in that
district was due solely to the proximity of this focus, for, still
farther to the west, at Antibes, the second part was again the
stronger.
There is thus a striking agreement in the inferences drawn from
observations on the direction, time of occurrence, and nature of the
shock. In the face of such concurring testimony, little doubt can
remain as to the existence of two foci, one to the south of Oneglia
and the other to the south of Nice, the initial impulse at the latter
being decidedly the weaker, and preceding that at the eastern focus by
an interval of some seconds, long enough at any rate for the resulting
vibrations to reach the Oneglia focus and to spread beyond it before
the vibrations from that focus started on their outward journey.
_Seismographic Records._--In 1887, the Riviera and the districts
adjoining it were unprovided with accurately constructed seismographs.
The observatories at Alessandria, Milan, Monza, Parma, Florence, and
other places in Italy contained seismoscopes and other pendulums, and
these all registered the fact that an earthquake had occurred, and in
many cases traced a series of elliptical or elongated curves. A record
of the shock was also given by a Cecchi seismograph at Perpignan in
France, but the distance from the epicentre was too great to allow
details to be shown. The most valuable record was that obtained from a
Cecchi seismograph at the observatory of Moncalieri, near Turin, about
ninety miles north of the principal epicentre.
In this seismograph, the pendulums are provided with pointers, the
tips of which touch vertical sheets of paper attached to the sides of
an upright rectangular box. When an earthquake occurs, this box is
made to descend slowly with a uniform velocity, while the moving
pointers trace curves upon the smoked paper. The north-and-south
component of the horizontal motion is inscribed on the sheet of paper
facing west, and the east-and-west component on the paper facing
south.
[Illustration: FIG. 36.--Seismographic record of the Riviera
earthquake at Moncalieri. (_Denza._)]
During the principal Riviera earthquake, the former pendulum furnished
an indistinct record, while the other traced the diagram reproduced in
Fig. 36. The movement, as here represented, began at about 6h. 21m.
50s. A.M. (mean time of Rome) with a series of small tremors, which
lasted for about twelve seconds. Then followed some large
oscillations, always in a nearly east-and-west direction, which at 6h.
22m. 21s. gave place to a second series of tremors similar to those at
the beginning of the shock, but of greater amplitude. These continued
for at least twelve seconds, at the end of which time the motion of
the smoked paper ceased. The total duration of the movement at
Moncalieri cannot therefore have been less than forty-three seconds.
Interesting as this record is, it is doubtful how far it represents
accurately the movement of the ground. The Moncalieri instrument was
erected before the modern type of seismograph was designed, in which
some part remains steady, or very nearly steady, during the
complicated movements of the ground that take place in an earthquake.
It will be noticed that the curve in Fig. 36 shows no sign of the
division of the shock into two distinct parts, and this may perhaps be
due to the swinging of the pendulum itself; in which case, the curve
described by the pointer would be the resultant of the oscillations of
the ground and the proper motion of the pendulum.
SOUND-PHENOMENA.
The sounds that preceded and accompanied the Riviera earthquake have
attracted but little study, although they seem to have been widely
observed. No attempt was made to define the limits of the area over
which they were audible; but Professor Mercalli states that in the two
outer zones (Fig. 33) the sound generally passed unobserved. It was,
however, heard near Piacenza in Lombardy and Reggio in Emilia, places
which are about 115 and 140 miles from the principal epicentre.
In the area in which the shock was most violent, the sound resembled
that of trains and vehicles in motion; while, outside this area it
generally appeared to be like the hissing of a violent wind. In only a
few places was it compared to detonations, the crashes of artillery or
distant thunder. Some observers describe the sound as appearing at
first as if a strong wind were rising, and then as the roaring of a
heavy railway-train passing.
Nearly all the observers, who were awake at the beginning of the
earthquake, agree in asserting that the sound distinctly preceded any
movement of the ground. From this, as in the case of the Andalusian
earthquake, Professor Mercalli infers that the sound-vibrations
travelled with the greater velocity; but, as will be shown in Chapter
VIII., the general precedence of the sound admits of another and more
probable explanation.
THE UNFELT EARTHQUAKE.
If the Andalusian earthquake first drew general attention to the
distant spread of unfelt earth-waves, the Riviera earthquake showed
that this was no isolated phenomenon. We know now that the propagation
of such waves is only limited by the surface of the earth, but in 1887
some doubt was felt at first as to the nature of the disturbance,
whether it was magnetic or mechanical in its origin.
In 1884, the only observatories at which magnetographs were disturbed
were those of Lisbon, Parc Saint-Maur (near Paris), Greenwich, and
Wilhelmshaven. In 1887, the magnetographs registered the Riviera
earthquake at these and several other observatories, the distribution
of which is shown in Fig. 37. In this sketch-map, the position of the
principal epicentre is represented by the small cross, while the
nearly circular line shows the boundary of the disturbed area.
[Illustration: FIG. 37.--Distribution of observatories at which
magnetographs were disturbed by the Riviera earthquake.]
Three of the observatories, those of Nice, Lyons, and Perpignan, lie
inside this area. At Nice (which is thirty-seven miles from the
principal epicentre), M. Perrotin states that the magnetograph curves
show nothing of any interest, except a notable magnetic perturbation
on the vertical force curve, the time of which, however, is not
stated.[49] At Lyons (211 miles), the declination, horizontal force
and vertical force, magnets were all disturbed at 6h. 25m. 47s. A.M.,
and Perpignan (264 miles), all three magnets, but especially those for
the declination and horizontal force, were set abruptly oscillating
at 6h. 25m. 20s.
Elsewhere in France, the disturbances were noticed at the
observatories of Parc Saint-Maur and Montsouris, near Paris (about 447
miles), and at Nantes (538 miles). At Parc Saint-Maur, all three
curves show a very clear trace of the earthquake at 6h. 25m. 35s., the
oscillations lasting several minutes, and at Montsouris they also
began at the same time. At Nantes, the perturbations were so slight
that they escaped notice on a first examination.
In Austria, disturbances were observed at Pola (295 miles) and Vienna
(506 miles), beginning at 6h. 28m. 35s. and 6h. 30m. 35s.,
respectively. They reached Brussels (522 miles) at 6h. 29m. 27s., and
Utrecht (600 miles) at 6h. 28m. 38s.[50] At Wilhelmshaven (690 miles),
only the vertical force magnet was affected, the oscillations
beginning at 6h. 30m. 35s., and lasting for fourteen minutes. At 6h.
27m. 55s., the declination and horizontal force magnets of Greenwich
observatory (642 miles) were set vibrating, but no similar
disturbances were revealed by the vertical force curve or by the two
earth-current registers. At Kew (652 miles), the horizontal force
magnetograph was moved by the earthquake at about 6h. 29m. 55s. The
curves at Stonyhurst and Falmouth show no sign of any disturbance, nor
do those at Pawlovsk in Russia, or Seville. At Lisbon (951 miles),
however, the three curves indicate disturbances at 6h. 32m. 35s., but
so feeble are they that they would have escaped discovery if the
occurrence of the earthquake had been unknown.
The effects registered on the magnetograms are quite different from
those which correspond to ordinary magnetic perturbations; but they
are not unlike those produced by the action of the momentary currents
which are used for making the hour-marks, except that the
earthquake-oscillations lasted several minutes (see Fig. 21). In each
case, then, the magnetic bars must have received a succession of
several or many impulses.
Now, the effect of these impulses on each magnet must depend on the
relations which exist between the period of oscillation of the magnet,
the rate of damping of such oscillations, and the interval between the
successive impulses. Also, the apparent commencement of the phenomena
may be delayed if two impulses of contrary sense should follow one
another before the bar is perceptibly displaced. It is therefore to be
expected, as M. Mascart points out, that the disturbances of the three
instruments need not be of the same order of magnitude, that with
different forms of apparatus the effects may be very variable, and
that the deflection of one instrument may precede that of another at
one and the same place.
In all the magnetographs, the record is made on photographic paper,
which travels so slowly that the time of a movement can only be
ascertained to the nearest minute. As the disturbances on the French
curves were apparently almost simultaneous, and as no two of the
others differed in time of occurrence by more than five minutes, there
is thus some colour for M. Mascart's contention that the magnetic
apparatus registered, not the movements of the ground, but the passage
of electric currents produced in the ground at a certain epoch of the
earthquake.[51]
On the other hand, it is important to notice that, in the central part
of the disturbed area, at Nice, two, if not all three, of the
magnetographs were unaffected at the time of the earthquake.
At first sight, this fact seems equally opposed to a mechanical
explanation of the disturbance. But, when the vibrations are very
rapid, as they are in the neighbourhood of the epicentre, the magnetic
bars, owing to their mode of suspension, have not sufficient time to
be sensibly deflected in the brief interval between successive phases
of the impulse. The magnetograms of the Montsouris observatory show,
for instance, hardly any perceptible trace of disturbance during the
passage of railway trains along two adjacent lines. The farther,
however, the earth-waves travel from the origin, the longer becomes
the period of their vibrations. In Switzerland, they were remarkable
for their slowness, even to the unaided senses. Thus, at places more
or less remote from the Riviera, the magnets would receive impulses at
intervals approximating to their own periods of vibration, and they
would then oscillate freely for some time.
Again, notwithstanding some variations, it will be remarked that on
the whole the retardation of the initial epoch of the disturbances
increases with the distance from the epicentre. It thus seems clear, I
think, that the cause of the disturbances must be sought in the shock
itself; although their initial epochs at different places are too
roughly defined for ascertaining the velocity with which the
earth-waves travelled.
EFFECTS OF THE EARTHQUAKE AT SEA.
The Riviera earthquake, owing to its submarine origin, was marked by
certain phenomena that were absent from the other earthquakes
described in this volume.
_Nature of the Earthquake at Sea._--At the time of the earthquake,
several vessels were close to the epicentral area. One, about three
miles off Diano Marina, was shaken twice at about 6.20 A.M., and so
violently that it seemed as if the masts would be broken off. Another,
about ten miles south of P. Maurizio, also experienced two shocks, a
few minutes apart, as if each time it had struck the bottom. These
observations are chiefly interesting in showing that the double shock
was felt at sea as well as on land. As transverse vibrations are not
propagated through water, it follows that the second part of the shock
cannot, as some maintain, have been composed of transverse vibrations.
_Destruction of Fishes._--During the days immediately following the
earthquake, a large number of deep-sea fishes were found dead or
half-dead either in shallow water or stranded on the beach, especially
in the neighbourhood of Nice. Among them were numerous specimens,
mostly dead and floating, of _Alepocephalus rostratus_, a typical
deep-sea form, several of _Pomatomus telescopium_, _Scopelus
elongatus_, and _S. humboldti_, and many of _Dentex macrophthalmus_
and _Spinax niger_. The death and flight of these fishes must have
been due to a sudden shock, almost like that caused by the explosion
of dynamite, and communicated simultaneously to the whole surface of
their bodies.
_Seismic Sea-Waves._--Immediately after the earthquake, the sea
retired a short distance, variously estimated at from ten to thirty
metres, laying bare some rocks that were usually immersed. At P.
Maurizio, the surface was lowered by a little more than a metre; and
after a few minutes it rose to nearly a metre above its original
level, returning to it after a series of continually-decreasing
oscillations. At San Remo, a fall of about the same amount took place,
the sea returning after five minutes, and a ship anchored in the
harbour broke from her moorings. Again, at Antibes, the sea was
suddenly lowered by about a metre, so that ships afloat in the harbour
were aground for some instants, and then returned with some
impetuosity to its original level.
[Illustration: FIG. 38.--Record of tide-gauge at Nice.
(_Issel._)]
The evidence of eye-witnesses is confirmed by that of the tide-gauges
at Nice and Genoa, the curves of which are reproduced in Figs. 38 and
39. At Nice, the first arrest of the curve in its usual course
occurred at 6.30 A.M.;[52] the sea-level sank somewhat abruptly, and
after a few marked oscillations gradually returned to its normal
position at 7.50 A.M. At Genoa, the shock caused the writing-pen of
the tide-gauge to dent the paper on which the record is made, and soon
afterwards the curve shows a series of irregular oscillations, about
eight taking place every hour, and gradually decreasing until they
ceased to be perceptible about two hours after the principal
earthquake.
[Illustration: FIG. 39.--Record of tide-gauge at Genoa.
(_Issel._)]
MISCELLANEOUS PHENOMENA.
_Connection between Geological Structure and the Intensity of the
Shock._--As with the Andalusian earthquake, faulty construction and
defective materials were responsible for much of the damage caused by
the Riviera earthquake. But, if we may judge from the sharp local
variations in its amount, the nature of the surface-rocks must have
exerted a still more potent influence. At Cervo, for example, the
injury to property amounted to less than £3 per head of the
population; at Diano Marina, only two or three miles to the west, it
rose to £22 per head. The death-rate at Cervo was about one-tenth, and
at Diano Marina about 8-1/2 per cent. Again, at Mentone, the damage
must have been considerable, for about 155 houses were rendered
uninhabitable; while Monte Carlo, only a few miles farther west,
escaped almost unharmed. Now, Mentone and Diano Marina are for the
most part built on clay or alluvial deposits, and Monte Carlo on a
foundation of limestone.
Even within the limits of a single town, variations no less striking
were perceptible. In Mentone, the greatest damage occurred to houses
of two storeys built on alluvial soil in the low-lying parts near the
sea and in the valleys. The effect of the foundation in this part was
well shown in the case of two equally well-built houses not more than
300 yards apart. One in the valley, with doubtful foundations, was
very much shattered; the other, built on rock, was uninjured. The
large hotels, especially those on high ground, suffered least, few of
them having their main walls seriously damaged. These buildings rise
to heights of from four to six storeys, and of necessity have a firm
and solid foundation.
Professors Taramelli and Mercalli have made a careful study of the
subject of this section. The general conclusions at which they arrive
are that the intensity of the shock was greatest at places built on
pliocene conglomerates, beds of clay superposed on compact old rocks,
patches of alluvium, miocene formations of some thickness formed of
repeated alternations of strata of incoherent marls and limestones or
compact sandstones, beds of chalk, or somewhat rotten dolomite.
The shock was also more destructive on the summits of isolated hills
and ridges and on the steep slopes of mountains. The influence of the
form of the ground was, however, subordinate to that exerted by the
nature of the subsoil. Thus, at Mentone, as we have seen, and also at
Nice and Genoa, houses built on rock in elevated positions suffered
much less than those situated on the plains below that are composed of
sand and recent alluvium.
_Observations of the Earthquake in Railway-Tunnels._--Observations
made in mines at various times and places have proved that an
earthquake is felt less strongly in deep workings, if felt at all,
than on the surface of the ground. In the railway-tunnels of the
Riviera, as Professor Issel has shown, the same result was established
during the earthquake of 1887.
On the line which runs northward from Genoa to Piedmont, a tunnel more
than five miles in length pierces the hilly ground between Ponterosso
and Ronco, the greatest thickness of rock above being about a thousand
feet. At the time of the earthquake, the tunnel was not everywhere
opened out to its full width, and men were at work in different
sections. Outside, the shock was strong enough to damage buildings.
Inside, at about 200 yards from the south end, only a feeble shock was
felt; at 1,350 and 1,625 yards, some bricks were seen to fall from the
facing, but the shock was not otherwise perceived, and only a few
yards farther nothing unusual was noticed by the men at work.
Again, in an unfinished tunnel, about three-quarters of a mile long,
between the harbour of Genoa and the eastern railway-station, the
vibrations were very slightly felt. Even in the tunnels traversed by
the coast railway from Genoa to Nice--that is, in those situated
within the meizoseismal area--the shock was either very weak or not
felt at all, and not one of the tunnels suffered the slightest injury.
To men at work inside a long tunnel, the conditions for observing
earthquakes are somewhat imperfect, but these facts, nevertheless,
bring out very clearly the inferior intensity of the shock at some
depth below the surface.
AFTER-SHOCKS.
While the unfelt earth-waves of the great earthquake were still
wending their way over the zone that surrounds the disturbed area, the
central regions were again shaken, at 6.29 A.M., by a shock strong
enough to produce fresh ruins in the stricken towns along the coast.
Nearly two and a half hours of quiet followed, broken only by a few
subterranean rumblings in the central part of the meizoseismal area.
Then, at 8.51 A.M., occurred another shock, short and sharp, and
inferior in strength only to the principal earthquake. Both of these
after-shocks were felt in Western Switzerland; indeed, they were
perceptible nearly as far as the great shock; the second, however, a
little farther than the first, for it alone was noticed at such places
as Vicenza, Forlì, and Florence. The shock at 6.29 was usually
described as long and its vibrations as undulatory only; that at 8.51
as rather subsultory than undulatory and of very brief duration. The
latter, however, was followed after an interval of a few seconds by
another shock so weak that it generally passed unobserved. Both shocks
were preceded by a rumbling sound.
During the next two days, tremors and earth-sounds were frequent in
the Riviera; once an hour, on an average, the greater part of the
meizoseismal area was shaken by vibrations more or less slight. But,
between one shock and another, at Diano Marina and Alassio, and even
as far as Nice, it only required attention from a careful observer to
perceive an almost continual throbbing of the ground.
Only one of these shocks, that of February 24th, at 2.10 A.M., was
strong enough to cause slight damage to buildings. It disturbed an
area, not exceeded by any of the later shocks, the boundary of which,
shown by the dotted line A in Fig. 33, extends to the north and east
as far as Piacenza and Spezia, while to the west it includes Cannes.
The centre of the curve so drawn lies on land, but, as the shock was
not felt in Corsica, there is no evidence as to the southerly
extension of the disturbed area; and it is probable, as Professor
Mercalli suggests, that the shock originated in the eastern or Oneglia
focus of the great earthquake.
After February 25th, slight shocks were felt during the next
fortnight, at the rate of three or four a day, until March 11th, when
the last after-shock resulting in slight damage occurred at about 3.12
P.M. The boundary of its disturbed area, represented in Fig. 33 by the
dotted line B, passes a little to the east of Savona, and then through
Alessandria, Moncalieri, and Marseilles. The shock, however, was not
observed in Corsica, so that the exact position of the epicentre is
unknown; but Professor Mercalli believes it to coincide with the
western or Nice epicentre of the principal earthquake. At the moment
of the shock, the sea was observed from Alassio to curl and to rise
slightly, while the tide-gauge at Nice, which had traced a continuous
curve earlier in the day, showed a characteristic notch about 3.7 P.M.
Of the remaining after-shocks, only two attained any notable degree of
strength. One, on May 20th at about 8.15 A.M., disturbed an area
nearly concentric with that of the great earthquake, and with a
boundary coinciding nearly with the isoseismal 2 in Fig. 33. Again, on
July 17th at 11.30 P.M., occurred a shock felt over an area nearly as
large as that disturbed on February 24th at 2.10 A.M., and situated in
the same part of the country.
Altogether, during the year following the Riviera earthquake,
Professor Mercalli records 190 after-shocks, most of them slight or
only just felt. With the exception of the first two (on February
23rd), none was observed outside the isoseismal 4 of the principal
earthquake (Fig. 33); and, of the rest, only the four whose dates are
given above disturbed an area of more than one-eighth of that of the
great shock. Some of them, like the shock of March 11th, were stronger
in the western part of the meizoseismal area; but the majority
affected most the eastern portion and seem to be closely associated
with the Oneglia focus.
From February 26th to April 20th, Professor Rumi made observations on
the after-shocks by means of the Foucault pendulum erected at Genoa
for demonstrating the rotation of the earth. In nearly every case, the
oscillations took place along a north-east and south-west line, or in
the same direction as the first great shock--a resemblance which
supports the inference that many of the after-shocks originated
within the Oneglia focus.
ORIGIN OF THE EARTHQUAKES.
_Recent Movements in the Riviera._--The earliest movements that
resulted in the great range of the Maritime Alps and the Ligurian
Apennines date from pre-Carboniferous times, when the central
crystalline massifs in part emerged. At the end of the Liassic epoch,
the secondary formations of the district were uplifted, and it was at
this time that the range assumed its characteristic curved form. Later
still, at the close of the Eocene period, an elevation of more than
9000 feet took place, for upper Eocene beds are found at this height
in the Maritime Alps.
Since that time, other important movements have occurred. Pliocene
deposits have been found in the Riviera at an altitude of 1,800 feet.
Recent soundings in the Gulf of Genoa have also shown that all the
valleys of the Riviera between Nice and Genoa are continued far below
the level of the sea to depths of not less than 3000 feet. Thus, at
the end of the Pliocene or beginning of the Quaternary period, there
was an elevation of nearly 5000 feet, accompanied or followed by the
erosion of the valleys which, later on, during the Quaternary period,
were submerged about 3000 feet. Even in still more recent times,
probably in the Palæolithic age, minor movements continued. Traces of
recent elevation, varying in amount from a few feet to sixty feet or
more, occur at the Balzi Rossi in the Alpes Maritimes, near Bergeggi,
and in Genoa; while evidences of submergence are to be found near
Monaco, at Beaulieu and at Diano Marina. It is important to notice
that the great movements dating from the end of the Eocene period are
almost confined to the Maritime Alps and the western portion of the
Riviera. In the parts of Piedmont lying to the north of Cuneo and in
the eastern Riviera, they produced hardly any sensible effect.
_Seismic History of the Riviera._--The movements just referred to are
those which, in course of time, have become sensible to the eye. They
represent the sum of a long-continued series of displacements that may
once have been on a large scale, but are now comparatively small. The
earthquakes that occur in the Riviera show, however, that the final
stage has not yet been reached. Their epicentres indicate the regions
in which slips are still taking place, and the magnitude of these
slips is roughly measured by the intensity of the resulting shocks.
The map in Fig. 40 is one of a series drawn by Professor Mercalli to
represent the distribution of seismic activity in Piedmont and the
Riviera. It corresponds to the period from 1801 to 1895. The whole
area is divided into a number of seismic districts, each of which is
distinguished by a particular degree of activity. In estimating this
quantity, Professor Mercalli takes intensity as well as frequency into
account. Thus, the lowest degree, represented by the lightest tint of
shading, corresponds to one or two strong earthquakes with a few
moderate or slight shocks; the eighth and highest to four or five
ruinous or disastrous earthquakes followed by trains of after-shocks.
The map shows very clearly that, during the last century, the seismic
activity was greatest in the Maritime Alps and the western
Riviera--that is, in the very districts in which the recent
mountain-making movements have been most conspicuous.[53]
[Illustration: FIG. 40.--Distribution of seismic activity in the
Riviera. (_Mercalli._)]
In all these districts, Professor Mercalli distinguishes several
well-marked seismic centres, to each of which he traces the origin of
two or more earthquakes. In the districts with which we are at present
concerned, those of the Alpes Maritimes and the western Riviera, the
most important centres are situated near Oneglia (in the sea), near
Taggia, in the valleys of the Vesubia and Tinea (near Nice), and in
the sea to the south of Nice. To the first of these centres belongs
the disastrous earthquake of February 23rd, 1887, as well as its
after-shocks on February 24th, May 20th, July 17th, and September 30th
of the same year, also the ruinous earthquakes of 1612 and 1854, and
several others of a lesser degree of intensity. All of these were
longitudinal earthquakes, the axes of their meizoseismal areas being
parallel to the neighbouring mountain-ranges. A few miles to the west
of Oneglia lies the Taggia centre, with which were connected the
disastrous earthquake of 1831, the violent earthquake of 1874, and
other strong or very strong shocks. These were for the most part
transversal earthquakes, their axes being perpendicular to those of
the Oneglia centre.
Some of the strongest earthquakes in this region originated in a
centre lying to the north of Nice in the valleys of the Vesubia and
Tinea. Among them may be mentioned the ruinous earthquakes of 1494,
1556, 1564, and 1644, and probably also the disastrous earthquake of
1227. A fourth centre, and one of considerable interest, is that which
lies at sea, a short distance to the south of Nice, and nearly along
the continuation of the valleys above-mentioned. This is the secondary
centre of the earthquake of 1887, and probably also of that of
December 29th, 1554. It is occasionally in action apart from the
Oneglia centre, as on November 27th, 1771, June 19th, 1806, and
December 21st, 1861; but such shocks, though rather strong, never
reach a high degree of intensity.
_Origin of the Earthquakes of 1887._--The most important feature in
the principal earthquake of 1887 is its origination in two distinct
foci, which are sometimes in action almost simultaneously, but more
often separately. The earthquakes belonging to the two foci differ
greatly in intensity and number, and the stronger part of the shock in
1887 originated in the focus associated with the more disastrous and
more frequent earthquakes.
The existence of two foci would of course give rise to a meizoseismal
area elongated in the direction of the line joining them. It is clear,
however, that the Oneglia focus was also extended in the same
direction; for, in the after-shock of February 24th, the isoseismals
drawn by Professor Mercalli are parallel to this line; and this was
also the case in the shock of March 11th. As both foci were under the
sea, it is difficult to locate them with precision; but it seems very
probable that they occupy portions of a submarine fault that runs
parallel or nearly so to the Apennine axis between the meridians of
Oneglia and Nice.
A brief period of preparation is a characteristic of the Riviera
earthquakes. In 1887, two at least of the preliminary shocks on
February 23rd (those of about 2 and 5 A.M.) originated in the Oneglia
focus. At 6.20 A.M. the first and weaker movement took place in the
western focus; and, a few seconds after the resulting vibrations
reached the eastern focus, the second and greater slip took place
there. The occurrence of seismic sea-waves is probably evidence of the
formation of a small, though sensible, fault-scarp in the same region.
To relieve the additional stresses thus brought into action along the
fault-surface, numerous small slips took place in different parts,
some as far to the west as the Nice focus, but the greater number
probably within or close to the focus in the neighbourhood of Oneglia.
REFERENCES.
1. BERTELLI, T.--"Osservazioni fatte in occasione di una escursione
sulle Riviera Ligure di ponente dopo i terremoti ivi seguiti
nell' anno 1887." _Boll. Mens. dell' Oss. di Moncalieri_,
vol. viii., 1888, Nos. 6, 7, 8.
2. CHARLON, E.--"Note sur le tremblement de terre du 23 février
1887." _Bull. del Vulc. Ital._, anno xiv., 1887, pp. 18-23.
3. DENZA, F.--_Alcune notizie sul terremoto del 23 febbraio 1887_
(Turin).
4. ISSEL, A.--"Il terremoto del 1887 in Liguria." _Boll. del R. Com.
Geol. d'Italia_, anno 1887, supplemento, pp. 1-207.
5. MERCALLI, G.--_I terremoti della Liguria e del Piemonte_.
(Naples, 1897, 146 pp.)
6. ODDONE, E.--"I dati sismici della Liguria in rapporto alla
frequenza ed alla periodicità." _Boll. della Soc. Sismol.
Ital._, vol. ii., 1896, pp. 140-151.
7. OFFRET, A.--"Sur le tremblement de terre du 23 février 1887.
Discussion des heures observés dans la zone épicentrale."
Paris, _Acad. Sci., Compt. Rend._, vol. civ., 1887, pp.
1150-1153.
8. ----. "Tremblements de terre du 23 février 1887. Heures de
l'arrivée des secousses en dehors de l'épicentre." _Ibid._,
pp. 1238-1242.
9. ROSSI, M.S. DE.--"Relazione sui terremoti del febbraio 1887."
_Bull. del Vulc. Ital._, anno xiv., 1887, pp. 5-17.
10. ----. "Bibliografia: Sul terremoto ligure del 23 febbraio 1887."
_Ibid._, pp. 60-62, 107-112, 115-128.
11. TARAMELLI, T., and G. MERCALLI.--"Il terremoto ligure del 23
febbraio 1887." _Annali dell' Uff. Centr. di Meteor. e di
Geodin._, vol. viii., parte iv., 1888. (Roma, 298 pp.)
12. UZIELLI, G.--_Le commozioni telluriche e il terremoto del 23
febbraio 1887_ (Turin).
13. _Nature_, vol. xxxv., 1887, pp. 438, 462, 534-535; vol. xxxvi.,
1887, pp. 4, 151-152.
14. Paris, _Acad. Sci. Compt. Rend._, vol. civ., 1887, pp. 556-557,
606-612, 634-635, 659-667, 744-745, 757-758, 759-760, 764-766,
822-823, 830-835, 884-890, 950-951, 1088-1089, 1243-1245,
1350-1352, 1416-1419; vol. cv., 1887, pp. 202-203; vol.
cviii., 1889, p. 1189; vol. cix.; 1889, pp. 164-166, 272-274,
660.
FOOTNOTES:
[47] The above times and all others in this chapter are given in Rome
mean time, which is 50m. earlier than Greenwich mean time.
[48] Professor Uzielli has also published a map of the isoseismal
lines for the Italian part of the disturbed area.
[49] It seems doubtful whether this movement was connected with the
earthquake. M. Offret does not include Nice in his list of
observatories at which magnetographs were disturbed.
[50] This is the time given by M. Offret. According to M. Mascart, it
should be 6h. 25m. 40s.
[51] In order to test the truth of this explanation, M. Moureaux
suspended a bar of copper at the Parc Saint-Maur observatory by two
threads in the same way as the horizontal force-magnet. The direction
of this bar was also registered photographically, and it remained
unmoved during the Verny earthquake of July 12th, 1889, and the
Dardanelles earthquake of October 25th, 1889, while one or more of the
magnets were disturbed. The experiment, however, was ineffective; for,
in order that the magnet may rest in a horizontal position, its centre
of gravity must be at unequal distances from the two points of
support.
[52] The hour-marks in Fig. 38 refer to Paris mean time, and those in
Fig. 39 to Genoa mean time.
[53] In the seventeenth century, the maximum seismic activity was
manifested in the neighbourhood of Nice, and in the eighteenth century
in Piedmont.
CHAPTER VII.
THE JAPANESE EARTHQUAKE OF OCTOBER 28TH, 1891.
Although several years have elapsed since the occurrence of the
greatest of Japanese earthquakes, the final report that will embody
the labours of all its investigators is yet to be written. Several
important contributions to it, however, have already been made.
Professor Koto, in an admirable memoir, has traced the course of the
great fault-scarp and discussed the origin of the earthquake;
Professor Omori, with equal care and thoroughness, has investigated
the unrivalled series of after-shocks; Mr. Conder studied the damaged
buildings from an architect's point of view; Professor Tanakadate and
Dr. Nagaoka devoted themselves to a re-determination of the magnetic
elements of the central district,[54] while, by the compilation of his
great catalogue of Japanese earthquakes during the years 1885-92,
Professor Milne has provided the materials for a further analysis of
the minor shocks that preceded and followed the principal earthquake.
The part of Japan over which the earthquake was sensibly felt is
shown in Fig. 41. The small black area in the centre is that in which
the shock was most severe and the principal damage to life and
property occurred. The other bands, more or less darkly shaded
according to the greater or less intensity of the shock, will be
referred to afterwards. Fig. 45 represents the meizoseismal area on a
larger scale; and, as the greater part of it lies within the two
provinces of Mino and Owari, the earthquake is generally known among
the Japanese themselves as the Mino-Owari earthquake of 1891.
[Illustration: FIG. 41.--Sketch-Map of Disturbed Area and
Isoseismal Lines. (_Masato._)]
THE MEIZOSEISMAL AREA.
More than half of the meizoseismal area occupies a low flat plain of
not less than 400 square miles in extent. On all sides but the south,
the plain, which is a continuation of the depression forming the Sea
of Isé, is surrounded by mountain ranges, those to the west, north,
and north-east being built up mainly of Palæozoic rocks, and those on
the east side of granite. A network of rivers and canals converts what
might otherwise have been unproductive ground into one of the most
fertile districts in Japan. A great garden, as it has been aptly
termed, the whole plain is covered with rice-fields, and supports a
population of about 787 to the square mile--a density which is
exceeded in only six counties of England. As a rule, the soil is a
loose, incoherent, fine sand, with but little clayey matter; and it
is, no doubt, to its sandy nature that the disastrous effects of the
earthquake were largely due. In the northern half of the district, the
meizoseismal area is much narrower, and here it crosses a great
mountain-range running from south-west to north-east and separating
the river-systems of the Japan sea from those of the Pacific. To the
north, the meizoseismal area terminates in another plain, in the
centre of which lies the city of Fukui, where the destructiveness of
the earthquake was only inferior to that experienced in the provinces
of Mino and Owari. There is also a detached portion of the area lying
to the east of Lake Biwa, but it is uncertain whether the exceptional
intensity there was due to the nature of the ground or to the
occurrence of a secondary or sympathetic earthquake in its immediate
neighbourhood.
[Illustration: FIG. 42.--General Plan of Geological Structure of
Meizoseismal Area. (_Koto._)]
The general plan of the geological structure of the central district
is represented in Fig. 42. The thick line, partly continuous and
partly broken, shows the course of the great fault, to the growth of
which the earthquake chiefly owed its origin; while the thin
continuous lines represent the changing direction of strike of the
Palæozoic rocks which surround the Mino-Owari plain, and the
arrowheads the direction of the dip. It will be seen that the
direction of the strike forms an S-shaped curve, and it is clear that
the present torsion-structure of the district could not have been
produced without the formation of many fractures at right angles and
parallel to the lines of strike. Professor Koto points out that the
regular and parallel valleys of the rivers Tokuno-yama, Neo, Mugi, and
Itatori, indicated by broken lines in Fig. 42, have probably been
excavated along a series of transverse fractures running from
north-west to south-east; while fractures which are parallel to the
line of strike may be responsible for the zigzag course of the
valleys.
DAMAGE CAUSED BY THE EARTHQUAKE.
The great earthquake occurred at 6.37 A.M., practically without
warning, and in a few seconds thousands of houses were levelled with
the ground. Within the whole meizoseismal area there was hardly a
building left undamaged. The road from Nagoya to Gifu, more than
twenty miles in length, and formerly bordered by an almost continuous
succession of villages, was converted into a narrow lane between two
long drawn-out banks of _débris_. "In some streets," says Professor
Milne, "it appeared as if the houses had been pushed down from the
end, and they had fallen like a row of cards." Or, again, a mass of
heaped-up rubbish might be passed, "where sticks and earth and tiles
were so thoroughly mixed that traces of streets or indications of
building had been entirely lost." At Gifu, Ogaki, Kasamatsu, and other
towns, fires broke out after the earthquake. In Kasamatsu the
destruction was absolutely complete; nothing was left but a heap of
plaster, mud, tiles, and charred timbers. At Ogaki, not more than
thirty out of 8000 houses remained standing, and these were all much
damaged. Within the whole district, according to the official returns,
197,530 buildings were entirely destroyed, 78,296 half destroyed, and
5,934 shattered and burnt; while 7,279 persons were killed, and 17,393
were wounded.
Next to buildings, the embankments which border the rivers and canals
suffered the most serious damage, no less than 317 miles of such works
having to be repaired. Railway-lines were twisted or bent in many
places, the total length demolished being more than ten miles. In
cuttings, twenty feet or more in depth, both rails and sleepers were
unmoved; it was on the plains that the effects of the earthquake were
most marked. The ground appeared as if piled up into bolster-like
ridges between the sleepers, and in many places the sleepers had moved
end-ways. When the line crossed a small depression in the general
level of the plain, the whole of the track was bowed, as if the ground
were permanently compressed at such places. "Effects of compression,"
says Professor Milne, "were most marked on some of the embankments,
which gradually raise the line to the level of the bridges. On some of
these, the track was bent in and out until it resembled a serpent
wriggling up a slope.... Close to the bridges the embankments had
generally disappeared, and the rails and sleepers were hanging in the
air in huge catenaries."
ISOSEISMAL LINES AND DISTURBED AREA.
The land area disturbed by the earthquake and the different isoseismal
lines are shown in Fig. 41. The "most severely shaken" district, that
in which the destruction of buildings and engineering works was
nearly complete, contains an area of 4,286 square miles, or about
two-thirds that of Yorkshire. This is indicated on the map by the
black portion. Outside this lies the "very severely shaken" district,
17,325 square miles in area, extending from Kobe on the west to
Shizuoka on the east, in which ordinary buildings were destroyed,
walls fractured, embankments and roads damaged, and bridges broken
down. The third or "severely shaken" district contains 20,183 square
miles; and in this some walls were cracked, pendulum clocks stopped,
and furniture, crockery, etc., overthrown. Tokio and Yokohama lie just
within this area. In the fourth region the shock was "weak," the
motion being distinctly felt, but not causing people to run
out-of-doors; and in the fifth it was "slight," or just sufficient to
be felt. These two regions together include an area of 51,976 square
miles.
Thus, the land area disturbed amounts altogether to 93,770 square
miles--_i.e._, to a little more than the area of Great Britain.
According to Professor Omori, the mean radius of propagation was about
323 miles, and the total disturbed area must therefore have been about
330,000 square miles, or nearly four times the area of Great Britain.
Considering the extraordinary intensity of the shock in the central
district, this can hardly be regarded as an over-estimate.
The isoseismal lines shown in Fig. 41 are not to be regarded as drawn
with great accuracy; for there is no marked separation between the
tests corresponding to the different degrees of the scale of
intensity. The seismographs at Gifu and Nagoya were thrown down
within the first few seconds, and failed to record the principal
motion. But a great number of well-formed stone lanterns and
tombstones were overturned, and, from the dimensions of these,
Professor Omori calculated the maximum horizontal acceleration
necessary for overturning them at fifty-nine places within the
meizoseismal area.[55] At five of these it exceeded 4000 millimetres
per second per second, an acceleration equal to about five-twelfths of
that due to gravity. Making use of these observations, Professor Omori
has drawn two isoseismal lines within the central district, which are
shown in Fig. 44. At every point of the curve marked 2, the maximum
acceleration was 2000 millimetres per second per second, and of that
marked 1, 800 millimetres per second per second. The dotted line
within the curve marked 2 represents the boundary of the meizoseismal
area, which, it will be observed, differs slightly from that given by
Professor Koto (see Fig. 45). The difference, however, is apparently
due to the standard of intensity adopted, Professor Koto's boundary
agreeing rather closely with the curve marked 2 in Fig. 44.
NATURE OF THE SHOCK.
Little has yet been made known with regard to the nature of the shock,
and the published records of the accompanying sound are so rare that
it seems as a rule to have passed unheard. The seismographs at Gifu
and Nagoya registered the first half-dozen vibrations, and were then
buried beneath the fallen buildings. In the following table, the data
from these two stations are therefore incomplete:--
PRINCIPAL MEASUREMENTS OBTAINED FROM SEISMOGRAPHIC RECORDS.
-------------------+------------+----------+----------+---------------
| | | | Tokio
| Gifu. | Nagoya. | Osaka. | (Imp. Univ.).
-------------------+------------+----------+----------+---------------
Maximum horizontal | | | |
motion | > 18 mm. | > 26 mm. | 30 mm. | > 35 mm.
| | | |
Period of ditto | 2.0 secs. | 1.3 sec. | 1.0 sec. | 2.0 secs.
| | | |
Maximum vertical | | | |
motion | > 11.3 mm. | 6.2 mm. | 8 mm. | 9.5 mm.
| | | |
Period of ditto | 0.9 sec. | 1.5 sec. | 1.0 sec. | 2.4 secs.
-------------------+------------+----------+----------+---------------
If the period of the principal vibrations were known, the observations
of Professor Omori on the overturning of bodies would enable us to
determine the range of motion at different places. For instance, the
maximum acceleration at Nagoya was found by these observations to be
2,600 millimetres per second per second, and if we take the period of
the greatest horizontal motion to be the same as that of the initial
vibrations--namely, 1.3 second, the total range (or double amplitude)
would be 223 millimetres, or 8.8 inches. With the same period, and the
maximum acceleration observed (at Iwakura and Konaki) of more than
4,300 millimetres per second per second, the total range would be
greater than 14.5 inches.[56]
In the meizoseismal area, many persons saw waves crossing the surface
of the ground. At Akasaka, according to one witness, the waves came
down the streets in lines, their height being perhaps one foot, and
their length between ten and thirty feet. To the north of the same
area, we are told that "the shoreline rose and fell, and with this
rising and falling the waters receded and advanced." Even at Tokio,
which is about 175 miles from the epicentre, the tilting of the ground
was very noticeable. After watching his seismographs for about two
minutes, Professor Milne next observed the water in an adjoining tank,
80 feet long and 28 feet wide, with nearly vertical sides. "At the
time it was holding about 17 feet of water, which was running across
its breadth, rising first on one side and then on the other to a
height of about two feet." Still clearer is the evidence of the
seismographs in the same city. Instead of a number of irregular waves,
all the records show a series of clean-cut curves. The heavy masses in
the horizontal pendulums were tilted instead of remaining as steady
points. They were not simply swinging, for the period of the
undulations differed from that of the seismograph when set swinging,
and also varied in successive undulations. It was ascertained
afterwards, by measurement with a level, that to produce these
deflections, the seismograph must have been tilted through an angle of
about one-third of a degree.
_Direction of the Shock._--Shortly after the earthquake, Professor
Omori travelled over the meizoseismal area and made a large number of
observations on the directions in which bodies were overturned, taking
care to include only those in which the direction of falling would not
be influenced by the form of the base, such as the cylindrical stone
lanterns so frequently found in Japanese gardens. At some places
these bodies fell in various directions, at others with considerable
uniformity in one direction. For instance, at Nagoya, out of 200 stone
lanterns with cylindrical stems, 119 fell between west and south, and
36 between east and north; the numbers falling within successive
angles of 15° being represented in Fig. 43. The mean direction of fall
is W. 30° S., coinciding with that in which the majority of the
lanterns were overturned. Similar observations were made at forty-two
other places within and near the meizoseismal area, and the resulting
mean direction for each such place in the Mino-Owari district is shown
by short lines in Fig. 44, the arrow indicating the direction towards
which the majority of bodies at a given place were overturned. It will
be seen from this map that the direction of the earthquake motion was
generally at right angles, or nearly so, to that of the neighbouring
part of the meizoseismal zone, and that on both sides of it, the
majority of overturned bodies at each place fell towards this zone.
[Illustration: FIG. 43.--Plan of Directions of Fall of Overturned
Bodies at Nagoya.]
[Illustration: FIG. 44.--Map of Mean Directions of Shock and
Isoseismal Lines in Central District. (_Omori._)]
VELOCITY OF THE EARTH-WAVES.
The times of the great earthquake and of sixteen minor shocks on
October 28th and 29th and November 6th were determined at the Central
Meteorological Observatory at Tokio, and at either two or three of the
observatories of Gifu, Nagoya, and Osaka, each of which is provided
with a seismograph and chronometer. The after-shocks referred to
originated near a point about 6 miles west of Gifu, and the difference
between the distances of Tokio and Osaka from this point is 89-1/2
miles, of Tokio and Nagoya 147 miles, and of Tokio and Gifu 165 miles.
The mean time-intervals between these three pairs of places were 67,
111, and 128 seconds respectively; and these give for the mean
velocity for each interval 2.1 kilometres (or 1.3 mile) per second.
Thus there appears in these cases to be no sensible variation in the
velocity with the distance from the origin.
As might be expected, an earthquake of such severity was recorded by
magnetometers at several distant observatories. Disturbances on the
registers of Zikawei (China), Mauritius, Utrecht, and Greenwich have
been attributed to the Japanese earthquake, but the times at which
they commenced are too indefinite to allow of any determination of the
surface-velocity of the earth-waves to great distances from the
origin.
THE GREAT FAULT-SCARP.
As in all disastrous earthquakes, the surface of the ground was
scarred and rent by the shock. From the hillsides great landslips
descended, filling the valleys with _débris_; and slopes which were
formerly green with forest, after the earthquake looked as if they had
been painted yellowish-white. Innumerable fissures cut up the plains,
the general appearance of the ground, according to Professor Milne,
being "as if gigantic ploughs, each cutting a trench from 3 to 12 feet
deep, had been dragged up and down the river-banks." But by far the
most remarkable feature of the earthquake was a great rent or fault,
which, unlike the fissures just referred to, pursued its course
regardless of valley, plain, or mountain. Although at first sight
quite insignificant in many places, and some time hardly visible to
the untrained eye, Professor Koto has succeeded in tracing this fault
along the surface for a distance of forty miles, and he gives good
reasons for believing that its total length must be not less than
seventy miles.
[Illustration: FIG. 45.--Map of Meizoseismal Area. (_Koto._)]
[Illustration: FIG. 46.--Ploughshare Appearance of the Fault near
Fujitani. (_Koto._)]
[Illustration: FIG. 47.--The Fault-scarp at Midori. (_Koto._)]
The general character of the fault-scarp changes with the surface
features. On flat ground, where the throw is small, it cuts up the
soft earth into enormous clods, or makes a rounded ridge from one to
two feet high, so that it resembles, more than anything else, the
pathway of a gigantic mole (Fig. 46). When the throw is
considerable--and in one place it reaches from 18 to 20 feet--the
fault-scarp forms a terrace, which from a distance has the appearance
of a railway embankment (Fig. 47). Or, again, where the rent traverses
a mountain ridge or a spur of hills, "it caused extensive landslips,
one side of it descending considerably in level, carrying the forest
with it, but with the trees complicatedly interlocked or prostrate on
the ground."
[Illustration: FIG. 48.--Displacement of Field Divisions by the
Fault near Nishi-Katabira. (_Koto._)]
At its southern end, the fault was seen for the first time crossing a
field near the village of Katabira. The field was broken into clods of
earth, and swollen up to a height of 5-1/2 yards, while a great
landslip had descended into it from an adjoining hill. A little
farther to the north-west, the ground was sharply cut by the fault,
the north-east side having slightly subsided and at the same time been
shifted horizontally through a distance of 3-1/4 to 4 feet to the
north-west Adjoining fields were formerly separated by straight mounds
or ridges running north and south and east and west, and these mounds
were cut through by the fault and displaced, as shown in Fig. 48. From
this point the fault runs in a general north-westerly direction, the
north-east side being always slightly lowered with respect to the
other and shifted to the north-west. Near Seki it takes a more
westerly direction, and continues so to a short distance east of
Takatomi, where the north side is lowered by five feet, and moved
about 1-1/4 feet to the west. At the north end of Takatomi, a village
in which every house was levelled with the ground, the fault is
double, and the continuous lowering towards the north has converted a
once level field into sloping ground. At this point, the small river
Toba, flowing south, is partially blocked by the fault-scarp, and an
area of about three-quarters of a square mile, on which two villages
stand, was converted into a deep swamp (Fig. 49), so that, as the
earthquake occurred at the time of the rice-harvest, the farmers were
obliged to cut the grain from boats. After passing Takatomi, the fault
again turns to the west-north-west, but, the throw being small, it
resembles here the track of an enormous mole. At Uméhara it crosses a
garden between two persimmon trees, appearing on the hard face of the
ground as a mere line; but the trees, which were before in an
east-and-west line, now stand in one running north and south, without
being in the least affected by the movement (Fig. 50). From here to
Kimbara, where the fault enters the Neo valley, the north side is
always depressed and shifted westwards by about 6-1/2 feet.
[Illustration: FIG. 49.--Map of Swamp formed by stoppage of River
Toba by Fault-scarp. (_Koto._)]
[Illustration: FIG. 50.--Shifting of Trees by fault at Uméhara.
(_Koto._)]
It was in the Neo valley that the supreme efforts of the earthquake
were manifested. Landslips were so numerous that the greater part of
the mountain slopes had descended into the valley, the whole
appearance of which had changed. "Unfamiliar obstacles," remarks
Professor Koto, "made themselves apparent, and small hills covered
with forest had come into sight which had not been seen before." But
the ground was not only lowered and shifted by the fault; it was
permanently compressed, plots originally 48 feet in length afterwards
measuring only 30 feet. In fact, "it appears," in the words of
Professor Milne, "as if the whole Neo valley had become narrower."
A few miles after entering the Neo valley, the throw of the fault
reaches its maximum at Midori. But instead of the relative depression
of the east side, which prevails throughout the rest of the line, that
side is here about 20 feet higher than the other. It is, however,
shifted as usual towards the north, by about 13 feet; and this
displacement is rendered especially evident by the abrupt break in the
line of a new road to Gifu (Fig. 47). That the east side has really
risen is clear, for, a little higher up, the river has changed from a
shallow rapid stream 30 yards wide into a small lake of more than
twice the width, and so deep that a boatman's pole could not reach the
bottom. At Itasho, about a mile north of Midori, both sides are nearly
on the same level, the fault appearing like a mole's track; and seven
miles farther, at Nagoshima, the east side is relatively depressed by
more than a yard, and at the same time shifted about 6-1/2 feet to the
north.
[Illustration: FIG. 51.--Daily frequency of after-shocks at Gifu
and Nagoya.]
At Nogo, the main Neo valley turns off at right angles to the east,
and the fault continues its course up a side valley, the east side,
with respect to the other, being continually depressed and shifted
towards the north. It was traced by Professor Koto through Fujitani
(Fig. 46), where there were many unmistakable evidences of the
violence of the shock, as far as the eastern shoulder of Haku-san; and
here, after following the fault for 40 miles, the lateness of the
season compelled him to return. There can be no doubt, however, that
it runs as far as Minomata; and it is probable, from the linear
extension of the meizoseismal area, that it does not entirely die out
before reaching the city of Fukui, 70 miles from its starting-point at
Katabira.
MINOR SHOCKS.
For some hours after the earthquake, shocks were so frequent in the
meizoseismal area that the ground in places hardly ever ceased from
trembling. Without instrumental aid, detailed record was of course
impossible; but fortunately the buried seismographs at Gifu and Nagoya
were uninjured, and in about seven hours both were once more in
working order. To the energy by which this result was accomplished, we
owe our most valuable registers of the after-shocks of a great
earthquake.
[Illustration: FIG. 52.--Monthly frequency of after-shocks at
Gifu. (_Omori._)]
Until the end of 1893--that is, in little more than two years--the
total number of shocks recorded at Gifu was 3,365, and at Nagoya
1,298. None of these approached the principal earthquake in severity.
Nevertheless, of the Gifu series, 10 were described as violent and 97
strong; while of the remainder, 1,808 were weak, 1,041 feeble, and 409
were sounds alone without any accompanying shock. The slight intensity
of most of the shocks is also evident from the inequality in the
numbers recorded at Gifu and Nagoya, from which it appears that nearly
two-thirds were imperceptible more than about 25 miles from the chief
origin of the shocks. Only 70 of the after-shocks during the first two
years were registered at Osaka, and not more than 30 at Tokio.
_Distribution of After-shocks in Time._--The decline in frequency of
the after-shocks was at first extremely rapid, the numbers recorded at
Gifu during the six days after the earthquake being 303, 147, 116, 99,
92, and 81, and at Nagoya 185, 93, 79, 56, 30, and 31; in fact, half
of the shocks up to the end of 1893 occurred by November 23rd at Gifu,
and by November 6th at Nagoya. The daily numbers at these two places
are represented in Fig. 51, in which the crosses correspond to the
numbers at Gifu, and the dots to those at Nagoya; and the curves drawn
through or near the marks represent the average daily number of shocks
from October 29th to November 20th. It will be seen that these curves
are hyperbolic in form, the change from very rapid to very gradual
decline in frequency taking place from five to ten days after the
great earthquake. Fig. 52 illustrates the distribution in time of the
after-shocks at Gifu to the end of 1893, the ordinates in these cases
representing the number of shocks during successive months.[57]
A similar rapid and then gradual decline in frequency characterises
the strong and weak shocks recorded at Gifu. Of the ten violent
shocks, only one occurred after the beginning of January 1892; and of
the 97 strong shocks, only three after April 1892. But at the
commencement of the series, feeble shocks (_i.e._, shocks that could
just be felt) and earth-sounds without any accompanying movement were
comparatively rare, and did not become really prominent until two
months had elapsed. Of the 308 after-shocks recorded in 1893, none
could be described as strong, only 10 were weak, while 263 were feeble
shocks and 35 merely earth-sounds.
The last two diagrams show at a glance that the decline in frequency
of after-shocks is very far from being uniform. Some of the
fluctuations are due to the occurrence of exceptionally strong shocks,
each of which is followed by its own minor train of after-shocks.[58]
Others seem to be periodic, and possibly owe their origin to external
causes unconnected with the earthquake.[59]
_Method of representing the Distribution of After-shocks in
Space._--The maps in Figs. 54-57 show the distribution of the
after-shocks in space during four successive intervals of two months
each. They are founded on Professor Milne's great catalogue of
Japanese earthquakes, which give, among other data, the time of
occurrence and the position of the epicentre for every shock until the
end of 1892. For the latter purpose, the whole country is divided by
north-south and east-west lines into numbered rectangles, each
one-sixth of a degree in length and breadth; and the position of an
epicentre is denoted by the number of the rectangle in which it
occurs. The area included within the maps is bounded by the parallels
34° 40' and 36° 20' lat. N., and by the meridians 2° 10' and 3° 50'
long. W. of Tokio, so that ten rectangles adjoin each side of the map.
The number of epicentres lying within each rectangle having been
counted, curves are then drawn through the centres of all rectangles
containing the same number of epicentres, or through points which
divide the line joining the centres of two rectangles in the proper
proportion. Taking, for example, the curve marked 5, if the numbers in
two consecutive rectangles are 3 and 7, the curve bisects the line
joining their centres; if the numbers are 1 and 6, the line joining
their centres is divided into five equal parts, and the curve passes
through the first point of division reckoned from the centre of the
rectangle in which six epicentres are found. Thus the meaning of the
curve marked, say, 5 may be stated as follows:--If any point in the
curve be imagined as the centre of a rectangle whose sides are
directed north-south and east-west, and are respectively one-sixth of
a degree of latitude and longitude in length; then the number of
epicentres within this rectangle is at the rate of 5 for the time
considered.
_Preparation for the Great Earthquake._--At first sight, there appears
to have been but little direct preparation for the great earthquake.
Except for a rather strong shock on October 25th, at 9.14 P.M., it
occurred without the warning of any preliminary tremors. But a closer
examination of the evidence shows, as we should indeed expect, that
there was a distinct increase in activity for many months beforehand.
The region had become "seismically sensitive." Of the hundred
rectangles included in the maps in Figs. 53-57, there are thirteen
lying along the meizoseismal area of the earthquake of 1891, in which
nearly all the after-shocks originated. During the five years 1885-89,
53 out of 125 earthquakes (or 42 per cent.) had their epicentres lying
within the thirteen rectangles; or, in other words, the average
frequency in one of the rectangles of the meizoseismal area was five
times as great as in one of those outside it. In 1890 and 1891 (until
October 27th), the percentage in the thirteen rectangles rose to 61,
and the average frequency in one of them to ten times that of one of
the exterior rectangles.
The curves in Fig. 53 illustrate the distribution of epicentres during
the latter interval. It will be seen that they follow roughly the
course of the meizoseismal area southwards to the Sea of Isé, and that
to the south-east they continue for several miles the short branch of
the meizoseismal area which surrounds the southern end of the
fault-scarp.
[Illustration: FIG. 53.--Distribution of preliminary Shocks in
Space. (_Davison._)]
Thus, the preparation for the great earthquake is shown, first, by the
increased frequency of earthquakes originating within its meizoseismal
area; and, secondly, by the uniformity in the distribution of
epicentres throughout the same region, the marked concentration of
effort which characterises the after-shocks being hardly perceptible
during the years 1890-91.
[Illustration: FIG. 54.--Distribution of After-shocks in Space
(November-December 1891). (_Davison_).]
_Distribution of After-shocks in Space._--We have seen that the
after-shocks were subject to a fluctuating decline in frequency, rapid
at first, and more gradual afterwards. It is evident, from Figs.
54-57, that a similar law governs the area within which the
after-shocks originated. During the first two months, epicentres occur
over nearly the whole of the meizoseismal area, but afterwards they
are confined to a smaller district, which slowly, though not
continually, decreases in size.
[Illustration: FIG. 55.--Distribution of After-shocks in Space
(January-February, 1892). (_Davison._)]
[Illustration: FIG. 56.--Distribution of After-shocks in Space
(March-April). (_Davison._)]
The most important feature in the distribution of the epicentres is
the central region of extraordinary activity; but there are also
districts of minor and more short-lived activity near the three
extremities of the meizoseismal band. The seat of chief seismic action
shifts slightly from one part to another of the epicentral region,
especially about the end of 1891, as will be seen by comparing the
innermost curves of Figs. 54 and 55. Thus, with the decline in
frequency of the after-shocks and the decrease in their sphere of
action, there took place concurrently a gradual but oscillating
withdrawal of that action to a more or less central region of the
fault.
[Illustration: FIG. 57.--Distribution of After-shocks in Space
(May-June, 1892). (_Davison._)]
_Sound Phenomena of After-shocks._--While comparatively few observers
seem to have noticed any noise with the principal earthquake, many of
the after-shocks were accompanied by sounds. Professor Omori describes
them as belonging to two types. They were either rushing feeble noises
like that of wind, or loud rumbling noises like those of thunder, the
discharge of a gun, or the fall of a heavy body. In the Neo valley,
sounds of the second type were most frequent and distinct, but they
either occurred without any shock at all, or the attendant tremor was
very feeble; while, on the other hand, severe sharp shocks were
generally unaccompanied by distinctly audible sounds.
It is remarkable, also, that sounds were less frequently heard with
the early than with the later after-shocks. In November 1891, the
percentage of audible shocks was 17, and from December to the
following April always lay between 10 and 12. In May the percentage
suddenly rose to 39, and until the end of 1892 was always greater than
32, while in November 1892, it rose as high as 49. This, of course,
agrees with Professor Omori's observation that sounds attended feeble
shocks more often than strong ones.
The distribution of the audible after-shocks in space is shown in Fig.
58. These curves are drawn in the same way as those in Figs. 53-57,
but they represent the percentages, not the actual numbers, of shocks
accompanied by sound. It will be noticed that all three groups of
curves lie along the meizoseismal area, or the continuation of the
south-east branch; while the axis of the principal group of curves
lies to the west of the central regions in which most after-shocks
originated.
[Illustration: FIG. 58.--Distribution of Audible After-shocks in
Space (November 1891-December 1892). (_Davison_.)]
The explanation of these peculiarities is no doubt connected with the
comparative inability of the Japanese people to perceive the deep
sounds which in Europe are always heard with earthquake shocks. The
sounds are rarely heard by them more than a few miles from the
epicentre.[60] We may therefore conclude that slight after-shocks
originated nearer the surface than strong ones, that the mean depth
of the foci decreased with the lapse of time, and that the axes of the
systems of curves in Fig. 58 mark out approximately the lines of the
growing faults. The separation of the two westerly groups of curves
appears to show that the main branch of the meizoseismal area is
connected with a fault roughly parallel to that traced by Professor
Koto, but of which no scarp (if it existed) could be readily
distinguished among the superficial fissures produced by the great
shock.
EFFECT OF THE EARTHQUAKE ON THE SEISMIC ACTIVITY OF THE ADJOINING
DISTRICTS.
So great and sudden a displacement as occurred along the fault-scarp
could hardly take place without affecting the stability of adjoining
regions of the earth's crust, and we should naturally expect to find a
distinct change in their seismic activity shortly after October 28th.
In Fig. 59 two such regions are shown, bounded by the straight dotted
lines. The district in which the principal earthquake and its
after-shocks originated is enclosed within the undulating dotted
lines. The continuous lines inside all three districts are the curves
corresponding to 10 and 5 epicentres for the years 1885-92. Not far
from the axes of the outer groups of curves there are probably
transverse faults, approximately parallel to the great fault-scarp and
the main branch of the meizoseismal band, and distant from them about
45 and 55 miles respectively.
[Illustration: FIG. 59.--Map of Adjoining Regions in which
Seismic Activity was affected by the Great Earthquake.
(_Davison._)]
In the district represented in the north-east corner of Fig. 59, 29
earthquakes originated between January 1st, 1885, and October 27th,
1891, and 30 between October 28th, 1891, and December 31st, 1892, 7 of
the latter number occurring in November 1891. In the south-west
district, the corresponding figures before and after the earthquake
are 20 and 36, 8 of the latter occurring in November 1891. Thus, in
the north-east district, for every shock in the interval before the
earthquake there were six in an equal time afterwards, and at the rate
of 10 during November 1891; and in the south-west district, for every
shock before the earthquake there were 10 afterwards, and at the rate
of 16 during November 1891.
Now, it is unlikely that the gradual increase of stress should be so
nearly proportioned everywhere to the prevailing conditions of
resistance as to give rise to a marked and practically simultaneous
change in seismic activity over a large area; whereas the paroxysmal
occurrence of a strong earthquake might alter the surrounding
conditions with comparative rapidity, and so induce a state of seismic
excitement in the neighbourhood. It therefore seems very probable
that the increased activity in the two districts here described was a
direct consequence of the occurrence of the great earthquake.
ORIGIN OF THE EARTHQUAKE.
The preponderance of preliminary earthquakes within the meizoseismal
area and the outlining of the fault-system by the frequency curves of
1890-91 (Fig. 53) point to the previous existence of the originating
fault or faults, and to the earthquake being due, not to the formation
of a new fracture, as has been suggested, but to the growth of an old
fault.
The last severe earthquake in the Mino-Owari plain occurred in 1859,
so that for more than thirty years there had been but little relief to
the gradually increasing stresses. Now, the distribution of stress
must have been far from uniform throughout the fault-system, and also
the resistance to displacement far from proportional to the stresses
at different places. At certain points, therefore, the effective
stress would be greater than elsewhere, and it would be at these
points that fault-slips would first occur. Such slips tend to remove
the inequalities in effective stress. Thus, the function of the slight
shocks of 1890 and 1891 was, briefly, to equalise the effective stress
over the whole fault-system, and so to clear the way for one or more
great slips throughout its entire length.
As to which side of the fault moved during the great displacement, or
whether both sides moved at once, we have no direct evidence but as
regards the neighbourhood of Midori, and there the conditions were
exceptional. Professor Koto thinks that it was probably the rock on
the north-east side that was generally depressed and always shifted to
the north-west But the disturbance in reality seems to have been more
complicated. That this was the case, that displacement occurred along
more than one fault, is probable from the branching of the
meizoseismal area, the isolation of the audibility curves of the
after-shocks (Fig. 58), and the sudden increase in seismic activity
both to the north-east and south-west of the epicentre. The detached
portion of the meizoseismal area near Lake Biwa may also point to a
separate focus. The whole region, indeed, was evidently subjected to
intense stresses, and the depression on the north-east side of the
fault-scarp can hardly fail to have been accompanied by other
movements, especially along a fault running near the western margin of
the main branch of the meizoseismal area.
The later stages of the movements are somewhat clearer. From a study
of the after-shocks, we learn that the disturbed masses began at once
to settle back towards the position of equilibrium. At first the slips
were numerous and took place over the whole fault-system, but chiefly
at a considerable depth, where no doubt the initial displacement was
greatest. After a few months, stability was nearly restored along the
extremities of the faults; slips were confined almost entirely to the
central regions, while a much larger proportion of them took place
within the superficial portions of the faults.
The official records bring down the history to the end of 1893. Since
that time more than one strong shock has been felt in the Mino-Owari
plain; but the stage of recovery from the disturbances of 1891 is
probably near its end, and we seem rather to be entering on a period
in which the forces are once more silently gathering that sooner or
later will result in another great catastrophe.
REFERENCES.
1. CONDER, J.--"An Architect's Notes on the Great Earthquake of
October 1891." _Japan Seismol. Journ._, vol. ii., 1893, pp.
1-91.
2. DAVISON, C.--"On the Distribution in Space of the Accessory
Shocks of the Great Japanese Earthquake of 1891." _Quart.
Journ. Geol. Soc._, vol. liii., 1897, pp. 1-15.
3. ---- "On the Effect of the Great Japanese Earthquake of 1891 on
the Seismic Activity of the Adjoining Districts." _Geol.
Mag._, vol. iv., 1897, pp. 23-27.
4. ---- "On the Diurnal Periodicity of Earthquakes." _Phil. Mag._,
vol. xiii., 1896, pp. 463-476, especially pp. 466-468.
5. ---- "On Earthquake-Sounds." _Phil. Mag._, vol. xlix., 1900, pp.
31-70--especially pp. 49-53, 60-61.
6. KOTO, B.--"The Cause of the Great Earthquake in Central Japan,
1891." _Journ. Coll. Sci. Imp. Univ. Japan_, vol. v., 1893,
pp. 295-353.
7. MASATO, H.--"Report on Earthquake Observations in Japan." _Cent.
Meteor. Obs. of Japan_ (Tokio, 1892), pp. 16-18, 41, and map
30.
8. MILNE, J.--"A Note on the Great Earthquake of October 28th,
1891." _Japan Seismol. Journ._, vol. i., 1893, pp. 127-151;
_Brit. Assoc. Rep._, 1892, pp. 114-128.
9. ---- "A Catalogue of 8,331 Earthquakes recorded in Japan between
1885 and 1892." _Japan Seismol. Journ._, vol. iv., 1895, pp.
1-367--especially pp. 134-234, 303-353.
10. ---- "On Certain Disturbances in the Records of Magnetometers
and the Occurrence of Earthquakes." _Brit. Assoc. Rep._, 1898,
pp. 226-251--especially pp. 227, 232, 234, 241, and 245.
11. MILNE, J., and W.K. BURTON.--"The Great Earthquake in Japan."
_Journ. Coll. Sci. Imp. Univ. Japan_, vol. v., 1893, pp.
295-352.
12. OMORI, F.--"On the After-shocks of Earthquakes." _Journ. Coll.
Sci. Imp. Univ. Japan_, vol. vii., 1894, pp. 111-200; abstract
in _Japan Seismol. Journ._, vol. iii., 1894, pp. 71-80.
13. ---- "A Note on the Great Mino-Owari Earthquake of October 28th,
1891." _Pub. Earthquakes Inves. Com. in Foreign Languages_,
No. 4, Tokio, 1900, pp. 13-24.
14. ---- "Sulla velocità di propagazione e sulla lunghezza delle
onde sismiche." _Ital. Soc. Sismol. Boll._, vol. i., 1895, pp.
52-60--especially pp. 52-57.
15. ---- "Sull' intensità e sull' ampiezza del movimento nel gran
terremoto giapponese del 28 ottobre 1891." _Ital. Soc. Sismol.
Boll._, vol. ii., 1896, pp. 189-200.
16. ---- "Note on the After-shocks of the Mino-Owari Earthquake of
Oct. 28th, 1891." _Pub. Earthquakes Inves. Com. in Foreign
Languages_, No. 7, Tokio, 1902, pp. 27-32.
17. ---- "Note on the relation between Earthquake Frequency and the
Atmospheric Pressure." _Tokyo Phys.-Math. Soc. Reports_, vol.
ii., 1904, No. 8.
18. TANAKADATE, A., and H. NAGAOKA. "The Disturbance of Isomagnetics
attending the Mino-Owari Earthquake of 1891." _Journ. Coll.
Sci. Imp. Univ. Japan_, vol. v., 1893, pp. 149-192.
FOOTNOTES:
[54] I have not referred to the results of this survey, for, though
changes in all the magnetic elements (especially in horizontal
intensity) have taken place between 1887 and 1891-92, these changes
cannot be ascribed with confidence to the earthquake in the absence of
a thorough knowledge of the secular variation.
[55] From the formula a=x*g/y, where _a_ is the maximum horizontal
acceleration, _g_ the acceleration due to gravity, _y_ the height of
the centre of gravity, and _x_ its horizontal distance from the edge
about which the body was overturned.
[56] These estimates are made, on the supposition of simple harmonic
motion, from the formula 2*a=alpha*t^2/(2*pi^2), where 2_a_ is the
total range or double amplitude, _a_ the maximum acceleration, and _t_
the period of the vibration.
[57] Professor Omori finds that the mean daily number of earthquakes
_y_ during the month _x_ (reckoned from November 1891) may be
approximately represented by the equation--
y = 16.9 / (x + 0.397);
or, taking the semi-daily earthquake numbers during the five days
between October 29th and November 2nd, 1891, by the equation--
y = 440.7 / (x + 2.314),
where _y_ denotes the number of earthquakes observed during the twelve
hours denoted by _x_, the time being measured from the first half of
October 29th. It is interesting to notice that, taking account of the
mean annual frequency of earthquakes in ordinary years, the number of
shocks observed at Gifu during the two years 1898-99 should, according
to the latter formula, be 163; the actual number recorded was 160.
[58] The last violent shock before the end of 1893 occurred on
September 7th, 1892, and its effects on the frequency of after-shocks
is shown by the daily numbers recorded at Gifu during the first
fortnight in September. These are--2, 2, 2, 3, 5, 5, 28 (on September
7th), 8, 8, 5, 4, 3, 2, 4, 3.
[59] The periodicity of after-shocks is discussed in the papers
numbered 4, 12, 16, and 17 at the end of this chapter. In these, the
existence of diurnal and other periods is clearly established.
Professor Omori also shows that the mean daily barometric pressure is
subject to fluctuations with maxima occurring on an average every
5-1/2 days, and that earthquakes are least frequent on the days of the
barometric maxima and minima, and more frequent in the days
immediately preceding and following them.
[60] Of the Japanese earthquakes of 1885-92 originating beneath the
land, twenty-six per cent. were accompanied by a recorded sound; but
less than one per cent. of those originating beneath the sea and not
more than ten miles from the coast.
CHAPTER VIII.
THE HEREFORD EARTHQUAKE OF DECEMBER 17TH, 1896, AND THE INVERNESS
EARTHQUAKE OF SEPTEMBER 18TH, 1901.
Among the earthquakes described in this volume, the Hereford and
Inverness earthquakes hold but a minor place. The damage to buildings,
though unusual for this country, was slight when compared with that
caused by the preceding shocks; there was no loss of life, not a
single person was injured by falling masonry. The interest of the
earthquakes lies entirely in the detailed study rendered possible by
numerous observations of the shock and sound,[61] and in the bearing
of this evidence on the general theory of the origin of earthquakes.
THE HEREFORD EARTHQUAKE OF DECEMBER 17TH, 1896.
The principal earthquake of this series occurred at 5.32 A.M. on
December 17th, and was preceded by at least nine minor shocks (the
first of which was felt at about 11 or 11.30 P.M. on December 16th),
and followed by two others on the same day, and by a third and last on
July 19th, 1897. The accounts of these preliminary movements will be
found on a later page, as their bearing will be more fully apparent
after the discussion of the principal shock.
[Illustration: FIG. 60.--Isoseismal and Isacoustic lines of
Hereford earthquake. (_Davison._)]
ISOSEISMAL LINES AND DISTURBED AREA.
On the map in Fig. 60, the continuous curves represent isoseismal
lines corresponding to the degrees 8, 7, 6, 5, and 4 of the
Rossi-Forel scale. The isoseismal 8, which is the most accurately
drawn of the series, is an elongated oval, 40 miles long, 23 miles
broad, and containing an area of 724 square miles. The longer axis is
directed W. 44° N. and E. 44° S. Within this curve, there are 73
places where buildings are known to have been damaged, 55 places being
in Herefordshire, 17 in Gloucestershire, and one in Worcestershire.
The most important damage occurred in the city of Hereford, which, in
1901, contained 4,565 inhabited houses. Here, no fewer than 218
chimneys had to be repaired or rebuilt. The Cathedral was slightly
injured. The finial of a pinnacle of the Lady Chapel was thrown down,
a fragment of a stone fell from one of the arches in the south
transept, and the three pinnacles of the western front were fractured.
Several churches suffered to a similar extent, while, at the Midland
Railway Station, all the seven chimney-stacks were shattered. At
Dinedor, Fownhope, Dormington, Withington, and a few other villages,
the damage was also relatively greater than elsewhere, these places
all lying within a small oval about 8-1/2 miles long, which surrounds,
not the centre, but rather the north-west focus, of the isoseismal 8.
The isoseismal 7, which includes places where the shock was strong
enough to overthrow ornaments, vases, etc., is also very nearly an
ellipse, whose axes are 80 and 56 miles in length, and whose area is
3,580 square miles. Its longer axis, running from W. 42° N. to E. 42°
S., is practically parallel to that of the inner curve. Next in
succession comes the isoseismal 6, surrounding those places where the
shock was strong enough to make chandeliers, pictures, etc., swing;
but, as most of the observers seem to have slept in darkened rooms,
the number of determining points for this curve is less than usual,
and its course is therefore laid down with a somewhat inferior degree
of accuracy. The error, however, is probably small, and we may
therefore regard the isoseismal 6 as another ellipse, 141 miles long,
116 miles broad, and containing an area of 13,000 square miles. Its
longer axis is again nearly parallel to those of the preceding
isoseismals.
The next two isoseismals are nearly circular in form. It will be
noticed that large portions of them, and especially of the isoseismal
4, traverse the sea. In these parts, the paths of the curves are to
some extent conjectural. In drawing them, the chief guides are their
trend before leaving the land and the known intensity along the
neighbouring coastlines. The isoseismal 5 bounds the area within which
the shock was perceptible as a sensible displacement and not merely a
quiver. Its dimensions are 233 miles from north-west to south-east,
and 229 miles from south-west to north-east, and its area 41,160
square miles. The isoseismal 4, which includes places where the shock
was strong enough to make doors, windows, etc., rattle, is 356 miles
from north-west to south-east, and 357 miles from south-west to
north-east, and 98,000 square miles in area; its centre coincides
nearly with that of the small oval area in the neighbourhood of
Hereford, where the damage to buildings was relatively greater than
elsewhere.
Outside the isoseismal 4, the earthquake was observed at several
places. The shock was certainly felt at Middlesbrough, 12-1/2 miles
from the curve, and probably at Killeshandra (in Ireland), 65 miles
distant. Thus, if we consider the boundary of the disturbed area to
coincide with the isoseismal 4, its area would be 98,000 square
miles, or 1-2/3 that of England and Wales; if it were a circle
concentric with the isoseismal 4, and passing through Middlesbrough,
its area would be 115,000 square miles, or nearly twice that of
England and Wales; while, if it passed through Killeshandra, its area
would be 185,000 square miles, or more than three times the area of
England and Wales.[62]
_Position of the Originating Fault._--The form, directions, and
relative positions of the isoseismal lines furnish important evidence
with regard to the originating fault. We conclude in the first place
that its mean direction is parallel to the longer axes of the three
innermost isoseismal lines--that is, north-west and south-east, or,
more accurately, W. 43° N. and E. 43° S.[63] In this case, the
elongated forms of the isoseismal lines cannot be attributed to
variations in the nature of the surface rocks. The district embraced
contains about 13,000 square miles, and it is improbable that the axes
of the three isoseismals should retain their parallelism over so large
an area, if these variations had any considerable effect. Moreover, in
the same district, an earthquake occurred in 1863, whose meizoseismal
area was elongated from north-east to south-west, or almost exactly
perpendicular to the direction in 1896.
Secondly, it will be noticed (Fig. 60) that the isoseismal lines are
not equidistant from one another. On the north-east side, they are
separated by distances of 20, 34, 55, and 51 miles; and on the
south-west side by distances of 13-1/4, 25, 60, and 77 miles. It
follows from this that the fault-surface must hade or slope towards
the north-east; for, near the epicentre, the intensity is greatest and
dies out more slowly on the side towards which the fault hades.
If we could ascertain any one place through which the fault passed,
its position would thus be completely determined. Unfortunately, there
is no decisive evidence on this point. There are, however, several
places to the south-west of Hereford where the intensity of the shock
was distinctly less than in the surrounding district, and it is
possible that this was due to their neighbourhood to the fault-line
(see p. 135). If so, the originating fault must have extended from a
point about a mile and a half west of Hereford for a distance of about
16 miles to the south-east; and a fault in this position would
certainly satisfy all the details of the seismic evidence.
NATURE OF THE SHOCK.
Throughout the disturbed area, considerable variations were observed
in the nature of the shock. These changes were due to the mere size of
the focus, to its elongated form and, as will be seen, to its
discontinuity, and also to the distance of the place of observation
from the epicentre.
At places near the epicentre, rapid changes in the direction of the
shock were observed owing to the large angle subtended by the focus;
while, at considerable distances, this angle being small, the changes
of direction were imperceptible. A further variation with the distance
was an increase in the period of the vibrations. Close to the
epicentre, the general impression was that of crossing the wake of a
steamer in a very short rowing-boat, or of riding in a carriage
without springs. At distances of a hundred miles or more, the movement
is described as being of a pleasant, gentle, undulating character,
like that felt during the rocking of a ship at anchor or in a carriage
with well-appointed springs.
The most remarkable feature of the shock, however, was its division
into two distinct parts or series of vibrations, separated by an
interval, lasting two or three seconds, of absolute rest and quiet.
And this was no mere local phenomenon. With the exception of a narrow
band that will be referred to presently, records of the double shock
come from nearly all parts of the disturbed area, even from districts
so remote as the Isle of Man and the east of Ireland. The two parts
differed in intensity, in duration, and in the period of their
constituent vibrations. For instance, at Oaklands (near Chard), a
shivering motion was first felt, and then, after about three or four
seconds, a distinct rocking from side to side. At Exeter, there was a
sudden tremor lasting about two seconds, followed, after two or three
seconds, by another and more severe shaking lasting four or five
seconds. Again, at West Cross (near Swansea), an undulatory movement
for about four seconds was followed soon after by a tremulous shock.
At Liverpool, the durations of the first part, interval, and second
part were respectively estimated at about six, two, and four seconds.
As a first result of the observations, then, it appears that in the
south-east half of the disturbed area, the second part of the shock
was the stronger, of greater duration and consisted of longer-period
vibrations (as at _a_, Fig. 61); while, in the north-west half, the
same features characterised the first part of the shock (_b_, Fig.
61). A closer examination of the records shows, however, that the
boundary between the two portions of the disturbed area was not a
straight line, but slightly curved, the concavity facing the
south-east. The broken line on the map (Fig. 60), which is hyperbolic
in form, represents roughly the position of this curved boundary.[64]
[Illustration: FIG. 61.--Nature of shock of Hereford earthquake.]
Along this hyperbolic boundary-line, or rather within a narrow band of
which it is the central line, the shock lost its double character, and
was manifested as a single series of vibrations gradually increasing
in intensity and then dying away. Close to the edges of this band,
careful observers were able to distinguish two maxima of intensity
connected by a continuous series of tremors (_c_, Fig. 61). Thus,
within the band, the two series of vibrations, which elsewhere were
isolated, must have been superposed on one another; while, near the
edges of the band, the concluding tremors of the first series
overlapped the initial tremors of the second.
_Origin of the Double Series of Vibrations._--The Hereford earthquake
thus belongs to the same class as the Neapolitan, Andalusian,
Charleston, and Riviera earthquakes. As in these cases, the hypothesis
of a single focus is inadmissible. The division of the disturbed area
into two regions of opposite relative intensity, duration, etc., is
sufficient proof that a single series of vibrations was not duplicated
by reflection or refraction, or by separation into longitudinal and
transverse waves. It is equally conclusive against a repetition of the
impulse within the same focus. We must therefore infer that the focus
consisted of two nearly or quite detached portions arranged along a
north-west and south-east line, and that the impulse at the north-west
focus was the stronger of the two. The only question that remains to
be decided is whether the impulses at the two foci were simultaneous
or not.
Now, if the impulses occurred at the same instant, the waves from the
two foci would travel with the same velocity, and would therefore
coalesce along a straight band which would bisect at right angles the
line joining the two epicentres. But we have already seen that this
band is curved, and it thus follows that the two impulses were not
simultaneous. Again, since the concavity of the hyperbolic band faces
the south-east, the waves from the north-west focus must have
travelled farther than those from the south-east focus before the two
met along the hyperbolic band; in other words, the impulse at the
north-west focus must have occurred two or three seconds before the
impulse at the other.
_Position and Dimensions of the Two Foci._--There can be little doubt
that the impulse at the north-west focus was responsible for the
greater damage to buildings at Hereford, Dinedor, Fownhope, etc. The
centre of its epicentral area must therefore lie about three miles
south-east of Hereford. It is probable, also, that the corresponding
centre of the other focus is similarly placed with respect to the
south-east portion of the isoseismal 8--that is, about two or three
miles north-east of Ross. These two points are eight or nine miles
apart. Now, since, as we shall see, the mean surface-velocity of the
earth-waves was about 3000 feet per second, and the mean duration of
the quiet interval between the two series was 3-1/2 seconds, the
nearest ends of the two foci must have been separated by a distance of
not less than two miles. Moreover, since the series of vibrations from
the north-west or Hereford focus lasted a few seconds longer than that
from the south-east or Ross focus, the former must have been about two
miles longer than the latter, and we may therefore estimate their
lengths at about eight and six miles respectively. Including the
undisturbed intermediate portion, this would give a total length of
focus of about 16 miles, a result we have already inferred from the
dimensions of the isoseismal 8.
DIRECTION OF THE SHOCK.
Although no question was asked with regard to the direction of the
shock, no fewer than 469 observers made notes on this point. As a
general rule, their determinations are extremely rough, few referring
to more than the eight principal points of the compass. Moreover, in
any one place, the directions assigned to the shock are very varied.
For instance, in the city and suburbs of Birmingham, eight observers
give the direction along a north and south line, eight east and west,
eleven north-west and south-east, and five north-east and south-west,
while there are five other intermediate estimates. But, when these
directions are plotted on a map of the district, it is seen at once
that they are either nearly parallel or perpendicular to the roads in
which the observers were living; that is, the apparent direction of
the shock was at right angles to one of the principal walls of the
house. This, of course, is a result to be anticipated, for, whatever
be the direction of the earthquake-motion, a house tends to oscillate
in a plane perpendicular to one or other of its walls.
It is extraordinary to how great a distance the direction of the shock
is perceptible. Records come from Brighton (137 miles from the
epicentre), Maldon in Essex (144 miles), Harrogate (147 miles),
Douglas in the Isle of Man (167 miles), Dublin (176 miles), and
Baltinglass in Co. Wicklow (180 miles).
Nevertheless, whatever the distance may be, the sense of direction
must be most perceptible in those houses whose principal walls are at
right angles to the true direction of the earthquake-motion, and we
should therefore expect to find the observations of direction most
frequently made in such houses, or in others which approximate to this
situation. Thus, the average of all the observations within a fairly
small area should give a result not very far from the true direction
of the shock; and, the smaller the area and the farther from the
epicentre, the more reliable should be the result. Now, in Birmingham
the mean direction of the shock is E. 39° N., which differs only by 2°
from the line joining the city to the epicentre; in London it is E.
21° S., the difference being again 2°. In other cases, the
observations from different counties are grouped together, and the
mean direction is taken to correspond to the centre of the county.
Yet, even then, there is often a close agreement between the mean
direction of the shock and the direction of the county-centre from the
epicentre; the difference being not more than two or three degrees in
the counties of Buckingham, Devon, Stafford, Warwick, and York. In
other cases, where the deviation exceeds this amount, either the
number of observations is small or the county is near the epicentre
and so subtends a large angle.
Two results of some importance follow from this analysis: (1) that
while, with a few isolated observations, the "method of directions" is
almost sure to fail, with a large number of observations closely
grouped, the position of the epicentre may be determined with a fair
approach to accuracy; and (2) that, at any rate outside a radius of
forty miles, the earth-waves travelled in approximately straight lines
outwards from the epicentre.
COSEISMAL LINES AND VELOCITY OF EARTH-WAVES.
Coseismal lines were defined by Mallet as long ago as 1849, but, owing
to the difficulty of ascertaining the correct time, they have so far
been of little service in the investigation of earthquakes. In the
case of the Hereford earthquake, the distances traversed by the
earth-waves are small; but, on the other hand, the time-records are
numerous and frequently trustworthy to the nearest minute. Rejecting
all estimates earlier than 5.32 A.M., and later than 5.36, as well as
a number at 5.35, there remain fairly good observations from 381
places, and exceptionally accurate ones from 33 places. The latter
were obtained from signalmen and other careful observers who were in
possession of Greenwich time, or who compared their watches shortly
afterwards with well-regulated watches.
With evidence so abundant, a new method of drawing coseismal lines
becomes possible. According to this method, each place of observation
is indicated on the map by a mark corresponding to the particular
minute recorded. If the records were quite correct, there would be a
central area occupied by the marks corresponding to 5.32 A.M.,
surrounded by a series of zones in which the times were respectively
5.33, 5.34, and 5.35. The curves separating these zones would be
coseismal lines corresponding to the times 5.32-1/2, 5.33-1/2, and
5.34-1/2.
Owing, however, to the inevitable inaccuracy of all the time-records,
these different zones intrude on one another, and the coseismal lines
have therefore to be drawn about half-way through the overlapping
regions, special weight being attributed to the apparently more
accurate observations.
[Illustration: FIG. 62.--Coseismal lines of the Hereford
earthquake. (_Davison._)]
The coseismal lines obtained in this manner are represented by the
continuous curves in Fig. 62. The isoseismals, which are added for the
sake of comparison, are indicated by the dotted lines. It will be seen
that the coseismal lines are elongated in the same direction as the
isoseismals, but to a less extent, and this no doubt is due to the
fact that the epoch selected by the majority of observers was one not
far from, and slightly preceding, that of the maximum intensity of
the shock.
Now, the average distance between the two inner coseismals is 32-3/4
miles, between the two outer ones (so far as drawn) 35-1/6 miles, and
between the first and third 67-1/6 miles. The mean surface-velocity
between the two inner coseismals is therefore 2,882 feet per second,
and between the two outer ones 3,095 feet per second. There is thus an
apparent increase in the velocity with the distance, but the accuracy
of the coseismal lines is unequal to establishing this as a fact. The
mean surface-velocity of 2,955 feet per second between the first and
third coseismals is probably, however, the most accurate estimate of
the surface-velocity yet made in a slight earthquake.
SOUND-PHENOMENA.
_Nature of the Sound._--The sound which accompanied the shock was of
the same character as that heard during all great earthquakes. It is
often described in such terms as a deep booming noise, a dull heavy
rumble, a grating roaring noise, or a deep groan or moan; more rarely
as a rustling or a loud hissing rushing sound. As a rule, it began
faintly, increased gradually in strength, and then as gradually died
away; and this no doubt is the reason why it sometimes appeared as if
an underground train or waggon were approaching quickly, rushing
beneath the observer, and then receding in the opposite direction.
Occasionally, the sound was very loud, being compared to the noise of
many traction-engines heavily laden passing close at hand, or to a
heavy crash or peal of thunder. But its chief characteristic was its
extraordinary depth, as if it were almost too low to be heard.
According to one observer, it was a low rumbling sound, much lower
than the lowest thunder; and another compared it to the pedal notes of
a great organ, only of a deeper pitch than can be taken in by the
human ear, a noise more _felt_ than heard. It will be seen presently
how the sound, from its very depth, was inaudible to many persons.
A few observers described the sound in terms like those quoted above,
but by far the larger number compared it to some more or less
well-known type, and in many cases the resemblance was so close that
the observer at first attributed it to the object of comparison. The
descriptions, which present great varieties in detail, may be
classified as follows: (1) One or several traction-engines passing,
either alone or heavily laden, sometimes driven furiously past; a
steam-roller passing over frozen ground or at a quicker pace than
usual; heavy waggons driven over stone paving, on a hard or frosty
road, in a covered way or narrow street, or over hollow ground or a
bridge; express or heavy goods trains rushing through a tunnel or deep
cutting, crossing a wooden bridge or iron viaduct, or a heavy train
running on snow; the grating of a vessel over rocks, or the rolling of
a lawn by an extremely heavy roller; (2) a loud clap or heavy peal of
thunder, sometimes dull, muffled or subdued, but most often distant
thunder; (3) a moaning, roaring, or rough, strong wind; the rising of
the wind, a heavy wind pressing against the house; the howling of wind
in a chimney, a chimney or oil-factory on fire; (4) the tipping of a
load of coal, stones, or bricks, a wall or roof falling, or the crash
of a chimney through the roof; (5) the fall of a heavy weight or tree,
the banging of a door, only more muffled, and the blow of a wave on
the sea-shore; (6) the explosion of a boiler or cartridge of dynamite,
a distant colliery explosion, distant heavy rock-blasting and the boom
of a distant cannon; (7) sounds of a miscellaneous character, such as
the trampling of many men or animals, an immense covey of partridges
on the wing, the roar of a waterfall, the passage of a party of
skaters, and the rending and settling together of huge masses of rock.
The total number of comparisons made was 1,264. Of these, 45.4 per
cent. refer to passing waggons, etc., 15.0 per cent. to thunder, 15.5
to wind, 3.9 to loads of stones falling, 2.7 to the fall of a heavy
body, 7.2 to explosions, and 10.3 per cent. to miscellaneous sounds.
Generally, the sound adhered throughout to one of the types mentioned
above, and, if it varied at all, varied only in intensity. At some
places, however, the character of the sound was observed to change.
For instance, one person described it as like the rumbling of a train
going over a bridge, with a terrific crash, such as is heard in a
thunderstorm at the instant when the shock was strongest, the rumbling
dying away afterwards for some seconds.
_Inaudibility of the Sound to some Observers._--The total number of
observers who give a detailed account of the earthquake is 2,681, and,
of these, 59 per cent. state that they heard the sound, 23 per cent.
give no information, while 18 per cent. distinctly say that they
heard no sound; that is, roughly, out of every five observers, three
heard the sound, one made no reference to it, and one failed to hear
the sound.
In a few cases, no doubt, this failure was due to the distance of the
observer, but this is far from being a complete explanation; for, in
Herefordshire, six out of 179, and in Gloucestershire 17 out of 227,
observers heard no sound. Nor is the peculiarity a local one, for at
Clifton two out of five observers who were awake did not hear the
sound, at Birmingham four out of 23, and in London, eight out of 18.
Even in the same house, it would happen that one observer would hear a
sound as of a heavily-laden traction-engine passing, while to another
it was quite inaudible.
Again, a large number of observers who heard the sound expressly state
that they were unconscious of any while the shock lasted. The noise at
first resembled the approach of a steam-roller or traction-engine up
the street, it became gradually louder, and then ceased more or less
suddenly as the shock began; while, to others in the same places, the
sound continued to grow in loudness until the strongest vibrations
were felt.
Even when observers in the same place agreed in hearing the sound, it
presented itself to them under different aspects. Thus, at Hereford, a
crash or bomb-like explosion was noticed by some, but not by all,
observers; at Ledbury, the sound according to one began like a rushing
wind and culminated in a loud explosive report, another heard a noise
like distant thunder, which ended when the shock began, while a third
heard no sound at all. At places more distant from the epicentre, the
same diversity, both in character and intensity, is manifested. Thus,
at Birmingham, the accounts refer on the one hand to the distant
approach of a train and the rising of the wind, on the other to the
reports of large cannons and to a noise as if tons of _débris_ had
been hurled against the wall of the house; at Bangor, to muffled
thunder, wind through trees, and a loud rumbling sound.
The first explanation of these apparent anomalies which presents
itself is inattention on the part of the observers; but it is one that
will not bear examination, though it may apply in some cases. The
sound is too loud, at any rate near the epicentre, to escape notice,
and it is generally heard before the shock begins to be felt.
Moreover, as described in the last chapter, three out of every four
earthquakes in Japan are unaccompanied by recorded sound, and the
Japanese as a race cannot be accused of such constant inattention. The
defect, it can hardly be doubted, is inherent to the observer, and not
dependent on the conditions in which he is placed.
That the higher limit of audibility varies with different persons has
long been known; and there can be no reason for doubting that there is
a similar variability in the lower limit. Thus, to some observers, the
sound remains inaudible throughout, however intently they may be
listening. Again, it is found that, the deeper the sound, the greater
must be the strength of the vibrations required to render them
audible. As the vibrations which reach an observer increase in period,
it may therefore happen that, sooner or later, the strength of some
does not attain or exceed that limiting value, and, at that moment,
the sound will cease to be heard. Moreover, for vibrations of a given
period, this limiting value varies for different persons. Thus, to one
observer, the sound may become inaudible, while another may continue
to hear it. Lastly, the vibrations which affect an observer at any
moment are of various strength and period. One may hear all perhaps,
while a second may be able to hear some and not others. Thus, to one
observer, the sound may be like a rising wind, to another like a heavy
traction-engine passing; one may hear the crashes which accompanied
the strongest part of the shock, while a second may be deaf to the
same vibrations; to one the sound may become continually louder and
cease abruptly, to another it may increase to a maximum and then die
away.
_Sound-Area._--While the sound was a very prominent feature of the
earthquake in and near the epicentral area, records at a great
distance are naturally difficult to obtain, and, on this account, the
number of stations for determining the boundary of the sound-area is
too small to allow of it being accurately drawn. As a rule, however,
it must lie between the isoseismals 5 and 4, but it is less nearly
circular than either of these lines. Its length, from north-west to
south-east, is 320 miles, its breadth 284 miles, and the area
contained by it about 70,000 square miles, or roughly two-thirds that
of the disturbed area.
_Isacoustic Lines._--The dotted lines in Fig. 60 represent isacoustic
lines--that is, lines which pass through all places where the
percentage of observers who recorded their perception of the sound is
the same. For instance, if we take any point in the line marked 80 and
describe a small circle with that point as centre, then 80 per cent.
of the observers within that circle would hear the earthquake-sound.
The isacoustic lines thus show how the audibility of the sound varies
throughout the sound area. To draw the curves with a close approach to
accuracy, the unit of area should be small and of constant dimensions;
but, in the present case, owing to the comparative paucity of the
observations, a smaller unit than the county would give unreliable
results.[65] At the centre of each county, the sound audibility may be
regarded as proportional to the percentage of the total number of
observers within the county who distinctly heard the sound. To draw
the curve marked 50, the centre of every county in which the average
percentage is less than 50 is joined to the centres of those adjoining
counties in which it is above 50, and these lines are then divided in
the proper ratio so as to give a point where the percentage would be
exactly 50. A number of points at which the percentage is 50 is thus
obtained, and the curve drawn through them is the required isacoustic
line. The percentage of audibility varies from 87 in Herefordshire to
23 in Essex and the east of Ireland, but the only isacoustic lines
which can be completely drawn are those that correspond to the
percentages between 80 and 50 inclusive.
The peculiar form of the isacoustic lines will be evident at a glance.
They bear little relation to the isoseismal lines. Their greatest
extensions are not along the axes of those lines, but in two
directions which are a little east of north-east and south of
south-west. They lie indeed along a hyberbolic line which, towards
the south-west, agrees closely with the curvilinear axis of the
hyperbolic band represented by the broken line in Fig. 60. Towards the
north-east, the coincidence is not so close, but this is chiefly owing
to the magnitude of the northern counties, which causes a deflection
of the isacoustic lines towards the north.
It will be remembered that the hyperbolic band is the area within
which the vibrations from the two foci were superposed. Now, the sound
accompanied each part of the shock, and ceased entirely during the
interval between them. Also, the stronger series of vibrations was
accompanied by the louder sound; but, while the difference in strength
was considerable between the two parts of the shock, it was very
slight between the two sounds. There is therefore no marked distortion
of the isoseismal lines when crossing the hyperbolic band, while the
isacoustic lines are completely diverted from their normal course.
Thus, the study of the isacoustic lines strongly confirms the
conclusions at which we have arrived above (p. 223)--namely, that
there were two distinct foci arranged in a north-west and south-east
line, and that the impulse at the former focus occurred a few seconds
earlier than that at the latter.[66]
_Variations in the Nature of the Sound throughout the Sound-area._--In
one respect, the sound exhibited a marked uniformity all over the
sound-area--namely, in its great depth; the word "heavy" being used in
one out of every four accounts of the sound, whether close to the
epicentre or near the boundary of the sound-area.
The type of comparison employed varies in different parts of the
sound-area. As we recede from the origin, the sound becomes on the
average less like thunder or explosions and more like wind. The
references to passing waggons, etc., are so numerous that it is
possible to draw curves, in the same way as isacoustic lines, which
represent equal percentages of comparison to this type out of the
total number of comparisons. The curves are somewhat incomplete, but
it is noteworthy that those corresponding to the higher percentages
cling to the extremities of the hyperbolic band, probably because the
uninterrupted duration of the sound is greater there than elsewhere.
The effect of distance from the epicentre, however, is most noticeable
in connection with changes in the character of the sound. It is only
on the immediate neighbourhood of the origin that the explosive
reports or crashes were heard in the midst of the rumbling sound. At a
moderate distance, the sound before and after the shock became
smoother, while the sound which accompanied the shock retained to a
certain extent its rougher and more rumbling or grating character.
Close to the boundary of the sound-area, the irregularities were still
further smoothed away, and the only sound heard was like the low roll
of distant thunder.
The explanation of these changes depends on the fact that, as we
recede from the epicentre, the vibrations of every period tend to
become inaudible. The limiting vibrations of the whole series will be
the first to be lost, especially those of the longest period. Thus,
near the epicentre, sound-vibrations of many different periods will be
heard, and the sound will be more complex than it is elsewhere. The
greater the distance, the narrower are the limits with regard to
period between which the audible vibrations lie, until, near the
boundary of the sound-area, the sound becomes an almost monotonous
deep growl of nearly uniform intensity.
_Time-relations of the Sound and Shock._--The principal epochs to be
compared are the beginning, the epoch of maximum intensity, and the
end. The beginning of the sound preceded that of the shock in 82 per
cent. of the observations on this epoch, coincided with it in 12, and
followed it in 6 per cent.; the epoch of maximum intensity preceded
that of the shock in 21 per cent. of the records, coincided with it in
73, and followed it in 6 per cent.; while the end of the sound
preceded that of the shock in 22-1/2 per cent., coincided with it in
27-1/2, and followed it in 50 per cent. Thus, as a general rule, the
beginning of the sound preceded that of the shock, the sound was
loudest when the shock was strongest, and the end of the sound
followed that of the shock. In other words, the duration of the sound
was in most cases greater than that of the shock.
MINOR EARTHQUAKES.
Of the twelve undoubted minor earthquakes, nine occurred before, and
three after, the principal shock, the times of the first eleven lying
between limits about seven hours apart. With three exceptions, the
records are insufficient to determine the positions of the epicentre
with any approach to exactness.
The first occurred at about 11 or 11.30 P.M. on December 16th. The
boundary of the disturbed area, which coincides nearly with that of
the fifth shock (E, Fig. 63), is 97 miles long from north-west to
south-east, 83 miles wide, and contains about 6,300 square miles. The
focus was apparently situated between the two foci of the principal
earthquake and partly coincided with them.
[Illustration: FIG. 63.--Map of minor shocks of Hereford
earthquake. (_Davison._)]
Then came three slight shocks (at about 1 A.M. on December 17th, 1.30
or 1.45 A.M., and 2 A.M.), about which little is known except that
they probably originated somewhere near the Ross focus.
The fifth shock (E, Fig. 63) occurred at about 3 A.M., and disturbed
an area 104 miles in length, 79 miles in width, and about 6,400 square
miles in area. Its boundary occupies approximately the position that
would be taken by an isoseismal of intensity between 7 and 6 of the
principal earthquake. We may therefore infer that this shock and the
principal earthquake were caused by slips along the same fault and in
about the same region of the fault. Also, as there is no evidence of
discontinuity in the vibrations of the minor shock, it is probable
that the focus was continuous, and occupied the space between the two
foci of the principal earthquake, as well as part or the whole of both
these foci.
The next four shocks occurred at about 3.30, 4, 5, and 5.20 A.M., and
were more closely associated with the Ross than with the Hereford
focus, and then followed the principal earthquake at 5.32 A.M.
A few minutes later, at 5.40 or 5.45 A.M., a very slight shock was
felt, the focus of which was possibly situated in the central region
between the two foci. The next, at about 6.15 A.M. (K, Fig. 63),
disturbed an area 41 miles long, 27 miles broad, and containing about
870 square miles. Its focus must have coincided approximately with the
Ross focus of the principal earthquake, and this was also the case
probably with the last shock of all, which occurred on July 19th,
1897, at 3.49 A.M.
ORIGIN OF THE EARTHQUAKES.
The greater part of the epicentral district is covered by a sheet of
Old Red Sandstone (Fig. 64), but, just to the north-east of the
position laid down for the originating fault (indicated by the
straight broken line), is the well-known Woolhope anticlinal, by which
Silurian beds are brought to the surface. The anticlinal axis runs
approximately north-west and south-east, and is thus roughly parallel
to the earthquake-fault. Moreover, the thinning-out and occasional
disappearance of some of the Silurian beds on the south-west side of
the anticlinal (as compared with those on the north-east side) is
suggestive of a north-west and south-east fault or rapid flexure at or
near the south-west junction of the Old Red Sandstone and the
Silurian strata. If it be a fault, it must hade to the north-east, and
would therefore satisfy two of the conditions determined by the
seismic evidence. It would lie, however, about two miles too far to
the north-east, being in fact to the north-east of the villages which
suffered most from the earthquake.
[Illustration: FIG. 64.--Geology of meizoseismal area of Hereford
earthquake. (_Davison._)]
But only a few miles to the south-east of the Woolhope anticlinal, and
almost in the same line with it, there is a second anticlinal, that of
May Hill. This is a triangular area, and is known to be bounded on all
three sides by faults. The fault on the north-east side has an average
north-west and south-east direction, and, if it were continued through
the Old Red Sandstone towards the north-west, but bending at first a
few degrees more to the west, it would pass through a point about
1-1/2 miles west of Hereford. It is worthy of notice that both this
fault and another nearly parallel to it, about half-a-mile farther
north-east, stop, according to the Geological Survey map, at the
points where they enter the Old Red Sandstone. The latter is an area
which has never been investigated with thoroughness by modern
stratigraphical methods, and in which it is difficult to trace faults.
It therefore appears not improbable that the earthquakes were due to
slips along a continuation of this fault.
Whether this be the case or not, however, it is clear that the
earthquake-fault must pass between the anticlinal areas of Woolhope
and May Hill, the former being on the north-east, and the latter on
the south-west, side of the fault. At the Hereford focus, the fault
must hade to the north-east; and, at the Ross focus, it is probable,
from the distribution of places where damage occurred to buildings,
that it hades to the south-west If this be the case, the fault must
change in hade between the two foci.
How long a time had elapsed since the last sign of growth in the
earthquake-fault took place, it is impossible to say; but it must be
many years in length. During this interval, the stresses tending to
produce movement along the fault-service had been gradually
increasing, until they were sufficient to overcome the resistance
opposed to them. It is worthy of notice that the earliest perceptible
movements were slight. Their function seems to have been to prepare
the way for the great slips by equalising the difference between
stress and resistance over a large area of the fault-surface. We
cannot trace with accuracy the transference of the seat of movement
from one part of the fault-surface to another. The first slip seems to
have taken place chiefly in the region between the two foci of the
principal earthquake; possibly it overlapped both of them partly. The
next three slips were apparently in the neighbourhood of the Ross
focus, and were followed by a fifth in the same area as the first.
Then came a series of small movements that we cannot locate further
than by saying that they were more closely connected with the Ross
focus than the other.
In consequence of the preliminary slips within and near the Ross
focus, the effective stress in that portion of the fault was
diminished; and this may be the reason why the first great slip took
place at the Hereford focus. The immediate result of such a movement
would naturally be an increase of stress in and beyond the terminal
regions, and the next slip might have been expected in an area partly
overlapping the Hereford focus, and either to the north-west or
south-east of it. Instead of this, for a distance of two miles in the
latter direction, there was not the least perceptible movement during
the principal earthquake, and the second great slip occurred in the
region beyond occupied by the Ross focus. This second slip, moreover,
occurred within two or three seconds after the other; that is, before
the earth-waves had time to travel from the Hereford to the Ross
focus. In other words, the slip at the Ross focus was not a
consequence of the slip at the Hereford focus; but both were due to a
single generative effort.
Now, a section drawn parallel to the earthquake-fault and on the
north-east side of it, would show an anticline near the Hereford focus
and a corresponding syncline near the Ross focus, with an undisplaced
portion in the intermediate region; while a parallel section on the
other side of the fault would show a syncline near the Hereford focus,
an anticline near the Ross focus, and again an undisplaced portion in
the intermediate region. If further movements tending to accentuate
such a structure were to occur (that is, if the anticlinals were to be
made more anticlinal and the synclines more synclinal), there would
therefore be two slips, one in each focus; while, along the
fault-surface between, there would be practically no displacement. At
any rate, the earlier stresses in that region may have been fully
relieved by two slight preliminary slips (those causing the first and
fifth minor earthquakes), and those resulting from the great
displacements by the first after-slip which followed in about ten
minutes.
Half-an-hour later, another slip took place at the Ross focus, and by
this the equilibrium of the rock-masses was almost completely
restored; for we have no certain evidence of any further movements
until seven months have elapsed (July 19th, 1897), when there was a
final slip in the same region of the fault.
THE INVERNESS EARTHQUAKE OF SEPTEMBER 18TH, 1901.
Between the north-east end of Loch Ness and the Moray Firth at
Inverness, there lies a tract of land not more than seven miles in
length, which is notable as one of those most frequently shaken by
earthquakes in the British Islands. In the intensity of its shocks it
is inferior to the south-east of Essex and the centre of
Herefordshire, and, in mere number, to the celebrated village of
Comrie in Perthshire. But, in the interest of its seismic phenomena,
in the light which they cast on the development of the earth's crust,
the neighbourhood of Inverness has no equal in Great Britain, and not
many superiors in any part of the world.
For this importance from a seismological point of view, the district
is indebted to the great fault which traverses Scotland along the line
of the Caledonian Canal, and to the fact that this fault, although it
dates from Old Red Sandstone times, has not yet finished growing. As
results of its formation, we have the almost straight cliff along the
south-east coast of Rossshire, and the long chain of lakes, beginning
with Loch Dochfour and Loch Ness, and ending with Loch Oich, Loch
Lochy, and Loch Linnhe. As evidences of its persistent though
intermittent growth, we have the slight tremors and earth-sounds
occasionally observed at and near Fort William, and the much stronger
shocks felt in the neighbourhood of Inverness.
During the nineteenth century there were three strong earthquake
shocks in this district. The first and most severe occurred on August
13th, 1816, and was felt over the greater part of Scotland; the second
on February 2nd, 1888; and the third and weakest on November 15th,
1890. This last shock was followed by several slighter ones, the
series ending with a rather smart shock on December 14th. Between this
date and the summer of 1901 no earthquakes seem to have been felt at
or anywhere near Inverness.
PREPARATORY SHOCKS.
The date of the first shock of 1901 is not quite certain. One is said
to have been felt at Aldourie (see Fig. 66) some time in June, and a
second at Dochgarroch in July. These may have been succeeded by others
too slight to attract much notice, but the first to be generally
observed occurred on September 16th at 6.4 P.M. A weak tremor,
accompanied by a faint sound, was perceived over a nearly circular
area about 12 miles in diameter, and with its centre about 1-1/2 miles
south of Dochgarroch. On the next day, at 11 P.M., a quivering lasting
two seconds was felt at Inverness, and a weak tremor, accompanied by
sound, at Dochgarroch at 1.15 A.M. on September 18th. Nine minutes
later, at 1.24 A.M., occurred the principal earthquake, the shock of
which would be called a strong one, even in Italy and Japan.
EFFECTS OF THE SHOCK.
In Inverness, the damage to buildings, though seldom serious, was by
no means inconsiderable. One brick building used as a smithy was
destroyed, several chimneys or parts of them fell, and many
chimney-cans were displaced or overthrown. At Dochgarroch and other
places within the meizoseismal area, walls were cracked, chimneys
thrown down, and lintels loosened.
But, for this country, an unusual effect of the earthquake was a long
crack made in the north bank of the Caledonian Canal near Dochgarroch
Lochs. It occurred in the middle of the towing-path, and could be
traced at intervals for a distance of 200 yards to the east of the
Lochs, and 400 yards to the west, being often a mere thread, and in no
place more than half-an-inch wide. Soon after its formation, however,
the fissure was obliterated by heavy showers of rain.
ISOSEISMAL LINES AND DISTURBED AREA.
The map (Fig. 65) shows the area over which the earthquake was
perceptible. The isoseismal lines are drawn partly continuous and
partly dotted--continuous where some confidence can be placed in their
accuracy, and dotted where their course must be regarded as doubtful,
owing to the rarity or absence of observations.
The innermost isoseismal (shown on a larger scale in Fig. 66)
corresponds to the intensity 8 of the Rossi-Forel scale, and includes
the places where the shock was strong enough to cause slight
structural damage to buildings. It is elliptical in form, 12 miles
long, 7 miles broad, and 67 square mile in area, with its centre at a
point about 1-1/2 mile east-north-east of Dochgarroch, and its longer
axis running N. 33° E. and S. 33° W.
[Illustration: FIG. 65.--Isoseismal lines of the Inverness
earthquake. (_Davison._)]
The remaining isoseismals are less accurately drawn, owing to the
scarcity of observations made in the west of Scotland. Except towards
the west, however, the course laid down for the isoseismal 7 may be
trusted. Its length is 53-1/2 miles, width 35 miles, and area 1,500
square miles. Its longer axis is almost exactly parallel to that of
the preceding isoseismal, but the distance between the two curves is 9
miles on the north-west, and 14 miles on the south-east, side. The
isoseismal 6 is 105 miles long, 87 miles wide, and contains 7,300
square miles; and the isoseismal 5, 157 miles long, 143 miles wide,
and about 17,000 square miles in area.
The isoseismal 4 may be regarded as the boundary of the disturbed area
of the earthquake, for, so far as known, the shock was not noticed at
any point outside it. Towards the north, it was felt at Wick,
Castletown, and other intermediate places; towards the west at
Tobermory in the island of Mull; and, towards the south, at Skelmorlie
(in Ayrshire), Paisley, Belsyde (near Linlithgow), Gullane (near North
Berwick), and Dunbar. Along the east coast of Scotland, between Wick
and Dunbar, there are few places of any size where the shock was not
felt. The disturbed area of the earthquake is thus 215 miles long from
north-east to south-west, 198 miles wide, and contains about 33,000
square miles.
_Position of the Originating Fault._--The only isoseismals which are
drawn accurately enough to determine the earthquake-fault are the two
inner ones, those marked 8 and 7; but these are sufficient for the
purpose. It is clear, from the direction of their longer axes, that
the average direction of the fault must be N. 33° E. and S. 33° W.
Again, the isoseismals are farther apart towards the south-east than
towards the north-west, implying that the fault hades to the
south-east. Lastly, as the intensity of the shock is greater on the
side towards which the fault hades, it follows that the fault-line
must lie a short distance (about a mile or so) on the north-west side
of the centre of the isoseismal 8.
Now, the great fault alluded to above occupies almost exactly the
position indicated by the seismic evidence. Its mean direction from
Tarbat Ness to Loch Linnhe is N. 35° E. and S. 35° W., it hades to the
south-east, and the fault-line passes through a point about
three-quarters of a mile to the north-west of the centre of the
isoseismal 8 (Fig. 66). There can be little doubt, therefore, that the
earthquake was caused by a slip of this fault; and the evidence of the
after-shocks, as will be seen, offers additional support to this
conclusion.
The region in which the slip took place may be determined roughly from
the position and form of the innermost isoseismal. Its centre must
have been close to the point marked A in Fig. 66, which corresponds to
a point about 1-1/2 mile east-north-east of Dochgarroch. In a
horizontal direction, its length must have been at least five or six
miles; otherwise, the isoseismal 8 would have been less elongated. It
must therefore have reached from about half-a-mile north-east of Loch
Ness to about half-a-mile south-west of Inverness. Its width, measured
along the dip of the fault-surface is unknown; but the small distance
between the centre of the isoseismal and the fault-line shows that the
principal movement took place at a depth which was probably under,
rather than over, one mile.
NATURE OF THE SHOCK.
We come now to the evidence afforded by the nature of the shock, in
which there was but little variation throughout the disturbed area. At
Inverness, a gentle movement was first felt, followed by an
extraordinary quivering, which increased in force for two or three
seconds, and then decreased for two or three seconds; just as the
quivering was about to cease, there was a distinct lurch or heave,
after which the vibration was much more severe than before and lasted
several seconds longer than the first part of the shock. Dalarossie
lies about fourteen miles south-east of Inverness, and here the first
indication was a loud sound, as of an express train, coming from the
east, rushing close to, and then under, the house; this lasted for a
few seconds, and towards the end of it the house vibrated. Then
succeeded an interval of quietness for about a second, followed by a
terrific burst or crash, not unlike the crash of a loud thunder peal,
of about two seconds' duration, during which the house distinctly
heaved up once and then sank back. After another brief interval of
quietness, there was a low rumble, like the sound of a dying peal of
thunder.
It will be noticed, in this account, that the two parts of the shock
were no longer consecutive. There was a short interval of rest between
them, the intermediate vibrations observed at Inverness being too weak
to be felt at Dalarossie. Still farther away, the extinction became
more marked. At Aberdeen, for instance, the shock consisted of two
parts, the first a tremble, followed, after an interval of a few
seconds, by a swinging movement of longer duration than the tremble.
In all parts of the disturbed area, the shock maintained the same
character of division into two parts, the second of which was of
greater duration and intensity than the first and consisted of
vibrations of longer period. A phenomenon of such wide occurrence was
clearly not due to local influences. It must have been caused by two
separate initial impulses, the stronger succeeding the other after an
interval of a few seconds and taking place in nearly the same region
of the fault.[67]
SOUND-PHENOMENA.
Outside the isoseismal 5, there are but few records of the
earthquake-sound; but it was heard faintly at Skelmorlie (in
Ayrshire), Belsyde (near Linlithgow), and Gullane (near North
Berwick). Towards the north, it was not observed beyond Wick and
Wathen (in Caithness). The boundary of the sound-area cannot be laid
down with any approach to accuracy, but it must have included a
district containing about 27,000 square miles.
Throughout the whole disturbed area, 84 per cent. of the observers
heard the sound. The percentage varies in different counties, from 93
in Inverness-shire to 77 in the counties of Perth and Aberdeen; but
the records in the more distant regions are too few to allow of the
construction of isacoustic lines.
In its character, the sound resembled that usually heard with strong
earthquakes, 39 per cent. of the observers having compared it to
passing waggons, traction-engines, etc., 25 per cent. to thunder, 14
to wind, 8 to loads of stones falling, 3 to the fall of heavy bodies,
4 to explosions or the firing of heavy guns, and 7 per cent. to
miscellaneous sounds. The intensity of the sound gradually diminished
outwards from the epicentre, and most rapidly near the isoseismal 7,
which abounds approximately the area in which the sound was very loud
from that in which it was distinctly fainter, and also includes nearly
all the places at which loud explosive crashes were heard with the
strongest vibrations.
In the time-relations of the sound and shock, the Inverness earthquake
resembles the Hereford earthquake of 1896. The beginning of the sound
preceded that of the shock in 72 per cent. of the records, coincided
with it in 20, and followed it in 8 per cent.; the epoch of maximum
intensity of the sound preceded that of the shock in 20 per cent. of
the records, coincided with it in 73, and followed it in 7 per cent.;
while the end of the sound preceded that of the shock in 15 per cent.
of the records, coincided with it in 34, and followed it in 52 per
cent.
Somewhat similar proportions hold over the greater part of the
disturbed area, the percentages being nearly the same in the counties
of Inverness, Ross, Nairn, Elgin, Banff, and the most distant
counties. But in Aberdeenshire an exception occurs, the three epochs
of sound and shock in most cases coinciding with one another. The
majority of the observations in this county come from the southern
part, and the line joining this district to the epicentre is nearly
perpendicular to the line of the earthquake-fault. This result has an
important bearing on the origin of the sound-vibrations. For, if the
general precedence of the sound with respect to the shock were due to
its superior velocity, the percentage of records in which the
beginning of the sound preceded that of the shock would vary only with
the distance, and not with the direction from the origin. Indeed,
with increasing distance from the origin, this percentage should
continually approach 100; while that in which the end of the sound
followed that of the shock should diminish to zero. There is, however,
no trace of either tendency, the sound being heard after the shock at
places close to the boundary of the sound-area. On the other hand, it
the sound-vibrations were to start simultaneously, or nearly so, from
all parts of the focus, but especially from its marginal regions,
then, in the greater part of the disturbed area, the sound would be
heard both before and after the shock; for the lateral margins of the
focus would be the portions nearest to, and farther from, most
observers; while, at places near the line through the epicentre at
right angles to the earthquake-fault, the three principal epochs of
the sound and shock should approximately coincide.
The inference that the sound-vibrations heard before and after the
shock come from the margins of the focus is also supported by the
observations on the relative duration of the sound and shock. If we
take only those records which are free from doubt, in 78 per cent. of
the total number, the duration of the sound was greater than that of
the shock; while, in Aberdeenshire, according to 93 per cent. of the
observers, the durations of sound and shock were equal.
We may imagine, then, that the slip within the seismic focus would be
greatest in a central region, and that it would die outwards in all
directions towards the edges. The friction arising from the slipping
in the central region would produce chiefly the comparatively large
oscillations that formed the perceptible shock; the evanescent creep
within the marginal regions would produce the small and rapid
vibrations that were sensible only as sound.
ORIGIN OF THE EARTHQUAKE.
While the seismic evidence enables us to determine the
surface-position and the horizontal dimensions of the seismic focus,
it unfortunately throws no light whatever on a point of some
importance--namely, the direction of the movement which caused the
earthquake. We cannot infer from it whether it was the rock on the
south-east or north-west side of the fault that slipped or whether
both sides slipped at once; nor, if that point had been settled, do we
know if the movement of the displaced side was upward or downward. In
the formation of the fault, however, it is clear that either the
south-east side has been depressed or the north-west side elevated;
and, as the bed of Loch Ness is below the level of the sea, that the
former movement has predominated. If the displacements which gave rise
to the earthquake were merely a continuation of the original series of
movements--and this is, to say the least, a very probable view to
take--then we may imagine that, for a distance of five or six miles,
and at a depth of about a mile or less, there was a sudden sag
downwards of the rock on the south-east side of the fault through a
distance which perhaps in no part exceeded a fraction of an inch.
Fig. 66 is an attempt to represent roughly the displacement which
caused the principal earthquake. The diagram makes no pretence to
accuracy, and the scale in the vertical direction is enormously
greater, perhaps a hundred thousand times greater, than that in the
horizontal direction. The straight line is supposed to represent a
straight line drawn before the earthquake on the surface of the rock
adjoining the fault on the south-east side and at a depth of about a
mile, and the curve the form of the same line after the earthquake.
[Illustration: FIG. 66.--Diagram to illustrate supposed
fault-displacement causing Inverness earthquake.]
The effect of this great slip would obviously be to relieve the stress
in the central region A, and to increase it suddenly in the parts
denoted by the letters B and C. It is, therefore, in these parts
especially that we should expect future slips to occur. Each slip
would of course give rise to an after-shock, and would in like manner
result in an increase of stress in its own terminal regions, though
chiefly on the side remote from the centre A.
THE AFTER-SHOCKS AND THEIR ORIGIN.
It is difficult to form any estimate of the total number of
after-shocks. The list, compiled from the records of careful observers
only, includes forty-six shocks and ten earth-sounds, the last of all
occurring on November 21st. But the list is certainly incomplete. It
contains, for instance, only one entry on September 18th between 3.56
and 9 A.M.; whereas, during the same interval, no fewer than eighteen
slight shocks were felt by one observer at Dochgarroch, while another
near Aldourie estimates the number of shocks up to October 23rd at
about seventy. The total number probably did not fall short of one
hundred.
The majority were certainly very slight, and, at another time, would
hardly have attracted any notice. There were, however, three of much
greater importance than the rest. These occurred on September 18th at
3.56 and 9 A.M., and on September 30th at 3.39 A.M. The isoseismal
lines of all three are elongated ovals, their longer axes are parallel
to the fault, and their centres lie on the south-east side of the
fault-line. The shocks were therefore evidently due to slips several
miles in length along the fault. At present, we are concerned more
with the position of their epicentres. These are indicated by the dots
lettered B, C, D in Fig. 67; the dot marked A denoting the centre of
the principal earthquake, and the continuous line the path of the
fault.
Thus, within two and a half hours, the great slip was followed by one
with its centre at B, near the south-west margin of the principal
focus. About five hours later, the scene of action was suddenly
transferred to a region with its centre at C on the north-east margin.
Both slips affected a portion of the fault-surface several miles in
length, and must therefore have increased the area of displacement,
slightly towards the north-east and considerably towards the
south-west. Only small movements occurred during the next twelve days
until 3.39 A.M. on September 30th, when another long slip took place,
with its centre at D, still farther to the south-west, and therefore
again extending the area and amount of displacement in this direction.
[Illustration: FIG. 67.--Map of epicentres of after-shocks of
Inverness earthquakes. (_Davison._)]
Turning now to the weaker after-shocks and earth-sounds, we find them
affecting chiefly three regions of the fault. One of these is close to
Dochgarroch, another near Inverness, and the third between Aldourie
and Drumnadrochit; the effects of the slips in the last two districts
being, as before, to extend the area of displacement a short distance
(perhaps half a mile) to the north-east and not less than six miles to
the south-west underneath Loch Ness.
The unequal division of the after-shocks between the two sides of the
principal centre (A, Fig. 67) is very marked. The positions of the
epicentres of forty-four shocks and earth-sounds can be determined
with more or less accuracy, and, of these, only ten lie to the
north-east of the principal centre, while thirty-four lie to the
south-west, six or seven of the latter being beneath Loch Ness.
One other point may be referred to before leaving these minor shocks.
So far as regards the stronger shocks, there was a continual decrease
in the depths of the seismic foci. This is shown by the progressive
approach of their epicentres towards the fault-line; the distances in
the three chief after-shocks being 1.7, 1.0, and 0.5 miles
respectively; and in one of the latest shocks (that of October 13th at
4.24 P.M., E, Fig. 67) the distance is no more than one-tenth of a
mile. The focus of this shock must, indeed, have been quite close to
the surface near Dochgarroch. This constant diminution in the depth of
the foci shows that the great slip was followed by a sudden increase
of stress upwards as well as laterally, and explains why that slip did
not leave any perceptible trace, either as fault-scarp or fissure, at
the surface.
SYMPATHETIC EARTHQUAKES.
It is remarkable that, of the 56 recorded after-shocks, at least six
were felt or heard only at Dalarossie and other places in the valley
of the Findhorn, a valley which lies about 13 or 14 miles to the
south-east of the great fault. That they had no connection with that
fault is certain, for two of them were so strong that, if they were so
connected, they could not have escaped the notice of one or more of
the watchful observers between Drumnadrochit and Inverness. The
probable explanation of these after-shocks is that they were due to
slips of a fault running along the Findhorn valley;[68] and that the
great displacement near Inverness on September 18th led to a sudden
increase of stress within the rocks for many miles around, which, at
and near Dalarossie, was sufficient to precipitate the slips referred
to.
CONCLUSION.
At first sight, two earthquakes could hardly be more unlike than the
Japanese earthquake of 1891 and the Inverness earthquake of 1901. In
the rice-fields of central Japan, as we have seen, the roads for many
leagues were edged with ruins, the fault-slip was prolonged up to the
surface and visible as a scarp forty, if not seventy, miles in length,
plots of ground were compressed and their boundaries altered, the
hillsides were scored by landslips, places can now be seen from one
another that formerly were hidden by a mountain ridge, and the total
number of after-shocks within little more than two years amounted to
above three thousand. On the other hand, when we examine the
distribution of the after-shocks in space, we find that, though no
part of the fault was exempt from slips, they favoured three regions
in particular--one, the most important, a central region, yet not
coincident with that in which the principal shock was most intense;
and the other two surrounding the extremities of the fault. With the
lapse of time, the after-shocks on the whole became weaker and
occurred less frequently, and the average depth of the foci gradually
diminished. Moreover, in two districts distant forty-five and
fifty-five miles from the fault, the frequency of the shocks during
the month succeeding the earthquake was suddenly increased to ten and
sixteen times the normal rate.
It is interesting to notice so close a similarity in character,
subsisting with so vast a difference in the scale of intensity. The
identity of the powers at work in shaping the structure of both
islands Is manifest. In Japan, we see the mountain-making forces
acting with violence and producing effects that are only too apparent
to the eye. In Scotland, whatever may have happened in former
geological epochs, the changes in surface-structure are now taking
place with almost infinite slowness, and hundreds or thousands of
years must elapse before Loch Ness makes any visible progress in its
march towards the sea.
REFERENCES.
1. DAVISON, C.--_The Hereford Earthquake of December 17, 1896._
(Birmingham, 1899.)
2. ---- "The Inverness Earthquake of Sept. 18, 1901, and its
accessory shocks." _Quart. Journ. Geol. Soc._, vol. lviii.,
1902, pp. 377-397.
FOOTNOTES:
[61] The study of the Hereford earthquake is based on 2,902 records,
coming from 1,943 places; that of the Inverness earthquake on 710
records from 381 places.
[62] The disturbed area of the Hereford earthquake of 1896 was
probably greater than that of any other British earthquake of the
nineteenth century; that of the Pembroke earthquake of 1892 being more
than 56,000 square miles, of the Pembroke earthquake of 1893 about
63,600 square miles, while that of the Essex earthquake of 1884 (a far
stronger shock in the meizoseismal area) is estimated at about 50,000
square miles.
[63] The approximate circularity of the two outer isoseismals is due
to the fact that the vibrations propagated to such great distances are
those which start from the comparatively small central region of the
focus.
[64] The above statement summarises the evidence of the majority of
the observers in each portion of the disturbed area. In this, as in
other similar cases, discrepancies in the observations are
unavoidable; but it is important to notice that they are least
frequent in the observations evidently made with the greatest care.
[65] Except in the case of Yorkshire, where the three Ridings are
regarded as separate counties.
[66] The Derby earthquake of March 24th, 1903, was also a twin
earthquake. The centres of the two foci were situated near Ashbourne
and Wirksworth, above eight or nine miles apart, along a line running
N. 33° E. and S. 33° W. The two parts of the shock coalesced along a
rectilineal band about five miles wide running centrally across the
lower isoseismals in a direction at right angles to their longer axes.
The isacoustic lines are also elongated in the direction of this band.
In this case, the impulses at the two foci must have taken place at
the same instant. (_Quart. Journ. Geo. Soc._, vol. lx., 1904, pp.
215-232.)
[67] If the foci of the two impulses had been detached, there would,
with so small an interval between the two parts, have been a variation
in the nature of the shock like that observed during the Hereford
earthquake.
[68] This part of Inverness-shire has not yet been mapped by the
Geological Survey, but a fault is known to exist in the Findhorn
valley near Drysachan Lodge, which lies about eleven miles down the
valley from Dalarossie.
CHAPTER IX.
THE INDIAN EARTHQUAKE OF JUNE 12TH, 1897.
Very different from the shocks of Britain was the earthquake that
overwhelmed so large a part of its great dependency on June 12th,
1897--an earthquake which, if it is not without a rival, is certainly
one of the most disastrous and most widely-felt of which we possess
any record. That it was of the first magnitude was evident at once in
Calcutta from the extensive injury to buildings, and its investigation
was undertaken without delay by the members of the Geological Survey
of India. The four officers who were at the headquarters in Calcutta
were despatched to the area of greatest damage, letters and circulars
were distributed as widely as possible, a large number of observers
were induced to co-operate by keeping records of the after-shocks,
and, later on, during the cold weather of 1897-98, Mr. R.D. Oldham,
one of the superintendents of the Survey, made a tour through the
epicentral district. To him, moreover, fell the much harder task of
discussing the very numerous observations collected by himself and
others; and the least that can be said of the valuable report prepared
by him is that it is worthy of a great subject. Professor Omori also
spent several months in studying the earthquake on behalf of the
Japanese Government; but the account, which is written in his own
language, unfortunately remains a sealed book to western
seismologists.
[Illustration: FIG. 68.--Isoseismal Lines of Indian Earthquake.
(_Oldham._)]
ISOSEISMAL LINES AND DISTURBED AREA.
In Fig. 68, which shows the area disturbed by the earthquake, Mr.
Oldham has drawn two series of curves. In the absence of detailed
records of the intensity--records that could not have been obtained
from some parts of the disturbed area, and would have been difficult
to procure in sufficient number from others--he has represented by the
dotted curves a group of isoseismals in the form which he believes
they would have assumed had the earth-waves been propagated in a
homogeneous medium. The first includes all places, such as Shillong
and Goalpara, where the destruction of brick and stone buildings was
practically universal; the second, those, like Darjiling, in which
damage to buildings was universal and often serious; the third,
places, like Calcutta, where the earthquake was strong enough to
injure all or nearly all brick buildings. Inside the fourth
isoseismal, the shock was strong enough to disturb furniture and loose
objects, but not to cause more than slight damage; within the fifth,
it was generally noticed; and, beyond this, and as far as the sixth
isoseismal, the earthquake was perceived only by a small number of
sensitive persons at rest. The approximation of the curves towards the
east and south-east, Mr. Oldham believes to be partly real, and not
due to imperfect information.
The continuous curves represent more closely the actual variation of
intensity. The innermost curve A indicates the probable boundary of
the epicentral tract, which is about 200 miles in length and more than
6000 square miles in area. This will be referred to afterwards in
greater detail. The next curve B bounds the region within which
serious damage to brick houses was common. Its irregular course is
closely connected with the geological structure of the country, and is
due to the fact, of which we have already met with several examples,
that earthquakes are more destructive to houses built on alluvial
ground than to those founded on rock. The area included within this
curve is not less than 145,000 square miles; and, if we include the
parts from which reports were not obtainable, it must amount to about
160,000 square miles.
The curve C represents the boundary of the disturbed area, so far as
known, for about one-third of the area lies in regions from which no
information was procurable, while another third is inhabited by
ignorant and illiterate tribes. But, notwithstanding this, the shock
is known to have been felt over an area of at least 1,200,000 square
miles. If we include the detached region to the west, near Ahmedabad,
the portion of the Bay of Bengal in which the shock would have been
felt had the sea been replaced by land, and a large part of Thibet or
Western China, from which no reports have come, but in which the shock
was certainly sensible, this estimate, great as it is, must be raised
to about 1,750,000 square miles.[69]
Figures, such as those given above, convey but little idea of the
vastness of the area concerned. Transferring them to countries with
which we are more familiar, we may say that the disturbed area was
only a little less than half the size of Europe; the region in which
serious damage occurred to masonry was more than twice as large as the
whole of Great Britain; while, if the centre of the epicentral tract
had been in Birmingham, nearly every brick and stone building between
York and Exeter would have been levelled with the ground.
NATURE OF THE SHOCK.
Few and slight were the forerunners of the greatest of modern
earthquakes. Early in June, faint tremors were felt by sensitive
persons at Shillong. Others at the same place heard a rumbling sound
for ten or fifteen seconds before the shock began, and at Silchar
birds were seen to rise suddenly from trees before the movement became
sensible to man. Except for these almost imperceptible warnings, the
earthquake broke abruptly over the whole district.
"At 5.15," writes one observer at Shillong, "a deep rumbling sound,
like near thunder commenced, apparently coming from the south or
south-west.... The rumbling preceded the shock by about two seconds
... and the shock reached its maximum violence almost at once, in the
course of the first two or three seconds. The ground began to rock
violently, and in a few seconds it was impossible to stand upright,
and I had to sit down suddenly on the road. The shock was of
considerable duration, and maintained roughly the same amount of
violence from the beginning to the end. It produced a very distinct
sensation of sea-sickness.... The feeling was as if the ground was
being violently jerked backwards and forwards very rapidly, every
third or fourth jerk being of greater scope than the intermediate
ones. The surface of the ground vibrated visibly in every direction,
as if it was made of soft jelly; and long cracks appeared at once
along the road.... The road is bounded here and there by low banks of
earth, about two feet high, and these were all shaken down quite flat.
The school building, which was in sight, began to shake at the first
shock, and large slabs of plaster fell from the walls at once. A few
moments afterwards the whole building was lying flat, the walls
collapsed, and the corrugated iron roof lying bent and broken on the
ground. A pink cloud of plaster and dust was seen hanging over every
house in Shillong at the end of the shock.... My impression at the end
of the shock was that its duration was certainly under one minute, and
that it had travelled from south to north.... The violence of the
shock may be imagined when it is stated that the whole of the damage
done was completed in the first ten or fifteen seconds of the shock."
Other estimates of the duration are generally higher than that given
above, ranging from three to five or even more minutes at Tura,
Dhubri, Silchar, Calcutta, and other places. In some cases, it is
possible that the immediately succeeding tremors were included as part
of the great shock; but, in the central area, it is probable that the
average duration of the shock did not differ much from three or four
minutes.
In this district, the movement was most complicated. Changes of
direction were frequently noticed. At Silchar, for instance, the
earthquake began with an undulatory movement from north to south, like
the swinging of a suspension bridge; it closed with a motion like
that of a boat tossed in a choppy sea, or by the crossing of great
waves which, whatever their dominant direction may have been,
certainly did not travel from north to south. The vertical component
of the motion must have been considerable; for, at Shillong, loose
stones lying on the roads were tossed in the air "like peas on a
drum," But this was even less pronounced than the horizontal movement,
the range of which was at least eight or nine inches, and during which
people felt as if they were being shaken like a rat by a terrier. The
period of these vibrations was estimated at about a second.
As they left the central region, the period of the waves lengthened,
so that, at a distance, the shock no longer consisted of short jerks,
but became a gentle rocking motion, causing in some people a sensation
of nausea. At Calcutta, the undulations were regular and resembled the
rolling of a mighty ship, the period being between one and two
seconds. At Balasor, the motion was a long rolling one, such as would
be felt on the deck of a ship in a fairly heavy sea; and, farther to
the south as far as the limit of the disturbed area, the same
undulatory movements were observed, gradually decreasing in intensity,
and usually compared to the easy motion of a ship in a gentle sea.
_Visible Earth-Waves._--A few examples have already been given of the
observation of visible waves on the surface of the ground. They were
seen at Charleston during the earthquake of 1886 (p. 110), and at
Akasaka and other places in the meizoseismal area during the Japanese
earthquake of 1891 (p. 186). But they were more than usually
prominent in the Indian earthquake; indeed, much of the difficulty
experienced in standing during the shock seems to have been due to the
passage of these surface-waves.
At Shillong, according to an observer quoted above (p. 266), the
surface of the ground vibrated visibly in every direction, as if it
were made of soft jelly. Another describes it as presenting "the aspect
of a storm-tossed sea, with this difference that the undulations were
infinitely more rapid than any seen at sea." Near Maimansingh,
earth-waves were watched approaching, exactly like rollers on the
sea-coast, and, as they passed, the observers had a difficulty in
standing. At Nalbari, the rice in the fields could be seen rising and
falling at intervals during the transit of the waves. In the Assam
valley, near Mangaldai, there were seen "waves coming from opposite
directions and meeting in a great heap and then falling back; each time
the waves seemed to fall back the ground opened slightly, and each time
they met, water and sand were thrown up to a height of about 18 inches
or so." Even as far as Midnapur, the ground was "distinctly billowy,"
and at Allahabad a series of waves was observed to cross the ground
from south-south-west to north-north-east.
It is obviously difficult to judge in any case of the magnitude of
such waves. In the epicentral area, Mr. Oldham believes that, on an
average, they were probably about thirty feet long and one foot in
height, though some may have been both shorter and higher. These
movements must have been comparatively slow, for their progress could
be easily followed by the eye; indeed, their rate, as one witness
remarks, "though decidedly faster than a man could walk, was not so
fast as he could run."
ELEMENTS OF THE WAVE-MOTION.
In his study of the Neapolitan earthquake, Mallet showed how the
amplitude and maximum velocity of the vibrations could be determined
roughly from the displacement, projection, or overthrow of various
bodies by the earthquake. Somewhat similar methods were employed by
Mr. Oldham in the absence of seismographs from the epicentral area.
His results are of course only approximate, but they lead nevertheless
to a conclusion of great value and interest.
[Illustration: FIG. 69.--Section of Tombs in the Cemetery at
Cherrapunji. (_Oldham._)]
_Amplitude._--The best measure of the amplitude was obtained at the
cemetery at Cherrapunji, situated near the southern margin of the
epicentral area. Here were two oblong masonry tombs (Fig. 69),
standing close together with their longer axes pointing north and
south. Their inner sides were partially destroyed. "On the outer
sides, they are almost intact, but the tombs have been driven bodily
down into the ground, and on either side to east and west, there is a
depression with a vertical side parallel to the outer surface of the
tomb and a smooth flat bottom over which the base of the tomb has
slid.... The edge of the western depression has the grass growing
undisturbed up to the edge of it, and along the edge small fragments
of lime and plaster show that this was originally in contact with the
edge of the tomb, which has now moved away to a distance of 18 inches.
On the east the edge of the depression is raised and the grass and
earth forced upwards by the thrust of the tomb against it; the breadth
of this depression is 10 inches."
During the movement of the ground, the tombs, owing to their inertia,
remained comparatively stationary, and the depressions were formed by
the backward and forward movement of the ground against them. The
movement on the east side was clearly arrested in some manner, and the
range therefore cannot have been less than 10 inches. It may have been
as much as 18 inches, and was probably, in Mr. Oldham's opinion, the
mean of these two amounts--namely, 14 inches. This would give an
amplitude of about 7 inches, a value which may be in excess of the
average amount elsewhere in the district, as the cemetery is situated
near the edge of a high sandstone scarp.
At Tura, also within the epicentral area, a range of not less than 10
inches was given by the sliding of a wooden house over the posts on
which it rested. Six months after the shock, Mr. Oldham frequently
noticed vacant spaces four or five inches across by the side of large
boulders scattered over the Khasi hills, and he infers that
"throughout the whole tract lying west of Shillong and Gauhati, as far
as the hills extend, and probably over a large area of the plains
besides, the amplitude of the wave-motion was nowhere less than 3
inches, while in many places it was over 6 inches."
_Maximum Velocity._--The most trustworthy measure of the maximum
velocity are those obtained from the projection of bodies. Mr. Oldham
selects the following as most deserving of notice:--At Goalpara, an
obelisk surmounting a tomb was broken off and thrown to one side,
giving a maximum velocity of not less than 11 feet per second. At
Gauhati, the coping of a small gate-pillar was shot off and fell at a
distance of 4 feet 4 inches from the centre of the pillar; in this
case the maximum velocity must have exceeded 8 feet per second. The
highest velocity, of more than 16 feet per second, was measured at
Rambrai, where a small group of monoliths were shot out of the ground,
one of them to a distance of 6-1/2 feet. Lastly, at Silchar, a bullet
was projected from the corner of a wooden post, acting as a rough form
of seismometer, from which a maximum velocity of at least 1-1/2 feet
per second was deduced.
_Maximum Acceleration._--Estimates of the maximum horizontal
acceleration were made from 28 overthrown pillars by means of
Professor West's formula (p. 184, footnote). The measures obtained at
the same place show some variation, but Mr. Oldham considers as fair
average values those of 14 feet per second per second at Goalpara, 12
at Gauhati, Shillong, and Sylhet, 10 at Cherrapunji, 9 at Dhubri, and
4 feet per second per second at Silchar.
Of the vertical component of the acceleration, not even the roughest
numerical estimate can be formed. We know, however, that at Shillong,
Gauhati, and indeed throughout the epicentral area, stones were
projected upwards, and this is evidence that the vertical component
was greater than that of gravity--namely, 32 feet per second per
second.
Violent as the shock was at the places just mentioned, it must have
been still greater in certain parts of the epicentral area. At Dilma,
in the Garo hills, the shock seems to have been strong enough to
disable men; and, in the neighbourhood of the faults that will be
described in a later section, forest trees were snapped in two.
Fortunately, as Mr. Oldham remarks, there were in these districts no
towns or populous settlements to feel the full power of the earthquake
to destroy.
_Anomalies in the above Measurements._--If the movements of the ground
followed the law of simple harmonic motion, any two of the four
elements (period, amplitude, maximum velocity, and maximum
acceleration) would suffice to determine the others (p. 4). Applying
the usual formulæ to the quantities obtained at Gauhati--namely, 8
feet per second for the maximum velocity and 12 feet per second per
second for the maximum acceleration, it follows that the amplitude
would be 5 feet and the period 4 seconds--values, which are evidently
inadmissible. Or, taking the maximum vertical component at 32 feet per
second per second, the corresponding values would be 2 feet and 1-1/2
seconds, that of the amplitude being still too great. Again, at
Rambrai, the maximum velocity was found to exceed 16 feet per second.
The other elements are unknown, but, if the amplitude were one foot,
Mr. Oldham shows that the maximum acceleration would be 256 feet per
second per second; or, taking the amplitude at the impossible amount
of two feet, that the maximum acceleration would be 128 feet per
second per second.
It follows, therefore, that only part of the high velocities at
Rambrai and elsewhere can be due to the elastic waves provoked by the
initial disturbances. The remaining portion must be attributed to a
bodily displacement of the earth's crust within the epicentral area--a
displacement of which the fault-scarps and other distortions of that
region furnish ample evidence.
SOUND-PHENOMENA.
In the epicentral area, the sound that accompanied the earthquake was
remarkable for its extraordinary loudness. At Shillong, the crash of
houses falling within thirty yards was completely drowned by the roar
of the earthquake.
The sound was generally compared to distant thunder, the passage of a
train or cart, etc.; but, whatever the type may be, it always implies
a sound of deep pitch, close to the lower limit of audibility--a
continuous rumbling or rattling noise, as a rule gradually becoming
louder and then dying away. There was the usual conflict in the
evidence of different observers due to the depth of the sound. In
Calcutta, which lies well within the sound-area, some persons asserted
that they heard a rumbling noise; others were positive that the only
noise was that caused by falling buildings and furniture. Some, again,
noticed that the shock was preceded by a loud roar; while others were
certain that there was no sound of any kind until the earthquake had
become severe.
As in the case of the disturbed area, it is impossible to define the
boundary of the region over which the sound was heard. Like the shock,
also, it seems to have been observed farther to the west than towards
the east. Leaving out of account records that are possibly doubtful,
the sound was heard for a distance of 330 miles to the west and
south-west, and 290 miles to the east of the epicentral area--that is,
allowing for the dimensions of that area, it must have been
perceptible over a region measuring not less than 800 miles from east
to west.
VELOCITY OF THE EARTH-WAVES.
It is somewhat doubtful whether a more accurate estimate of the
velocity is to be obtained from a violent earthquake or from one of
moderate intensity. In the former case, the vast distances to which
the shock is noticed lessen the effects of errors in the
time-determinations, but this advantage is to a great extent
compensated by the considerable duration of the shock and the
consequent uncertainty whether all observers have timed the same phase
of the movement. Also, in the Indian earthquake, there are further
sources of error in the variety of standard times employed throughout
the country and in the magnitude of the epicentral area.
Of the numerous time-records collected by Mr. Oldham, the best are
those which were obtained from a few self-recording instruments, from
the more busy telegraph offices, from the larger railway stations, and
in some cases from private individuals. All records were in the first
place subjected to a rigid process of selection; a large number were
rejected on various grounds, and those only were retained which bore
internal evidence of accuracy, due either to the conditions of the
reporter's occupation or to the care taken by him to ensure exactness.
To guard against any unconscious bias in making the selection, this
process was carried out before the distances were calculated, and even
before the position of the epicentral area was known.
The boundary of this area is shown by the continuous line A in Fig.
68. Its greatest length being about 200 miles from east to west, it is
necessary in the first place to fix upon an equivalent centre within
it, which may be regarded for this special purpose as the point of
departure of the earth-waves. The more natural course perhaps would be
to assume this point to coincide with the centre of the area. But, as
the rate at which the initial movement spread over that area would
probably differ little from the velocity of the earth-wave, and as all
the time-stations lie towards the west, Mr. Oldham regards a point
near the western boundary of the area (in lat. 25° 45' N. and long.
90° 15' E.) as a sufficiently exact approximation to the position of
the equivalent centre.
The nearest place at which good time-observations were made is
Calcutta, distant 255.5 miles from the assumed centre. One is
indicated on the recording tide-gauge by a sudden rise of the water,
while the others were obtained from the central telegraph office, the
terminal railway stations, and from two careful readings by interested
observers. They vary from 4h. 27m. 0s. to 4h. 28m. 37s. P.M., all
being liable to an error of half-a-minute. The arithmetic mean for the
beginning of the shock is 4h. 27m. 49s., and this is probably as
accurate an estimate as the conditions allow.[70]
Bombay lies outside the disturbed area, 1208.3 miles from the
equivalent centre; and, for the time of arrival in that city, we have
to depend on the records of the barograph and the three magnetographs.
The horizontal force magnet was set in motion two and a half minutes
before the others, no doubt by the advance tremors. The times given by
the barograph and the vertical force-instrument differ by only one
minute, and the best result seems to be that obtained by taking their
mean--namely, 4h. 35m. 43s., which is probably accurate to within a
minute.
Assuming, then, that the time-interval between Calcutta and Bombay
does not err by more than half-a-minute, it follows that the
intervening velocity must lie between 2.8 and 3.2 kilometres per
second, its probable value being 3 kilometres, or 2 miles, per second.
The remaining records, which are of less value than those obtained in
these cities, fall into two groups, the first consisting of a number
of stations along a line running north and south between Calcutta and
Darjiling or within a hundred miles on either side of the same, and
the second a long series of stations crossing Northern India in a
nearly westerly direction. The observations made at the Burmese
stations were unfortunately affected by an error arising from the
retardation of the Madras time-signals through frequent repetition
along the line.
[Illustration: FIG. 70.--Time-curve of Indian earthquake.
(_Oldham._)]
Individually, these records are not exact enough to be used in
determining the velocity, but they may be employed collectively for
the construction of the time-curve in Fig. 70. In this diagram,
distances in hundreds of miles from the equivalent centre are
represented along the horizontal line, and the time of occurrence in
minutes past 4 P.M. along the perpendicular line. The small circles
represent the observations at Calcutta and Bombay, the dots those at
places lying nearly west of the origin, and the crosses those at
places situated to the south or north-west. The continuous curve
passes in an average manner through the series of points, and probably
does not differ much from the true curve of the time of arrival of the
shock at different places. The curve, it will be noticed, is at first
concave, and afterwards convex, upwards; indicating that the times
required to traverse successive equal distances at first increased,
and then decreased. Thus, if the curve is an accurate representation
of the facts, it would follow that the surface-velocity was subject to
a continual decrease outwards from the centre, until it was a minimum
at a distance of about 280 miles, after which it increased.
The deviation of the curve from a straight line is, however, so slight
that we cannot feel much confidence in this conclusion. If we join the
points corresponding to Calcutta and Bombay by a straight line (drawn
dotted in Fig. 70), it does not in any part vary from the continuous
line by a distance equivalent to more than half-a-minute. Indeed, if a
very few discordant records are excluded, and if less weight is given
to those times which are multiples of five minutes, the straight line
represents the mean quite as fairly as the curved line does; and that
this is the more probable interpretation will appear from the
observations on the unfelt earthquake described in the next section.
We may therefore conclude that the earth-waves travelled along the
surface at an approximately uniform rate of 3 kilometres per second,
or about 120 miles a minute--a result which Mr. Oldham considers may
be accepted as accurate to within five per cent.
If the two time-curves in Fig. 70 are continued to the right until
they meet the time-scale, it will be seen that they intersect it near
the point corresponding to 4.26 P.M., implying that this would be
approximately the time at which the shock was felt within the
epicentral area. This agrees closely with the observed times of about
4.25 at Parbatipur and Kuch Bihar, 4.26 at Siliguri, and 4.27 at
Shillong and Goalpara; and it is probable that the error is not more
than a quarter of a minute in defect or half-a-minute in excess. Thus,
the time of arrival of the first sensible waves at the surface would
lie between 4h. 25m. 45s., and 4h. 26m. 30s. P.M., Madras time, or
between 11h. 4m. 45s. and 11h. 5m. 30s. A.M., Greenwich mean time.
THE UNFELT EARTHQUAKE.
Of the crowd of vibrations that agitate the ground during an
earthquake, part only combine to form the perceptible shock. Some are
insensible owing to their small amplitude, others to the slowness of
the motion. An interesting observation belonging to the latter class
was made by an engineer near Midnapur, a place which lies just within
the area of damage. At the time of the earthquake, he was taking
levels on a railway bank, and was about to take a reading when he
noticed the bubble of the level oscillating. In five or ten seconds
the shaking began and appeared to last three or four minutes; but, for
more than five minutes after it had apparently ceased, the level
showed that the ground continued to rock.
Again, in Burmah, at a place nineteen miles east of Tagaung and close
to the border of the disturbed area, the water in a shallow tank,
about 300 yards in length, was seen lapping up against the side in a
manner that was at first attributed to elephants bathing. No shock was
felt, but the shaking of the trees at the same time showed that the
disturbance was due to the earthquake.
Far beyond the limits of the disturbed area, however, the earthquake
was recorded by many of the delicate instruments which have been
employed during the last few years for the registration of distant
shocks. Among the more important of these instruments are long
vertical pendulums, horizontal pendulums of various forms, and
magnetographs. In the vertical, and some of the horizontal,
pendulums, especially in those used in the Italian observatories, the
masses carried are heavy, and the movements of the ground are
magnified by lightly-balanced levers ending in points which trace
their records on bands of smoked paper driven by clockwork. In the
other horizontal pendulums and in the magnetographs, the method of
registration is photographic. The paper required for the mechanical
records being inexpensive, a high velocity (half-an-inch or more per
minute) can be given to it, and the resulting diagrams are open and
detailed. The Italian instruments also respond more readily than the
others to the earlier and slighter tremors: while the apparatus in
which photographic methods are used are sometimes so violently
disturbed by the later undulations that the spot of light fails to
leave any trace on the photographic paper. It is therefore from the
Italian observatories that the more interesting records come. One of
these, given by a horizontal pendulum at Rocca di Papa near Rome, is
reproduced in Fig. 71; while the curve of the bifilar pendulum at
Edinburgh (Fig. 72) is a good example of those obtained by the
photographic method of registration.[71]
All over Italy, from Ischia and Catania in the south to Pavia in the
north, the different instruments employed began, one after the other,
to write their records of the movement as the unfelt earth-waves sped
outwards from the centre. Italy passed, the tale was taken up by
magnetographs at Potsdam and Wilhelmshaven, Pawlovsk (near St.
Petersburg), Copenhagen, Utrecht, and Parc St. Maur (near Paris); by
horizontal pendulums at Strassburg and Shide (in the Isle of Wight),
and by a bifilar pendulum at Edinburgh. Shide is 4,891 miles from the
centre of disturbance, but, as we shall see, the movement could be
traced for a distance greater even than this.
[Illustration: FIG. 71.--Seismographic Record of Indian
Earthquake at Rocca di Papa. (_Cancani._)]
In the more complete records, and especially in those given by the
Italian apparatus, Mr. Oldham distinguishes three phases of motion.
The first consists of rapid and nearly horizontal movements of the
ground. In Italy, it begins at about 11.17 A.M.--that is, about 12-1/2
minutes after the commencement of the shock at the epicentre (Fig. 71,
_a_). Without any break in the movement, and after a further interval
of about 8-1/2 minutes, the second phase begins; the vibrations are
similar to the preceding, but they are larger and more open, and are
accompanied by an unmistakable tilting of the surface of the ground
(Fig. 71, _b_). Lastly, after the lapse of about twenty minutes more,
the second phase gives place, without interruption, to the third (Fig.
71, _c_),[72] consisting of well-marked slow undulations, which have
been aptly compared by Professor Milne to the movements caused by an
ocean-swell. As they travelled across Europe, the surface of the
ground was thrown into a series of flat waves, 34 miles in length, and
20 inches in maximum height, the complete period of each wave being 22
seconds. This phase is by far the longest of the three; in the more
sensitive instruments, two or three hours elapsed before their traces
ceased to show any sign of movement.
[Illustration: FIG. 72.--Seismographic Record of Indian
Earthquake at Edinburgh. (_Heath._)]
Knowing the distances of the different observatories from the
epicentre, and the times taken by each phase to reach them, we can
form some idea of the rates at which they travelled. If the early
tremors moved in straight lines, their mean velocity for the first
phase was 9.0, and for the second 5.3, kilometres per second; but, if
they moved along curved paths through the body of the earth, their
mean velocities must have exceeded these amounts. For the first
undulations of the third phase, the velocity would be 2.9 kilometres
per second if they travelled along straight lines, and 3.0 kilometres
per second if they were confined to the surface of the earth.
The existence of the second phase was noticed for the first time by
Mr. Oldham in the records of the Indian earthquake, but he has since
detected it in those of other shocks. He believes, in common with most
seismologists, that the first phase corresponds to waves of elastic
compression, or longitudinal waves, travelling through the body of the
earth; and the second phase he attributes to waves of elastic
distortion, or transversal waves, travelling in the same way, in which
the particles move at right angles to the direction in which the wave
travels, thus causing a slight tilting of the surface. It is probable
that the waves of both phases move along curved, rather than straight,
lines through the earth, that the curves are concave towards the
surface, and that the velocity of the waves increases with the depth
of their path below the surface.
On the other hand, the surface-velocity of the first undulations of
the third phase is practically constant for all distances from the
epicentre, and, in the case of the Indian earthquake, it agrees almost
exactly with that obtained for the velocity within the disturbed area,
and as far as Bombay. It is therefore difficult to resist the
conclusion that the third phase consists of undulations which travel
along the surface of the earth. Diverging in two dimensions only, they
fade away much more slowly than the vibrations of the other two
phases.
We may thus imagine these surface-undulations speeding outwards from
the epicentre in ever-widening circles until they have passed over a
quarter-circumference of the earth, when they should begin to converge
towards the antipodes. Here they should cross each other, and again
spread out as circular waves, once more in their course passing the
same observatories where they were first recorded, but in the opposite
order. It has been reserved for the most violent earthquake of modern
times to verify this interesting conclusion. Faint, but decided, are
the traces of the second crossing. At Edinburgh, they occur at 2.6
P.M., at about the same time at Shide, at Leghorn 2.10, Catania
2.12-3/4, while at Ischia there are several movements between 2 and 3
P.M. At Rocca di Papa, near Rome, the time is slightly earlier, but
the undulations, like those at the first crossing, have a complete
period of about 20 seconds. The distances traversed by the waves are
more than 20,000, instead of less than 5000 miles; but the mean
velocity with which they travelled is almost exactly the same as at
first--namely, 2.95 kilometres per second.
EARTH-FISSURES, SAND-VENTS, ETC.
_Earth-Fissures._--Among the superficial effects of the earthquake,
none take a more important place than the fissures formed in alluvial
plains. Not only were they remarkably abundant, more so than in any
other known earthquake, but they occurred over an unusually wide area.
Wherever the necessary conditions prevailed, they were found to be
numerous over a district bounded approximately by the isoseismal 1
(Fig. 68), and measuring about 400 miles from east to west, and about
300 miles from north to south; and they were present, though in
smaller numbers, over an area nearly 600 miles long in an
east-north-east and west-south-west direction. They were naturally
more frequent near river-channels and reservoirs, on account of the
absence of lateral support, and as a rule were parallel to the edge of
the bank, a few hundred yards in length, and in width varying from
some inches to four or five feet.
Fissures in such positions are formed with every violent earthquake,
and even with some of those more moderate shocks that visit the
British Islands (see p. 247). But an interesting point established by
the Indian earthquake is that they also occurred at a distance from
any water-channel or excavation, often running parallel to, and along
either side of, a road or embankment. In other situations, they showed
a distinct tendency to range themselves parallel to one another; and,
in these cases, it is possible that their formation was connected with
the passage of the visible surface-waves. In an account already quoted
(p. 247), it is stated that these waves came from opposite directions
and that, as they separated after meeting, the ground opened slightly.
Among the Khasi and Garo hills (see Fig. 75), wherever the alluvium of
the plains runs up to the foot of the hills, another form of fissure,
represented in Fig. 73, was constantly noticed. Close to the
junction, there was a sudden drop, as at _a_, of from one to five
feet, the vertical face having the appearance of a fault, but
distinguished from one by following the windings of the hills. Then
came a depressed band _b_, from ten to twenty feet wide, and outside
this a low rounded ridge _c_ raised above its former level, and
merging beyond at _d_ into the undisturbed plain. When Mr. Oldham
visited the district in March 1898, the natives had flooded the
rice-fields, and the features described were clearly depicted by the
gathering of the water in the depression and the isolation of the
ridge.
[Illustration: FIG. 73.--Displacement of alluvium at foot of a
hill. (_Oldham._)]
The explanation of these peculiarities is evidently that given by Mr.
Oldham. During the passage of repeated waves of compression, the
thrust of the hill and plain against one another caused the heaping up
of the alluvium in the ridge _c_; while the return movements resulted
in the tearing of the alluvium away from the hillside, leaving the
scarp _a_ and the depression _b._
_Displacements of Alluvium._--Many other remarkable evidences of
compression were observed. Telegraph posts, originally set up in a
straight line, were displaced, occasionally as much as ten or fifteen
feet; sometimes without any apparent connection with neighbouring
river-channels. In one part of the Assam-Bengal Railway, for nearly
half a mile, the whole embankment, including borrow-pits and trees on
either side, was shifted laterally without any sign of wrenching from
the adjoining ground, the maximum distance amounting to 6-3/4 feet. As
the displacement took place parallel to the only river-course in the
neighbourhood, Mr. Oldham attributes it to the sliding of the
surface-layers over some yielding bed beneath. Again, throughout large
areas of Northern Bengal, Lower Assam, and Maimansingh, rice-fields,
which had been carefully levelled so that they might be uniformly
flooded, were thrown into gentle undulations, the crests of which were
occasionally two or three feet above the hollows. The piers of bridges
were also moved parallel to, as well as towards, the streams, showing
that the displacements extended to the depth of the foundations.
The buckling of railway lines was often violent and took place over a
large area. In the Charleston earthquake, every such bend was
accompanied by a corresponding extension elsewhere (p. 113); but, in
the Baluchistan earthquake of 1892, the neighbouring fish-joints were
jammed up tight.[73] In the one case, there was merely local
compression; in the other, a permanent displacement of the earth's
crust. The distortion of the Indian lines seems to belong to the
former class. Repairs were of course generally made without delay; but
all the information that could be obtained on this point showed that
the compression causing the crumpling of the lines was accompanied by
a compensating expansion, generally at a distance of about 300 yards.
_Sand-Vents._--Shortly after the earthquake, large quantities of water
and sand issued from fissures in the ground. At Dhubri, "innumerable
jets of water, like fountains playing, spouted up to heights varying
from 18 inches to quite 3-1/2 or 4 feet. Wherever this had occurred,
the land was afterwards seen to occupy a sandy circle with a
depression in its centre. These circles ranged from 2 to 6 and 8 feet
in diameter, and were to be seen all over the country. In some places,
several were quite close together; in others they were at a distance
of several yards." Near Maimansingh, they seem to have been almost as
numerous, fifty-two, of four feet and less in diameter, being counted
within an area 100 yards long and about 20 feet wide.
The sand and water were ejected from the vents with some force. A few
observers estimated the height of the spouts at about 12 feet, but
this probably refers to stray splashes. It is clear, however, that the
sand and water were forced not only up to the surface, but even in a
continuous stream to heights of from two to ten feet above it. In many
districts, trunks of trees or lumps of coal and fossil resin were
washed up with the water, and even, in one or two cases, pebbles of
hard rock weighing as much as half-a-pound.
The origin of the sand-vents is to be sought in the presence of a
water-bearing bed situated not far below the surface. In the central
area, where there was a marked vertical component in the motion, this
bed during the earthquake was compressed between those above and below
it, and the resulting pressure was in places sufficient to force the
water and sand, through the fissures formed by the earthquake, up to
and beyond the surface. The gradual settling of the upper layer, cut
up by the fissures, into the underlying quicksand, prolonged the
process for some time after the shock was over; and, when the pressure
was at last relieved, some of the water was sucked back and so
produced the crateriform hollows.
_Rise of River-Beds, etc._--Over a large area, river-channels, tanks,
wells, etc., were filled up, partly by the outpouring of the sand from
vents, but chiefly, as shown by the forcing up of the central piers of
bridges, by the elevation of the beds of the excavations. In the
lowlands which lie between the Garo hills and the Brahmaputra, there
were numerous channels from 15 to 20 feet in depth, the beds of which
were pressed up until they became level with the banks, while a
compensating subsidence took place close to the streams on either
side. The general tendency of the earthquake was thus to obliterate
the surface inequalities, so that, when the rivers rose later on, the
district was extensively flooded.
Besides these deferred floods, there occurred immediately after the
earthquake a sudden rise in many rivers, amounting to from two to ten
feet, followed by a gradual decline to the former state in two or
three days. At Gauhati, for instance, the river-gauge showed that, at
about three-quarters of an hour after the earthquake, the water stood
7 feet 7 inches higher than on the morning of June 12th; at 7 A.M. on
June 13th it had fallen to 5 feet 8 inches, and at the same time on
the two following days to 2 feet 7 inches and 6 inches, showing that
the water had returned nearly to its original level after the lapse of
two and a half days.
In most of the large rivers, the rise of water was due to the
formation of partial dams formed by the local elevation of the
river-beds described above. As the barriers were composed of loose
sand, they were gradually scoured away and the material was spread
over the bottom so as to leave the water at a level slightly higher
than that which it maintained before the earthquake.
LANDSLIPS.
The distribution of landslips shows that their formation depends
almost as much on local conditions as on the violence of the shock.
The effect of the latter is manifested by their limitation to a
certain central area. To the east of the North Cachar hills, few, if
any, were to be seen; but, as far as Kohima, cracks or incipient
landslips were formed on the hillsides. The Sylhet valley and a line
to the west of Darjiling form the southern and western boundaries of
the landslip area, which was therefore not less than 300 miles in
length from east to west.
Within this area, however, local conditions asserted their
superiority. Among the more important may be mentioned the
constitution of the hills and the presence of a thick superficial
layer of subsoil or rock with an inner bounding surface of weak
cohesion, the slope of the hillsides, and their height from base to
crest. Thus, though the epicentral area was situated chiefly to the
south of the Brahmaputra valley (Fig. 75), the east and west range of
the landslips was more extensive in the Himalayas on the north side
than in the Garo and Khasi hills on the south. In many places, the
steep sides of the Himalayan valleys exist always in a critical
condition of repose, and the effect of the Indian earthquake was such
that all along the north side of the Brahmaputra valley, the range is
scarred by landslips, even to the east of Tezpur.
Again, along the southern edge of the Garo and Khasi hills, landslips
were unusually prevalent. "Viewed from the deck of a steamer sailing
up to Sylhet," says Mr. Oldham, "the southern face of these hills
presented a striking scene. The high sandstone hills facing the plains
of western Sylhet, usually forest-clad from crest to foot, were
stripped bare, and the white sandstone shone clear in the sun, in an
apparently unbroken stretch of about 20 miles in length from east to
west." At Cherrapunji, also, the deep valleys were so scored that,
from a distance, there appeared to be more landslip than untouched
hillside.
But in no part, probably, were landslips more strikingly developed
than in the small valley of the Mahádeo, which forms an amphitheatre
about four miles long from east to west, and a mile and a half across,
lying to the south of the Bálpakrám and Pundengru hills. "Here,"
remarks Mr. Oldham, "everything combined to favour the formation of
landslips. The hills were composed of soft sandstone, they were
steep-sided, high, and narrow from side to side, and consequently were
doubtless thrown into actual oscillation as a whole; while the range
of motion of the wave particle was not less than eight inches near the
edge of the precipices. The result ... has been to produce an
indescribable scene of desolation. Everywhere the hillsides facing the
valley have been stripped bare from crest to base, and the seams of
coal and partings of shale could be seen running in and out of the
irregularities of the cliffs with a sharpness and distinctness which
recalled the pictures of the cañons of Colorado. At the bottom of the
valley was a piled-up heap of _débris_ and broken trees, while the old
stream had been obliterated and the stream could be seen flowing over
a sandy bed, which must have been raised many feet above the level of
the old watercourse."
In the sandstone districts of the area here considered, the landslips
had some important secondary effects. Along the southern edge of the
Garo and Khasi hills, great sand-fans spread over the fields, and the
exposure of the hillsides formerly protected by forest left free scope
for future denudation. Every stream of any size has in this way
devastated many square miles of country. Among the hills themselves,
more sand was brought down than the streams could carry away, and
everywhere their beds were raised. "Ordinarily, the beds of these
rivers, which are raging torrents when in flood, consist of a
succession of deep pools separated by rocky rapids. After the rains of
1897, it was found that the pools had been filled up, and the rapids
obliterated by a great deposit of sand, over which the rivers flowed
in a broad and shallow stream."
A few valleys were for a short time barred across by landslips. In
one, on the northern foot of the Garo hills, a landslip crossed the
drainage channel and formed a shallow pond, which was not filled up by
sand until the end of January 1898. Near Sinya, in the northern Khasi
hills, an unusually large landslip formed a barrier, of which the
remains are more than 200 feet above the level of the river-bed.
Behind this, the water accumulated in a great lake until the beginning
of September 1897, when the barrier burst and a flood of water rushed
down the valley.
ROTATION OF PILLARS, ETC.
A curious effect of earthquakes strong enough to damage buildings is
that pillars, monuments, etc., may be fractured and the upper part
rotated over the lower without being overthrown. Even in Hereford and
the surrounding villages, several pinnacles and chimney-stacks were
twisted by the earthquake of 1896. The interest of the phenomenon,
which has been known, since 1755,[74] is mainly historical, for the
endeavour to discover its cause was the origin of Mallet's views on
the dynamics of earthquakes. Partly, also, it lies in the difficulty
of finding a satisfactory explanation, or rather in deciding which of
three or four possible explanations is the true one in any particular
case.
[Illustration: FIG. 74.--Twisting of monument at Chhatak.
(_Oldham._)]
The Indian earthquake offered exceptional opportunities for studying
the phenomenon in the large number of examples observed and the
variety of objects rotated. None could be more striking than the
twisted monument to George Inglis, represented in outline in Fig. 74.
Chhatak, where this is situated, lies close to the southern boundary
of the epicentral area. The monument is an obelisk, built of broad
flat bricks or tiles on a base of 12 feet square, and originally more
than 60 feet high. It was split by the earthquake into four portions.
The two upper, about six and nine feet long, were thrown down; while
the third, 22 feet high, remains standing, but is twisted through an
angle of 30° with respect to the lowest part, which is unmoved. The
upper of these two parts had evidently rocked on the lower, as the
corners and edges were splintered, and below the fracture a slice of
masonry about 15 inches thick, which was not bonded into the main
mass, was split off by the pressure on its upper end. The plan of the
parts still standing is shown in the lower part of Fig. 74.
The possible explanations of the phenomenon are at least three in
number. According to the first, which was given by Mallet in 1846, the
adhesion of the twisted portion to its base is not uniform, and the
resultant resistance to motion is not in the same vertical plane as
the wave-movement.[75] Some years later, Mallet offered another
explanation. The body, he imagined, might be tilted on one edge by the
earthquake, and, while still rocking, a second shock oblique to the
first might twist it about that edge.[76] In 1880, Professor T. Gray
suggested that the column might be tilted on one corner and then
twisted round it by later vibrations of the same shock.[77]
None of these theories, Mr. Oldham argues, can give by itself a
complete explanation of the phenomena observed in the central district
of the Indian earthquake; and he therefore favours an extension of the
second theory, which, though first proposed in 1882,[78] was thought
out independently and in greater detail by himself. When the focus is
of considerable dimensions, the shock at neighbouring places is
constantly varying in direction, owing to the arrival of vibrations
from different parts of the focus. Thus, instead of the two separate
shocks required by Mallet's second explanation, we have a number of
closely successive impulses frequently changing in direction and
giving rise to what is known in the South of Europe as a vorticose
shock. And, instead of a single twist of the pillars about one centre
only, we have a series of small twists round a number of different
centres, accompanied in consequence by a much smaller displacement of
the centre of gravity than would have occurred had the same rotation
been accomplished in one operation.
The theory, it will be seen, accounts for the twisting of the pillar
without overthrow, and for the splintering of the edges during the
rocking of the column. It explains why in any district a number of
similarly placed objects are generally twisted in the same direction.
Moreover, a low column rocks to and fro more rapidly than a tall one
similar in form and position, so that, at the instant when a later
impulse comes from a different direction, two such columns might
happen to be tilted on opposite edges, and would then be twisted in
opposite directions. In certain cases, then, as occurred at several
places during the Indian earthquake, an object may rotate in one
direction, while others, similar in every respect but size, may be
twisted in the opposite direction.
AFTER-SHOCKS.
_Frequency of After-Shocks._--For some days after the great
earthquake, the after-shocks by their very frequency and by their wide
distribution baffled close inquiry. During the first 24 hours,
hundreds were felt at all points of the epicentral area; indeed, it is
not too much to say that for several days the ground was never
actually at rest. At the Bordwar tea-estate, which is traversed by one
of the great fractures to be described in the next section, the
surface of a glass of water on a table was for a whole week in a
constant state of tremor; and at Tura a hanging lamp was kept
continually swinging for the first three or four days.
Most of these shocks were, of course, very slight; but, interspersed
among them, were others of greater strength, and a few of considerable
violence. One, on June 13th, about eight hours after the earthquake,
was sensible beyond Allahabad--that is, for more than 520 miles from
the epicentre; and another on the same day was felt in Calcutta,
distant 255 miles. On June 14th, 22nd, and 29th, and again on August
2nd and October 9th, shocks were noticed in that city; but, after the
latter date, the disturbed area of no shock reached to so great a
distance.
To form any estimate of the total number of after-shocks is
impossible, even for any one station. At first, lists were kept at
isolated places, such as Shillong, Maimansingh, Dhubri, and a few
others. Then, from July 15th, through Mr. Oldham's efforts, the
records became more numerous until the end of the year, after which
interest in the subject declined. Mr. Oldham's catalogue closes with
the year 1898; but the register of a roughly-constructed seismograph,
erected at Shillong in July 1897, continues to the present day.
Imperfect as all non-instrumental registers must be, they nevertheless
furnish some idea of the frequency of the after-shocks. Thus, until
the end of June, 679 shocks were recorded at Rangmahal (North
Gauhati), 135 at Maimansingh, 89 at Kuch Bihar, and 83 at Kaunia
(omitting those on June 12th). Again, from August 1st to 15th, 182
were felt at Goalpara, 151 at Darangiri, 124 at Tura, 105 at Bijni, 94
at Lakhipur, 94 at Krishnai, 48 at Dhubri, 28 at Rangpur, and 12 at
Kuch Bihar; while at Borpeta, 113 shocks were reported during the
first nine days of August. Turning to the registers of longer
duration, we find that at Maophlang (near Shillong) 1,194 shocks were
felt by one observer from September 12th, 1897, to October 7th, 1898;
at the neighbouring station of Mairang, 1,065 from September 7th,
1897, to December 31st, 1898; and at Tura, in the Garo hills, 1,145
shocks from July 21st, 1897, to December 31st, 1898. The total number
of earthquakes registered by the seismograph at Shillong from August
1897 to the end of 1901 amounts to 1,274, and all of these were
probably strong enough to arouse the observer from sleep. Outside the
epicentral area, Mr. Oldham's list includes 88 shocks from June 12th
to July 15th, about 950 from July 16th to December 31st (the period
when the after-shocks were most carefully observed), and 296 shocks
during the year 1898.
_Geographical Distribution of After-Shocks._--When we endeavour to
compare the lists of after-shocks at different places, we are at once
met by two serious difficulties,--the imperfection of the records and
the approximate character of the times of occurrence. Making every
allowance, however, for these deficiencies, it is evident that very
few of the shocks felt at any one station were perceptible at its
neighbours; in other words, that the shocks originated in a large
number of foci scattered over a very wide area.
For instance, two of the most carefully kept registers of after-shocks
are those compiled at Maophlang (near Shillong), and at Mairang, only
11 miles to the north-west. Now, between September 12th and September
28th, 1897 (both dates inclusive), 92 shocks were felt at Maophlang
and 83 at Mairang. Of the former, 37 were described as smart, 45
slight, and 10 feeble; of the latter, 6 as smart, 9 slight, 65 feeble,
and 3 very feeble. But, of the total number, only 20 were felt at both
places at recorded times that were not more than fifteen minutes
apart; 13 being described as smart--one at both places, one at Mairang
alone, and the remaining 11 at Maophlang alone. When shocks occur so
frequently, as in these cases, it is inevitable that, even if all were
independent, some should coincide approximately in time of occurrence.
It is therefore probable that only one in every eight shocks, and
possibly only one in every twelve, was felt at both places.
The actual numbers of shocks felt within stated periods at different
places are perhaps hardly comparable, owing to the obvious
imperfection of the records and the probably varying standards adopted
by the reporters. But there can be little doubt that certain districts
were more subject to after-shocks than others, especially such places
as North Guahati, Shillong, and neighbouring villages, Tura,
Darangiri, Goalpara, Bijni, Borpeta, Kaunia, and Rangpur. On the other
hand, they seem to have been unusually scarce at Dhubri and in the
district to the north-west, and they became rare at Gauhati long
before they ceased to be frequent at Borpeta. In the plain to the
south of the Garo and Khasi hills, they were also uncommon, the
combined records for Sylhet and Sonamganj for August 1-15 giving only
20 shocks, and, neither to the east nor to the west of these places,
is there any sign of greater frequency.
_Sound-Phenomena of After-Shocks._--Many of the after-shocks were
accompanied by sound, or else consisted of sound-vibrations only; and
Mr. Oldham notices that such sounds were equally frequent both on the
rocky ground of the hills and on alluvial plains nearly all the shocks
that originated under the Borpeta plain being attended by distinctly
audible rumblings.
During his tour in the epicentral area in the winter of 1897-98, Mr.
Oldham had many opportunities for observing these earth-sounds. They
were, he says, close to the lower limit of audibility, less a note
than a rumble, and very like distant thunder, though sometimes they
consisted of a rapid succession of short sounds, such as is caused by
a cart when driven rapidly over a rough pavement. "As a rule, they
began as a low, almost inaudible rumble, gradually increasing in
loudness, though to a very varying degree, and then gradually dying
out after having lasted anything from 5 to 50 seconds. It cannot be
said that there was any connection between the duration and the
loudness of the sounds, some of the most prolonged never becoming
loud, and some of those which lasted a shorter period being as loud as
ordinary thunder at a distance of two or three miles."
Mr. Oldham records an interesting fact in connection with the
distribution of the earth-sounds. At Naphak, in the Garo hills and
about five miles south of Samin, 48 distinct rumbles were heard during
23 hours on January 21-23, 1898, only seven of them being accompanied
by a perceptible shock. At Samin, which was visited next, they were
much less frequent, not more than 8 or 10 a day, and most of them
attended by tremors. At Damra, a few miles to the north-east, they
again became frequent; while, in the Chedrang valley, very few were
heard, and only a small proportion of them were unaccompanied by
sensible shocks. In the next section, it will be seen that the most
conspicuous fault-scarps known in the epicentral area pass close by
Samin and along the Chedrang valley. Thus, though the statement
perhaps requires further confirmation, it would appear that
earth-sounds were more common where the surface of the ground had been
merely bent than where fractures extended right up to the surface.
STRUCTURAL CHANGES IN THE EPICENTRAL AREA.
We come now to the important features which assign the Indian
earthquake to a small class apart from nearly every other shock. Most
earthquakes are due to movements that are entirely deep-seated. If
strong enough, they may precipitate landslips or fissure the alluvial
soil near river-channels. In the Neapolitan, Andalusian, and
Charleston earthquakes, there were many such effects of the shock
within the meizoseismal areas. In all three, however, the disturbances
produced were superficial; no structural change, no fissuring that did
not die out rapidly downwards, was in any place perceptible. In the
Riviera earthquake, the seismic sea-waves point to a small
displacement of the ocean-bed; but it is only in the long fault-scarp
of the central Japanese plain that we find a rival of the
mountain-making movements that gave rise to the Indian earthquake.
The boundary of the epicentral area, to the growth of which these
distortions contributed, is represented by the curve marked A in Fig.
68, and on a larger scale by the continuous line A in Fig. 75. A great
part of the district is occupied by a group of hills known by various
names locally, but which are conveniently included under the general
term of the Assam range. To avoid the confusion of hill-shading, only
the boundary of the range is indicated (by the broken line) in the map
in Fig. 75. The Garo hills form the western part, and the Khasi and
Jaintia hills the central and western parts, of the range as there
depicted. They are formed chiefly of crystalline gneissic and granitic
rocks and some metamorphic schists and quarzite, with cretaceous and
tertiary rocks of varying thickness along its southern edge.
Three stages have been distinguished in the history of the range.
During the earliest, an old land-surface was worn down by rain and
rivers till they were almost incapable of producing any further
change. Traces of this surface are still visible in the plateau
character of the mass. It was then elevated, not uniformly, but along
a series of faults, so that it now consists of a succession of ranges,
the face of each range being a fault-scarp, and its crest the edge of
an adjoining plateau sloping away from the summit. With this elevation
began the third and last stage. The streams were able to work again,
and deep gorges were cut out of the range, so that in parts its
original character was nearly effaced. But the retention of that
character in other districts is of course evidence of the
comparatively recent date of the final elevation.
[Illustration: FIG. 75.--Epicentral Area of Indian Earthquake.
(_Oldham._)]
Owing to the great size of the epicentre and to the thickness of the
forests which cover so much of its area, a comparatively small part of
it could be traversed by Mr. Oldham during his tour in the winter of
1897-98. The positions of the more important structural changes are
indicated in Fig. 75. Of these, the fault-scarps are represented by
continuous straight lines, the Bordwar fracture by the dotted straight
line, pools and lakes not due to faulting by black ovals, reported
changes in the aspects of the hills by circles, and the principal
stations of the revised trigonometrical survey by crosses.
_Fault-Scarps._--The most important fault-scarp is that called by Mr.
Oldham the Chedrang fault, after the stream which coincides roughly
with a great part of its course. The longer straight line in Fig. 75
represents its position and general direction, and the sketch-map in
Fig. 76 gives the plan of its southern half. From these, it will be
seen that the fault follows on the whole a nearly straight path from
south-south-east to north-north-west for not less than twelve miles,
and that its throw, as indicated by the numbers to the right in Fig.
76; is very variable, being zero in some places, and in one as much as
35 feet or more. The upthrow is uniformly on the eastern side of the
fault.
At its southern end, as mapped in Fig. 76, there is no perceptible
throw at the surface, but various marks of violence are manifested in
the fissuring of the hillside and the snapping of small trees. About a
quarter of a mile from this point, the fault crosses a tributary
stream, where the throw amounts to two feet, and the same distance
farther on it meets the Chedrang river, the bed of which it crosses
many times in its short course.
[Illustration: FIG. 76.--Plan of Chedrang fault. (_Oldham._)]
Mr. Oldham describes the fault in detail, as observed by him in
February 1898. Here, it will be sufficient to refer to its more
important features, and to its effects on the superficial drainage of
the district. At the spot marked _a_ (Fig. 76) the river, after
running on the west or down-throw side of the fault for nearly half a
mile, meets the scarp, and is ponded back by it for about a quarter of
a mile upstream. For the next half-mile, the river keeps to the
upthrow side of the fault, the scarp of which blocks the tributary
streams from the west, forming a number of small pools. At the last of
these, the total throw is not less than 25 feet. A little farther on,
the fault crosses the Chedrang and causes the waterfall at _b_, the
height of which, owing to the fall of dislodged fragments, does not
exceed nine feet. The fault then runs along the old and now dry bed of
the river, while the stream itself flows in a depression on the
down-throw side. About a quarter of a mile below the waterfall, the
fault crosses the river, and soon after enters a large sheet of water
at _c_, half a mile long, from 300 to 400 yards wide, and with a
maximum depth of 18 feet. At first, the pool spreads on both sides of
the fault, but the inequalities due to the scarp are evidenced by
soundings. At the point where the fault leaves the pool, its throw is
reduced to nothing, and it is just here that the water attains its
greatest depth. To the north the throw increases rather rapidly, to 25
feet in a quarter of a mile. But the peculiarity of this pool is that
it is not, like the others mentioned above, dammed back by the
fault-scarp. There is no barrier at its northern end, where the river
escapes, except that formed by the gradually increasing throw of the
fault. The pool is simply due to the reversal of the natural slope of
the river-bed, caused by the formation of a roll or undulation in the
ground on the upthrow side of the fault. Its recent origin is evident
from the number of dead trees and bamboo clumps still standing in the
water.
For a mile after the fault leaves the pool, its throw varies
considerably. It rises, as already mentioned, from zero to 25 feet. A
little farther on, the fault runs up the side of a spur, the throw
increasing to 31 feet; and, in this part, the violence of the shock
was shown by the dislodgment of blocks of granite as much as 20 feet
in diameter, and by the overthrow or destruction of many trees. After
crossing the spur, the fault returns to the neighbourhood of the
river, and crosses its bed four times, forming pools (_e_, _g_) or
waterfalls (_d_, _f_) according as the scarp occurs on the downstream
or upstream side. The throw of the fault then changes considerably
within little more than half a mile, from 18 feet to zero and again to
20 feet, the undulation so formed producing a large pool (_h_)
entirely on the upthrow side of the fault.
At the point marked _i_ on the map, the river once more crosses the
fault; but the bottom of the valley is filled with alluvium, and,
instead of a waterfall, a large sandy delta spreads down the stream.
The scarp is, however, readily traced on the east side of the river, a
throw of 32 feet being measured. After this, the alluvium becomes of
considerable thickness, and the continuation of the fault is marked by
a short slope, which tilts over the trees when it traverses
forest-land. Leaving the valley of the Chedrang, the fault crosses an
open plain, and is followed with some difficulty to the neighbourhood
of Jhira, where, owing to the thick bed of alluvium, it forms a gentle
roll or undulation of the surface, crossing the main channel of the
Krishnai to the north-east of Jhira. On the west side of this barrier
a large sheet of water, a mile and a half in length, three-quarters of
a mile wide, and 12 feet in depth, gathered over the village of Jhira.
"On the east side of the Jhira lake," says Mr. Oldham, "there is ample
evidence of change of level, for part of the dry land was formerly ...
perpetually under water, and at one place the remains of an old
irrigation channel can be seen.... At the northern end of the lake the
drainage now makes its escape in a broad and shallow sheet of water
over what was once high land covered with _sal_ forest."
This is the last marked feature due to the Chedrang fault. Beyond the
north of Jhira the throw rapidly diminishes, and perhaps dies out
altogether before reaching the low hills lying to the north of that
village.
In several ways, this fault-scarp differs from that formed with the
Japanese earthquake of 1891. Throughout its course the down-throw,
wherever it is perceptible, is invariably to the west; in no place
could any trace of horizontal shifting be detected; and the plane of
the fault, when it traversed rock, is practically vertical.
Whether the scarp was formed by the elevation of the rock to the east
of the fault, or by the depression of that to the west, or by both
such movements at once, there is no decisive evidence; but there are
very good reasons for believing the first alternative to be the true
one. The undulations in the ground which gave rise to the large pools
at _c_ and _h_ (Fig. 76) occur on the east side of the fault. Also,
between the outlet of the lake at Jhira and the point where the
Krishnai rejoins its original channel, the gradient of the river
approaches that of a mountain stream, although the new bed consists of
alluvium, and not of rock. Now, the alluvial plain of this district is
raised so slightly above the sea-level that no subsidence great enough
to have caused the existing gradient could have occurred without the
depressed area being flooded with water. Though some movements may
have taken place on the west side of the fault, it seems clear, then,
that elevation of the rock on the east side was the predominant, if
not the sole, cause of the fault-scarp.
As the Chedrang fault has been described somewhat fully, a brief
reference to the rest will be sufficient The only other known scarp of
any consequence lies about ten miles to the south of the Chedrang
fault, and runs by the village of Samin, with an average course from
E. 30° S. to W. 30° N. Its total length does not exceed 2-1/2 miles.
The down-throw is uniformly to the north, and the throw, which
amounts to ten feet near its centre, gradually diminishes to zero at
either end. Several pools are formed along the course of the
fault-scarp by the blocking of small streams.
_The Bordwar Fracture._--In the map of the epicentral area (Fig. 75),
this remarkable fracture is represented by a dotted straight line. It
is apparently an incipient fault. Though traceable for a distance of
about seven miles, at no point is there any decisive evidence of
either vertical or horizontal displacement; and, even if some doubtful
indications of a change of level should be real, the throw must
certainly be less than one foot. Yet, in the immediate neighbourhood
of the fracture, the violence of the shock was extreme. "Trees have
been overthrown or killed as they stood; a huge mass of rock,
dislodged from near the crest of the hills, has rolled down the slope,
scoring the side of the hill. On the opposite side an equally large
block has been dislodged, and in its downward course cleared a
straight track down the hill; and on the summit a gap has been cleared
by the overthrow of trees along the line of fracture." Being only a
few inches in width where it has rent the solid rock, the fracture was
difficult to follow in many parts of its course. But, through
forest-clad land, its track was marked by "a well-defined band of
about half a mile broad, in which overturned trees are much more
abundant than on either side, and towards the centre of this band the
overturned trees are not only more numerous, but many of the smaller
ones, up to six inches in diameter, have been snapped across by the
violence of the shock."
_Lakes and Pools not due to Faulting._--A few miles to the south of
the Chedrang and Samin faults, and also of the Bordwar fracture,
occurs a group of lakes or pools, represented on the map of the
epicentral area (Fig. 75) by small black ovals. In the gradual
increase in depth from either end, they resemble the two large sheets
of water along the course of the Chedrang fault (_c_ and _h_, Fig.
76), but they differ from them in having no direct connection with any
apparent fault.
One of these pools lies in the valley of the Rongtham river, to the
south of the Samin fault. It seemed, at first sight, to be nothing
more than an ordinary pool, such as may be seen on any mountain
stream. On the bottom, and close to the outlet, however, are coarse,
partially rounded boulders, exactly resembling those farther down the
river; and, as the old bed was followed up, these became coated with a
slight deposit of sand and mud, pointing clearly to a change in the
conditions under which they were formed. The water gradually deepened,
until trees were met standing in the water, but killed by the recent
submergence of their roots. The pool is nearly a quarter of a mile
long, and its greatest depth (12 feet) occurs near the middle, just
where the former stream, with an average depth of about a foot, was
crossed by the track from Darangiri. Towards the upper end, the water
shallows as gradually as it deepens at the other, and ends in a delta
of boulders brought down by the stream above. As no fault could be
discovered in the neighbourhood of the pool, it is evident that its
formation was due to a bend of the river-bed, the maximum change of
level, taking into account the river-slope, being not less than 24
feet.
Similar features characterise the other pools that were examined, some
of which are smaller, and others larger, than that described above.
One, higher up the valley of the Rongtham, has a length of about 1-1/2
mile and a maximum depth of 18 feet. Others of the same type, but of
smaller size, were observed among the Khasi hills, about fifteen miles
south of the Bordwar fissure; and it is probable that many others
would have been found in the intermediate district, which Mr. Oldham
was unable to visit.
_Changes in the Aspects of the Hills._--There are, again, other facts
of considerable interest which point to changes of level over a wide
area; the places where they were noticed being indicated by small
circles in Fig. 75. For instance, from Maophlang, near Shillong, a
road leads to the neighbouring station of Mairang. Before the
earthquake, only a short stretch of this road could be seen from the
former place, as it rounded a spur about three miles away. Now, a much
longer stretch is visible, and it can also be seen passing round the
next, and previously hidden, spur. In this district the movements seem
to have continued with the after-shocks; for, before the earthquake,
the crest only of a ridge about a mile and a half to the west was
visible; while, after it, a considerable portion could be seen, and
much more some months later than immediately after the shock.
Again, from a spot near the southern end of the Chedrang fault, it
used to be only just possible to see the Brahmaputra over an
intervening hill; whereas, now, the whole width of the river has come
into view.
Lastly, at Tura, which is 95 miles west of Maophlang, a battalion of
military police were accustomed to signal by heliograph with another
station, Rowmari, 15 miles farther to the west. This, formerly, could
just be done by means of a ray which grazed a hill between the two
places; it can now be done quite easily, and, in addition, a broad
stretch of the plains east of the Brahmaputra is visible from the same
spot.
_Revision of the Trigonometrical Survey._--The movements described in
the preceding pages are of course referred to points which may
themselves have been displaced, and only a revision of the
trigonometrical survey of the epicentral area and of part of the
surrounding district could determine their absolute magnitude. During
the cold weather of 1897-98, some of the triangles were re-measured by
a member of the trigonometrical survey; but, as the time at his
disposal was short, they were confined to the eastern part of the
epicentral area, as the focus at that time was supposed to lie under
the Khasi hills. The positions of some of these stations are indicated
by crosses in Fig. 75; and in Fig. 77 the more important triangles are
shown. In the revised work, all tower stations, consisting of brick
towers built on alluvium, were omitted, as it could not be assumed
that they had been undisturbed by displacements of the superficial
beds.
In re-calculating the lengths of the sides, the side
Rangsanobo-Taramun Tila was adopted as the initial base, and the
height of Rangsanobo as the initial height; a choice which later
experience showed to be unfortunate, for Taramun Tila probably lies
just outside, and Rangsanobo within, the epicentral area. Of the 16
sides, whose old and new lengths were compared, only one was found to
be apparently unchanged, two were shortened by an inch or two, while
the others were all lengthened by amounts varying from one to eight
or nine feet, the numbers affixed to the sides in Fig. 77 denoting the
calculated increases in feet. Assuming the new base-line to be
unaltered by the earthquake movements, these changes imply the
following displacements of the principal stations:--Thanjinath 6 feet,
Mun 4, and Laidera 2, feet to the north; Mopen 5, Dinghei 9, Landau
Modo 12, and Umter 11, feet to the north-west; and Mosingi 3, and
Mautherrican 5, feet to the west. At the same time, the height of
most of the stations was found to be increased with reference to that
of Rangsanobo: Mun by 2 feet, Thanjinath and Umter by 3, Mosingi by 4,
Taramun Tila and Laidera by 6, Dinghei by 7, Landau Modo by 17, and
Mautherrican by 24, feet; while the height of Mopen seems to have been
diminished by 4 feet. Thus, at first sight, these calculations appear
to indicate "a general elevation and extension of the hills, such as
might follow on a bulging upwards of the surface due to the extension
of a large mass of molten matter underground."
[Illustration: FIG. 77.--Re-triangulation of Khasi hills.
(_Oldham._)]
Unfortunately, as Mr. Oldham shows, a very different, and more
probable, interpretation may be given of these results; for all the
calculated changes are rendered uncertain by the choice of the two
stations which form the ends of the new base-line. One at least may
have been displaced by the structural movements within the epicentral
area; and, moreover, the line joining them runs nearly north and
south. As compression in this direction is to be expected, it is
probable that this line was shortened; and the assumption that its
length was unchanged would therefore lead to an apparent expansion of
all the other sides.
The calculated changes seem to favour this explanation to a great
extent. The sides joining Mopen, Rangsanobo, and Thanjinath run nearly
east and west, and are apparently lengthened by 4.9 and 3.4 feet
respectively; while, of the four sides joining these stations to
Mosingi and Mun, lying next to the north, two are nearly or quite
unchanged, and the others increased by 2.3 and 3.2 feet. Again, the
estimated increase of the Mosingi-Mun line is 4.4 feet; while the four
sides joining these stations to the next northerly group are
increased by small amounts--namely, 1.2, 2.6,-0.3, and 2.4 feet. Thus,
the apparent expansion that should have occurred in these more or less
northerly sides is lessened, or roughly compensated, probably by a
compression of the whole region in a meridianal direction.
For a similar reason, the slight general upheaval of the hills
indicated by the repeated calculations, must be regarded as doubtful,
for it depends on the assumed fixity of the station of Rangsanobo,
whereas it is more probable that it was the height of Taramun Tila
that remained unchanged. Reducing the calculated heights of all the
other stations by six feet (the assumed rise of the latter), it
follows that, on the whole, the height of the Khasi hills underwent
but little change, except at Mautherrican and Landau Modo, and the
secondary stations of Mairang and Kollong Rock, near Maonoi. The
apparent elevations of 24, 17, 11, and 15 feet at these places exceed
the probable error of the observations; and it is worthy of notice
that all four stations lie close to the edge of fault-scarps, while
Landau Modo is not far from two of the pools formed by distortion of
the surface unaccompanied by faulting.
If, then, the revised triangulation of the Khasi hills has failed to
provide absolute measures of the displacements in the epicentral area,
it has, nevertheless, proved that important movements, both horizontal
and vertical, have taken place.
_Distribution of the Structural Changes._--The boundary of the
epicentral area, as drawn in Figs. 68 and 75, lays no claim to great
accuracy; but its departure from the true line is probably in no place
considerable. It must evidently include all the districts where
marked structural changes occurred, and must therefore extend east of
Maophlang and west of Tura. Towards the north, these changes have been
traced to the foot of the Garo hills, and there is some, though not
very certain, evidence of alterations of level along the course of the
Brahmaputra. The very large number of after-shocks recorded at Borpeta
and Bijni also points to an extension of the epicentral area beyond
these places. To the east, the course of the boundary becomes
doubtful, but it must pass close to Gauhati and east of Shillong, and
probably ends a short distance beyond Jaintiapur. The southern
boundary must coincide nearly with the north edge of the alluvial
plains of Sylhet, for there is no evidence of its intrusion into the
plains. On the west side, the epicentral area includes the Garo hills
and part of the alluvial plain to the west; and, from the large number
of after-shocks felt at Rangpur and Kaunia, and the great violence of
the shock at the former, we may infer that both places lie within the
boundary-line. If, then, there is no great error in the mapping of
this line, it follows that the epicentre was about 200 miles long from
east to west, not less than 50, and possibly as much as 100, miles in
maximum width, and contained an area of at least 6000 square miles.
Near the boundary, the permanent displacements must have been
comparatively small; but they were certainly marked in the northern
part of the Assam hills for a distance of 100 miles from east to west.
At the limits of the latter area, as Mr. Oldham remarks, "the evidence
points to the changes being of the nature of long, low rolls, the
change of slope being insufficient to cause any appreciable change in
the drainage channels. Then comes a zone in which the surface changes
are more abrupt, the slopes of the stream beds have been altered so as
to cause conspicuous changes in the nature of the streams, but any
fracture or faulting which may have taken place has died out before
the surface was reached. And north of this, close to the edge of the
hills, the rocks have been fractured and faulted right up to the
surface."
ORIGIN OF THE EARTHQUAKE.
Almost every feature of the great earthquake points to an origin very
different from that of the others described in this volume. The
suddenness with which the shock began, its unusual duration, and the
occurrence of many maxima of intensity, are inconsistent with a simple
fault-displacement. Again, the excessive velocities of projection at
Rambrai and elsewhere, the existence of isolated fault-scarps and
fractures, the local changes of level, the compression indicated by
the revised trigonometrical survey, the wide area over which these
structural changes took place, and the numerous distinct centres of
subsequent activity, all these phenomena demonstrate the intense and
complex character of the initial disturbances, as well as the
widespread bodily displacement of the earth's crust within the
epicentral area. There may, it is conceivable, have been a number of
foci, nearly or quite detached from one another, and giving rise to a
group of nearly concurrent shocks. Or--and this is a far more probable
supposition--there may have been one vast deep-seated centre, from
which off-shoots ran up towards the surface, each partaking to a
greater or less degree in the movement within the parent focus.
As Mr. Oldham points out, we have recently become acquainted with a
structure exactly corresponding to that which is here inferred. The
great thrust-planes, so typically developed in the Scottish Highlands,
are only reversed faults which are nearly horizontal instead of being
highly inclined; and they are accompanied by a number of ordinary
reversed faults running upwards to the surface. In Fig. 78, the main
features of a section drawn by the Geological Survey of Scotland are
reproduced; T, T, representing thrust planes, and _t_, _t_, minor
thrusts or faults. A great movement along one of the main
thrust-planes would carry with it dependent slips along many of the
secondary planes. Direct effects of the former might be invisible at
the surface, except in the horizontal displacements that would be
rendered manifest by a renewed trigonometrical survey; whereas the
latter might or might not reach the surface, giving rise in the one
case to fissures and fault-scarps, in the other to local changes of
level, and in both to regions of instability resulting in numerous
after-shocks.
[Illustration: FIG. 78.--Diagram of Thrust-planes.]
The enormous dimensions of the parent focus will be obvious from the
phenomena that have been described above. Mr. Oldham has traced the
probable form of the epicentre. It may in reality be neither so
simple nor so symmetrical as is represented in Fig. 75, but there are
good reasons for thinking that it does not differ sensibly either in
size or form from that laid down. The part of the thrust-plane over
which movement took place must therefore have been about 200 miles
long, not less than 50 miles wide, and between 6000 and 7000 square
miles in area. With regard to its depth, we have no decisive
knowledge. It may have been about five miles or less; it can hardly
have been much greater.
It is a strain on the imagination to try and picture the displacement
of so huge a mass. We may think, if we will, of a slice of rock three
or four miles in thickness and large enough to reach from Dover to
Exeter in one direction and from London to Brighton in the other; not
slipping intermittently in different places, but giving way almost
instantaneously throughout its whole extent; crushing all before it,
both solid rock and earthy ground alike; and, whether by the sudden
spring of the entire mass or by the jar of its hurtling fragments,
shattering the strongest work of human hands as easily as the
frailest. Such a thrust might well be sensible over half a continent,
and give rise to undulations which, unseen and unfelt, might wend
their way around the globe.
REFERENCES.
1. AGAMENNONE, G.--"Notizie sui terremoti osservati in Italia
durante l'anno 1897 (Terremoto dell' India poco dopo il
mezzogiorno del 12 giugno)." _Ital. Sismol. Soc. Boll._, vol.
iii., pte. ii., 1897, pp. 249-293.
2. ---- "Il terremoto dell' India del 12 giugno 1897." _Ibid._, vol.
iv., 1898, pp. 33-40.
3. ---- "Eco in Europa del terremoto indiano del 12 giugno 1897."
_Ibid._, vol. iv., 1898, pp. 41-67. (See also the same volume,
pp. 167-172.)
4. BARATTA, M.--"Il grande terremoto indiano del 12 giugno 1897."
_Ital. Soc. Geogr. Boll._, vol. x., 1897, fasc. viii.
5. CANCANI, A.--"I pendoli orizzontali del R. Osservatorio
geodinamico di Rocca di Papa, ed il terremoto indiano del 12
giugno 1897." _Ital. Sismol. Soc. Boll._, vol. iii., 1897, pp.
235-240.
6. HEATH, T.--"Note on the Calcutta Earthquake (June 12th, 1897) as
recorded by the bifilar pendulum at the Edinburgh Royal
Observatory." _Edinb. Roy. Soc. Proc._, 1897, pp. 481-488.
7. OLDHAM, R.D.--"Report on the Great Earthquake of 12th June 1897."
_Mems. Geol. Surv. of India_, vol. xxix., 1899, pp. i.-xxx.,
1-379, with 44 plates and 3 maps.
8. ---- "List of After-shocks of the Great Earthquake of 12th June
1897." _Ibid._, vol. xxx., pt. i., 1900, pp. 1-102.
9. ---- "On Tidal Periodicity in the Earthquakes of Assam." _Journ.
Asiat. Soc._, vol. lxxi., 1902, pp. 139-153.
FOOTNOTES:
[69] According to some reports, the earthquake was felt in Italy. At
Livorno, the first movements were registered by seismographs at 11.17
A.M. (G.M.T.), and tremors were noticed by some persons at rest at
about 11.15 A.M. At Spinea, a sensible undulatory shock from
south-east to north-west, and lasting about four seconds, was felt at
the moment when all the seismographs were set in motion by the Indian
earthquake. In spite of the great distance, the perception of the
earthquake in Italy is not impossible, but the records seem to me to
refer to local tremors rather than to the very slow evanescent
oscillations of a very distant earthquake.
[70] All the times in this section are referred to Madras mean time,
which is 5h. 20m. 59.2s. in advance of Greenwich mean time. In the
next section it will be found convenient to use the latter standard.
[71] It may be useful to give references to works in English in which
the principal instruments for registering distant earthquakes are
described. For Cancani's vertical pendulum, see _Brit. Assoc. Rep._,
1896, pp. 46-47; Darwin's bifilar pendulum, _Brit. Assoc. Rep._, 1893,
pp. 291-303, and _Nature_, vol. 1., 1894, pp. 246-249; Milne's
horizontal pendulum, _Seismology_, pp. 58-61; Rebeur-Paschwitz's
horizontal pendulum, _Brit. Assoc. Rep._, 1893, pp. 303-308.
[72] The beginnings of the second and third phases are shown more
clearly in the record of the vertical pendulum at Catania, a record,
however, that will not bear the reduction necessary for these pages.
[73] _Geol. Mag._, vol. x., 1893, pp. 356-360.
[74] _Irish Acad. Trans._, vol. xxi, 1848, p. 52.
[75] _Irish Acad. Trans._, vol. xxi., 1848, pp. 55-57.
[76] _Neapolitan Earthquake of 1857_, vol. i., 1862, pp. 376-378.
[77] _Japan Seismol. Soc. Trans._, vol. i., pt. II., 1880, pp. 33-35.
[78] _Geol. Mag._, vol. ix., 1882, pp. 257-265.
CHAPTER X.
CONCLUSION.
In this concluding chapter, I propose to give a summary of the results
at which we have arrived from the study of recent earthquakes, and
this can, I think, be done best by describing what may be regarded as
an average or typical earthquake, though it may be convenient
occasionally to depart slightly from such a course. Few shocks have
contributed more to our knowledge than the majority of those described
in this volume; but, on certain points, we gain additional information
from the investigation of other earthquakes, and these are referred to
when necessary for the purpose in view.
FORE-SHOCKS.
At the outset, we are met by a question of some interest and great
practical importance--namely, whether there are any constant signs of
the coming of great earthquakes by means of which their occurrence
might be predicted and their disastrous effects mitigated.
Excluding the Ischian earthquakes, which belong to a special class, it
is evident that there is generally some slight preparation for a great
earthquake. For a few hours or days beforehand, weak shocks and
tremors are felt or rumbling noises heard within the future
meizoseismal area. But, unfortunately, it has not yet been found
possible to distinguish these disturbances from others of apparently
the same character which occur alone, so that for the present they
fail to serve as warnings.
In Japan, where the organisation of earthquake-studies is more
complete than elsewhere, it is possible that a vague forecast might be
made, if the distribution of the fore-shocks of the earthquake of 1891
should prove to be a general feature of all great earthquakes. It was
at first supposed that this earthquake occurred without preparation of
any kind; but a closer analysis of the records shows that during the
previous two years there was a very decided increase in the seismic
activity of the district, and also that the distribution of the
epicentres marked out the future fault-scarp, and at the same time
exhibited a tendency to comparative uniformity over the whole
fault-region.
For the present, then, the only warning available is that given by the
preliminary sound, which may precede the strongest vibrations by as
much as five or ten or even more seconds. Though two or three seconds
may elapse before its character is recognised, the fore-sound thus
allows time for many persons to escape from their falling houses. Some
races, however, are less capable of hearing the sound than others, and
this may be one reason why Japanese earthquakes are so destructive of
human life.
DISTURBED AREA.
It is usual with some investigators to measure the intensity of an
earthquake roughly by the extent of its disturbed area. The depth of
the seismic focus must of course have some influence on the size of
this area, and this condition is only neglected because we have no
precise knowledge of the depth in any case. Thus, Mr. Oldham regards
the Indian earthquake of 1897 as rivalling the Lisbon earthquake of
1755, which is generally considered to hold the first place, because
its disturbed area was not certainly exceeded by that of the latter.
That disturbed area is, however, an untrustworthy measure of intensity
will be evident from the following table, in which the earthquakes
described in this volume (omitting those of Ischia) are arranged as
nearly as may be in order of intensity, beginning with the
strongest:--
Earthquake. Disturbed Area
in Sq. Miles.
Indian 1,750,000
Japanese 330,000
Neapolitan 39,200
Charleston 2,800,000
Riviera 219,000
Andalusian 174,000
Hereford 98,000
Inverness 33,000
Here we see that the Charleston earthquake was perceptible over a
greater area than the Indian earthquake, while the Neapolitan
earthquake was inferior to that of Hereford in this respect. The
explanation of course is that the boundaries of the disturbed areas
are isoseismal lines corresponding to different degrees of intensity,
the inhabitants of Great Britain and the United States being evidently
more sensitive to weak tremors, or more observant, than those of
Italy, Spain, or Central Asia. The only disturbed areas that are
bounded by isoseismals of the same intensity are the two last. Very
roughly, then, we may say that the intensity of the Hereford
earthquake was three times as great as that of the Inverness
earthquake.
POSITION OF THE EPICENTRE.
One of the first objects in the investigation of an earthquake is to
determine the position and form of the epicentre. In a few rare cases,
as in the Japanese and Indian earthquakes, when the fault-scarp is
left protruding at the surface, only careful mapping is required to
ascertain both data. But, in the great majority of earthquakes, the
fault-slip dies out before reaching the surface and the position of
the epicentre is then inferred by methods depending chiefly on the
time of occurrence or on the direction or intensity of the shock.
At first sight, methods that involve the time of occurrence at
different places seem to be of considerable promise. No scientific
instruments are so widely diffused as clocks and watches; but, on the
other hand, few are so carelessly adjusted. It is the exception,
rather than the rule, to find a time-record accurate to the nearest
minute; and, as small errors in the time may be of consequence,
methods depending on this element of the earthquake are seldom
employed. If, however, the number of observations is large for the
size of the disturbed area, the construction of coseismal lines may
define approximately the position of the epicentre. In the Hereford
earthquake of 1896, the centre of the innermost coseismal line (Fig.
62) is close to the region lying between the two epicentres.
The method of locating the epicentre by means of the intersection of
two or more lines of direction of the shock was first suggested by
Michell in 1760,[79] and has been employed by Mallet in investigating
the Neapolitan earthquake, by Professors Taramelli and Mercalli in
their studies of the Andalusian and Riviera earthquakes, as well as by
other seismologists. The diversity of apparent directions at one and
the same place caused its temporary neglect, until Professor Omori
showed in 1894 that the mean of a large number of measurements gives a
trustworthy result (p. 19). His interesting observations should
reinstate the method to its former place among the more valuable
instruments at the disposal of the seismologist.
No observations, however, are at present so valuable for the purpose
in view as those made on the intensity of the shock. For many years,
it has been the custom to regard the epicentre as coincident with the
area of greatest damage to buildings; and, when the area is small, the
assumption cannot be much in error. It is of course merely a rough way
of obtaining a result that is generally given more accurately by means
of isoseismal lines; but there are exceptional cases, such as the
Neapolitan and Ischian earthquakes, when the destruction wrought by
the earthquake furnishes evidence of the greater value.
A single isoseismal accurately drawn not only gives the position of
the epicentre with some approach to exactness, but also by the
direction of its longer axis determines that of the originating fault.
When two or three such lines can be traced, the relative position
supplies in addition the hade of the fault (p. 219). The successful
application of the method requires, it is true, a large number of
observations, and these cannot as a rule be obtained except in
districts that are somewhat thickly and uniformly populated, such as
those surrounding the cities of Hereford and Inverness. In the
Charleston earthquake, also, the position and form of the epicentres
were deduced from the trend of isoseismal lines based on the damage to
railway-lines and various structures within a sparsely inhabited
meizoseismal area.
In a few cases, of which the Indian earthquake may be regarded as
typical, a fourth method has recently been found of service. The
numerous after-shocks which follow a great earthquake originate for
the most part within the seismic focus of the latter; and, as they
usually disturb a very small area, it is not difficult to ascertain
approximately the positions of their epicentres. Some, as in the
Inverness after-shocks of 1901, result from slips in the very margin
of the principal focus; but, as a rule, the seat of their activity
tends to contract towards a central region of the focus. Bearing in
mind, then, that some of the succeeding shocks originate at and beyond
the confines of the focus, and that others may be sympathetic shocks
precipitated by the sudden change of stress, it follows that the
shifting epicentres of the true after-shocks map out, in part at any
rate, the epicentral area of the principal earthquake.
DEPTH OF THE SEISMIC FOCUS.
It is much to be regretted that we have no satisfactory method of
determining so interesting an element as the depth of the seismic
focus. That it amounts to but a few miles at the most is certain from
the limited areas within which slight shocks are felt or disastrous
ones exhibit their maximum effects. Nor can we suppose that the rocks
at very great depths are capable of offering the prolonged resistance
and sudden collapse under stress that are necessary for the production
of an earthquake.
The problem is evidently beyond our present powers of solution, and
its interest is therefore mainly historical. All the known methods are
vitiated by our ignorance of the refractive powers of the rocks
traversed by the earth-waves. But, even if this ignorance could be
replaced by knowledge, most of the methods suggested are open to
objection. Falb's method, depending on the time-interval between the
initial epochs of the sound and shock, is of more than doubtful value.
Dutton's, based on the rate of change of surface-intensity, is
difficult to apply, and in any case gives only an inferior limit to
the depth. Time-observations have been employed, especially in New
Zealand; but the uncertainty in selecting throughout the same phase of
the movement, and the large errors in the estimated depth resulting
from small errors in the time-records, are at present most serious
objections. There remains the method devised by Mallet, and, though he
claimed for it an exaggerated accuracy, it still, in my opinion, holds
the field against all its successors. When carefully applied, as it
has been by Mallet himself, by Johnston-Lavis and Mercalli, we
probably obtain at least some conception of the depth of the seismic
focus.
Professor Omori and Mr. K. Hirata have recently[80] lessened the chief
difficulty in the application of Mallet's method. They have deduced
the angle of emergence from the vertical and horizontal components of
the motion as registered by seismographs, instead of from the
inclination of fissures in damaged walls. In two recent earthquakes
recorded at Miyako in Japan, they find the angle of emergence to be
7.2° and 9° respectively, the corresponding depths of the foci being
5.6 and 9.3 miles. These are probably the most accurate estimates that
we possess, and it will be noted that they differ little from the mean
values obtained for the Neapolitan, Andalusian, and Riviera
earthquakes--namely, 6.6, 7.6, and 10.8 miles.
NATURE OF THE SHOCK
In one respect, the earthquakes described above fail to represent the
progress of modern seismology. They furnish no diagrams made by
accurately constructed seismographs within their disturbed areas. The
curve reproduced in Fig. 36, as already pointed out, is no exception
to this statement. For another reason, the records that were obtained
in Japan of the earthquake of 1891 are trustworthy for little more
than the short-period initial vibrations; for, owing to the passage of
the surface-waves, visible in and near the meizoseismal area, the
Japanese seismographs registered the tilting of the ground rather than
the elastic vibrations that traversed the earth's crust.
Notwithstanding this defect, personal impressions of an
earthquake-shock give a fairly accurate, if incomplete, idea of its
nature. Nearly all observers placed under favourable conditions agree
that an earthquake begins with a deep rumbling sound, accompanied,
after the first second or two, by a faint tremor which gradually, and
sometimes rapidly, increases in strength until it merges into the
shock proper, which consists of several or many vibrations of larger
amplitude and longer period, and during which the attendant sound is
generally at its loudest; the earthquake dying away, as it began, with
tremors and a low rumbling sound.
[Illustration: FIG. 79.--Seismographic Record of Tokio Earthquake
of 1894. (_Omori._)]
The vibrations that produce the sensible shock are by no means all
that are present during an earthquake. The Indian earthquake, for
instance, seemed to last about three or four minutes at Midnapur; but
the movements of the bubble of a level showed that the ground
continued to oscillate for at least five minutes longer (p. 280). Many
of these unfelt waves are rendered manifest by seismographs, although
there are still others that elude registration either from the extreme
shortness or the great length of their periods.
In Fig. 79 is shown the principal part of a diagram obtained at Tokio
during the Japanese earthquake of June 20th, 1894 (p. 18), the curve
representing the N.E.-S.W. component of the horizontal motion during
the first 25 seconds of the record. The instrument employed is one
specially designed for registering strong earthquakes, and is
unaffected by very minute tremors. Those which formed the commencement
of this earthquake lasted for about 10 seconds, as shown by ordinary
seismographs, and the vibrations had attained a range of a few
millimetres before they affected the instrument in question. For the
first 2-1/2 seconds, they occurred at the rate of four or five a
second. The motion then suddenly became violent, and the ground was
displaced 37 mm. in one direction, followed by a return movement of 73
mm., and this again by one of 42 mm., the complete period of the
oscillation being 1.8 seconds. The succeeding vibrations were of
smaller amplitude and generally of shorter period for a minute and a
half, then dying out during the last three minutes as almost
imperceptible waves with a period of two or more seconds.[81]
Though incomplete in some respects, this diagram illustrates clearly
the division of the earthquake-motion into three stages--namely, the
preliminary tremors, the principal portion or most active part of an
earthquake, and the end-portion or gradually evanescent slow
undulations. In all three stages, however, both tremors and slow
undulations may be present; and, as the latter, owing to their long
period, are more or less insensible to human beings, the ripples of
the final stage give the impression of a tremulous termination as
described above. The duration of each stage varies considerably in
different earthquakes. Thus, in a valuable study of 27 earthquakes
recorded at Miyako, in Japan, during the years 1896-98, Messrs. Omori
and Hirata show[82] that the duration of the preliminary stage varies
from 0 to 26 seconds, with an average of about 10 seconds; that of the
principal portion from 0.7 to 26 seconds, also with an average of
about 10 seconds; and that of the end portion from 28 and 105 seconds,
with an average of about one minute. The total apparent duration,
however, depends on the instrument employed; one of the earthquakes,
that of April 23rd, 1898, disturbing the seismograph at Miyako for two
minutes; while, at Tokio, a horizontal pendulum designed by Professor
Omori oscillated for at least two hours. The periods of both ripples
and slow undulations, again, vary from one earthquake to another; but
it is worthy of notice that the average period of the undulations is
almost constant in all three stages of the motion, being 1.1, 1.3, and
1.3 seconds, respectively, for the east-west component of the
horizontal motion, and 1.0 second throughout for the north-south
component. For the ripples, the average period is .08 second in the
preliminary stage, .10 second in the principal portion, and .08 second
again in the end portion; those of the principal portion being
slightly larger in amplitude, as well as longer in period, than the
ripples of the first and third stages.
SOUND-PHENOMENA.
Besides the ripples already mentioned, there are others of still
smaller amplitude and shorter period that are sensible, but as a rule
only just sensible, to us as sounds. All the known evidence points to
the extraordinary lowness of the earthquake-sound. According to some
observers, it seems as if close to their lower limit of audibility;
while others, however intently they may listen, are unable to hear the
slightest noise. In other words, the most rapid vibrations present in
an earthquake do not recur at a rate of much more than about 30 to 50
per second; or, if they do, they are not strong enough to impress the
human ear.
To most observers, the sound seems to increase and decrease in
intensity with the shock, and so gradually and smoothly does this
change take place that the sound is frequently mistaken for that of an
underground train approaching the observer's house, passing beneath
it, and receding in the opposite direction. Some persons, especially
if situated within the meizoseismal area, hear also loud crashes in
the midst of the rumbling sound and simultaneously with the strongest
vibrations. At a moderate distance, say from 30 to 40 miles, the sound
becomes more harsh and grating while the shock is felt; and, at a
greater distance, even this change disappears, and nothing is heard
but an almost monotonous sound like the low roll of distant thunder.
The explanation of this is that the sound-vibrations are of different
periods and varying amplitude, and the limiting vibrations tend to
become inaudible with increasing distance, the lower on account of
their long period, the higher owing to their small amplitude.
The magnitude of the sound-area depends, even more than that of the
disturbed area, on the personal equation of the observers. The lower
limit of audibility varies not only in different individuals, but also
in different races. In Great Britain, it is doubtful whether an
earthquake ever occurs unaccompanied by sound; and in the meizoseismal
area the noise is heard by nearly all observers. With Italians, the
average lower limit of audibility is higher than with the Anglo-Saxon
race; slight shocks frequently occur without noticeable sound, but
with strong ones, the larger number of observers is sure to include
one or more capable of hearing the rumbling noise. The Japanese are,
however, seldom affected by the most rapid earthquake-vibrations, and
the strongest shocks may be unattended by any recorded sound. The
result is manifest in the size of the sound-area in different
countries. In the Hereford earthquake, the sound-area contained 70,000
square miles; in the Neapolitan earthquake, about 3,300 square miles;
while, in Japanese earthquakes, the sound is rarely heard more than a
few miles from the epicentre.
Another effect of this personal equation of the observers is that the
sound-vibrations apparently outrace those of longer period. The
Italians, for instance, generally hear the sound that precedes the
shock, and more rarely the weaker sound that follows it. In Japan,
only the earlier sound-vibrations, if any, seem to be audible. In
Great Britain, on the contrary, the fore-sound is perceptible to four,
and the after-sound to three, out of every five observers; and these
proportions are maintained roughly to considerable distances from the
epicentre. It follows, therefore, that the sound-vibrations and those
which constitute the shock must travel with nearly, if not quite, the
same velocity; and that the greater duration of the sound is due
either to the prolongation of the initial movement or to the
overlapping of the principal focus by the sound-focus. Neither
alternative can be regarded as improbable, but observations made on
British earthquakes point to the latter explanation as the true one.
It will be sufficient to refer to two phenomena in support of this
statement. In the first place, the percentage of observers who hear
the fore-sound varies with the direction from the epicentre. Thus,
during the Inverness earthquake of 1901, the majority of observers in
Aberdeenshire regarded the sound as beginning and ending with the
shock; while, in counties lying more nearly along the course of the
great fault, the sound was generally heard both before and after the
shock (p. 253). In this case, then, the initial and concluding sound
vibrations must have come chiefly from the margins of the seismic
focus; and those from the margin nearest to an observer would be more
sensible than those from the farther margin. Again, in slight
earthquakes, such as the Cornwall earthquake of April 1, 1898,[83] the
curves of equal sound intensity, while their axes are parallel to
those of the isoseismal lines, are displaced laterally with respect to
these curves, owing to the arrival of the strongest sound-vibrations
from the upper margin of an inclined seismic focus.
When a fault-slip occurs, the displacement is obviously greatest in
the central region, and dies out gradually towards the margins of the
focus. The phenomena described above show that the evanescent
displacement within these margins generate sound-vibrations only; and
that the greater slip within the central region produces also the more
important vibrations that compose the shock. As the former are
perceptible over a limited district, while the latter may be felt
through half a continent, it is clear that the sound-area should bear
no fixed relation in point of size to the disturbed area, but should
be comparatively greater for a slight shock than for a strong one.
VELOCITY OF THE EARTH-WAVES.
If we consider only the earthquakes here described, we see at once how
great is the diversity in the estimated velocity of the earth-waves.
On the one hand, we have a value as high as 5.2 kms. per sec. for the
Charleston earthquake, and, at the other end of the scale, a value of
0.9 km. per sec. for the Hereford earthquake. Between them, and
equally trustworthy, lie the estimates of 3.0 km. per sec. for the
Indian earthquake, and 2.1 kms. per sec. for the Japanese earthquake
and its immediate successors.
It is difficult to account entirely for such discordance. Errors of
observation may be responsible for a small part of the differences.
The initial strength of the disturbance appears to have some effect,
and the nature of the rocks traversed must be a factor of consequence
when the distances in question are not very great. In the Japanese and
Hereford earthquakes, all three may have combined to produce the
divergent results, the distance in these cases being only 275 and 142
kms. respectively.
In the Indian and Charleston earthquakes, the distances are much
greater (1944 and 1487 kms.), and the variety of rocks traversed must
tend to give a truer average. In the former, the result obtained (3.0
kms. per sec.) agrees so closely with the velocity of the long-period
undulations of distant earthquakes as to suggest that it was these
waves that were timed at the stations west of Calcutta and disturbed
the magnetographs at Bombay.[84]
Omitting, then, the Indian estimate, we find that, for the Japanese
and Charleston earthquakes, the velocity increases with the distance
as measured along the surface. To a certain extent, such a result
might have been expected, had we assumed the earthquake-waves to
travel along the chords joining the focus to very distant places of
observation.
The wave-paths that penetrate the earth are straight lines, however,
only when the conditions that determine the velocity are uniform
throughout, and such uniformity we have no reason to expect. From what
we know of the earth's interior, there can, indeed, be little doubt
that the velocity of earthquake-waves increases with the depth below
the surface, and that the wave-paths in consequence are curved lines
with their convexity downwards. It would be out of place to state more
than the principal result of the recent investigations by Dr. A.
Schmidt[85] and Prof. P. Rudzki[86] on this subject. These are based on
the assumptions that the velocity increases with the depth below the
surface, and that it is always the same at the same depth. From the
focus of the earthquake, wave-paths diverge in all directions. Those
which start horizontally curve upwards, and intersect the surface of
the earth in a circle dividing the whole surface into two areas of very
unequal size. Within the small area, the surface-velocity is infinite
at the epicentre, and decreases outwards until it is least on the
boundary-circle. In the larger region beyond, the surface-velocity
increases with the distance from the epicentre, until, at the antipodes
of that point, it is again infinite. But, as the depth of the focus is
always slight compared with the radius of the earth, the small circular
area surrounding the epicentre is practically negligible, and we may
regard the surface-velocity of the waves that traverse the body of the
earth as a quantity that continually increases with the distance from
the epicentre.
How fully this interesting theoretical result has been confirmed is
well shown in Mr. Oldham's recent and very valuable investigation on
the propagation of earthquake-motion to great distances.[87] A study
of the records of the Indian earthquake revealed the existence of
three series of waves, the first two consisting in all probability of
longitudinal and transversal waves travelling through the body of the
earth, and the third of undulations spreading over its surface (pp.
282-285). Extending his inquiries to ten other earthquakes originating
in six different centres, Mr. Oldham distinguishes the same three
phases in their movements; the third phase being the most constantly
recorded, the second less so, while the first phase is the most
frequently absent. With the exception of a few very divergent records,
the initial times of these phases and the maximum epoch of the third
phase are plotted on the accompanying diagram (Fig. 80), in which
distances from the epicentre in degrees of arc are represented along
the horizontal line and the time-interval in minutes along the
perpendicular line. The dots near the two lower curves refer to the
records of the heavily weighted Italian instruments, and the crosses
to those of the light horizontal pendulums, which respond somewhat
irregularly to the motion of the first two phases (p. 282). In the
third phase, there is less divergence between the indications of the
two classes of instruments, and dots are used in each case for the
initial, and crosses for the maximum epoch.
[Illustration: FIG. 80.--Time-curves of principal epochs of
earthquake-waves of distant origin. (_Oldham._)]
Of the smoothed curves drawn between these series of points, those
marked A, B, and C represent the time-curves of the beginnings of the
first, second, and third phases respectively, while D is the
time-curve for the maximum of the third phase.
The concavity of the two lower lines towards the horizontal base-line
shows that the surface-velocity of the corresponding waves increases
rapidly with the distance, far more so than would be possible with
rectilinear motion. The rates at which these waves travel through the
earth therefore increase with the depth, and the wave-paths must in
consequence be curved lines convex towards the centre of the earth.
If the time-curves A and B were continued backwards to the origin,
their inclinations at that point to the horizontal line give the
initial velocities of the corresponding waves, which prove to be about
5 and 3 kms. per sec. respectively. Now, according to recent
experiments made by Mr. H. Nagaoka on the elastic constants of
rocks,[88] the mean velocity of seven archaean rocks is 5.1 kms. per
sec. for the longitudinal waves, and 2.8 kms. per sec. for the
transversal waves--values which agree so closely with those obtained
for the first two series of earthquake-waves as to leave little doubt
with regard to their character.
The other time-curves, C and D, corresponding to the initial and
maximum epochs of the third phase, are practically straight lines.
Some of the records are slightly discordant for the average curve,
especially for the initial epoch; but it is often difficult to define
the commencement of this phase with precision. At any rate, the
observations show no distinct sign of an increase in the
surface-velocity of these waves with the distance from the origin. It
may therefore be concluded that they travel along the surface with
velocities which are practically constant for each individual
earthquake, the largest waves at the rate of about 2.9 kms. per sec.,
and the advance waves with a velocity of about 3.3 kms. per sec.,
rising occasionally to over 4.0 kms. per sec.
STRUCTURAL CHANGES IN THE EPICENTRAL AREA.
Changes of elevation have long been known as accompaniments of great
earthquakes, though many of the earlier observations and measurements
left much to be desired in accuracy and completeness. The Japanese
earthquake of 1891, however, placed the reality of such movements
beyond doubt, and revealed the existence of a fault-scarp, with a
height in one place of 18 or 20 feet, and a length of at least 40, if
not of 70, miles. In the Indian earthquake of 1897, the fault-scarps
were shorter, though more pronounced in character, the largest known
(the Chedrang fault) being about 12 miles long, and having a maximum
throw at the surface of 35 feet. In some other recent earthquakes,
also, remarkable fault-scarps have been developed. After the great
shocks felt in Eastern Greece on April 20th and 27th, 1894, a fissure
was traced for a distance of about 34 miles, running in an
east-south-east and west-north-west direction through the epicentral
district, and varying in width from an inch or two to more than three
yards. That it was a fault, and not an ordinary fissure, was evident
from its great length, its uniform direction, and its independence of
geological structure. The throw was generally small, in no place
exceeding five feet.[89] Again, in British Baluchistan, after the
severe earthquake of December 20th, 1892, a fresh crack was observed
in the ground running for several miles in a straight line parallel to
the axis of the Khojak range. It coincided almost exactly with a line
of springs, and was clearly produced by a fresh slip along an old line
of fault, for before the earthquake it had the appearance of an old
road, and the natives assert that the ground has always cracked along
this line with every severe shock. In 1892, the change in relative
height of the two sides of the fault was small, in one place where it
was measured being only two inches.[90]
But other changes, besides those in a vertical direction, occasionally
take place; though, owing to their recent discovery, comparatively few
examples are as yet known. While the throw of the Japanese fault
varied greatly in amount, and once even in direction, there was also a
constant shift towards the northwest of the ground on the north-east
side of the fault, the displacement at one spot being as much as 13
feet. In the fault-scarp formed in 1894 in Eastern Greece, a similar
shift took place, though to what extent is unknown. There is,
moreover, evidence of actual compression of the earth's crust at right
angles to the fault-line. The Neo valley, traversed by the Japanese
fault, was apparently narrower after the earthquake than it was
before, and plots of ground were reduced from 48 to 30 feet in
length--_i.e._, by nearly 40 per cent. In British Baluchistan, the
formation of the fissure referred to above was accompanied both by
compression perpendicular, and by shifting parallel, to the fault. The
actual displacement in each direction is unknown, but the resultant
was not less than 27 inches.
There can be no doubt that a fault-scarp is formed in the first place
with great rapidity. So abrupt, indeed, were the structural
displacements in the epicentral area of the Indian earthquake, that
they contributed very materially to the intensity of the shock, giving
rise to the excessive velocities observed at Rambrai and elsewhere (p.
273). The growth of the scarp does not, however, always cease with the
first great earthquake, though it may take place in a contrary sense,
as in the elevation connected with the Conception earthquake of 1835.
The principal shock, according to Darwin, was followed during the few
succeeding days "by some hundred minor ones (though of no
inconsiderable violence), which seemed to come from the same quarter
from which the first had proceeded; whilst, on the other hand, the
level of the ground was certainly not raised by them; but, on the
contrary, after an interval of some weeks, it stood rather lower than
it did immediately after the great convulsion."[91]
AFTER-SHOCKS.
A series of after-shocks, more or less long, is a constant attendant
on every great tectonic earthquake, and few are the earthquakes of
any degree of strength that can be regarded as completely isolated.
Even in those which visit this country, after-shocks are seldom
absent. For instance, confining ourselves to the last few years, the
Pembroke earthquake of 1892 was followed by 8 shocks, the Inverness
earthquake of 1890 by at least 10, and possibly by 19 shocks, and that
of the same district in 1901 by 15 well-defined after-shocks in
addition to many others recorded by one observer. Of 300 Italian
earthquakes strong enough to cause some damage to buildings, Dr.
Cancani finds that every one was either preceded or followed, and
chiefly followed, by its own train of minor shocks.
For some hours, and even for days, after a great earthquake, the
shocks are so numerous that it is often impossible to keep count of
them. Many local centres spring into activity in different parts of
the epicentral area; and, though only the strongest shocks can be
identified elsewhere, it is clear that as a rule the shocks felt at
any one station are quite distinct from those observed at another.
The enormous number of after-shocks that follow some earthquakes can
only be realised when they are subjected to continuous seismographic
registration; and, even then, countless earth-sounds and the slightest
tremors must escape detection. The shocks may, indeed, succeed one
another so rapidly that one begins before another ends, and the result
is an almost incessant tremulous motion rendered manifest by the
quivering of water-surfaces or the swinging of chandeliers. Of the
total number of after-shocks, we may form some idea from recent
records in Japan. After the Mino-Owari earthquake of 1891, 3,365
shocks were recorded within little more than two years at Gifu, and
1,298 at Nagoya, but neither of these figures includes the shocks felt
within the first few hours. Of the Kumamoto earthquake of July 28th,
1889, the after-shocks recorded at Kumamoto until the end of 1893
amount to 922; and those of the Kagoshima earthquake of September 7th,
1893, recorded at Chiran until the end of January 1894, to 480. During
the first 30 days, the numbers recorded were 1,746 at Gifu, 340 at
Kumamoto, and 278 at Chiran; showing, as Professor Omori remarks, that
the after-shocks diminish in frequency with the size of the disturbed
areas,[92]--_i.e._, roughly with the initial intensity of the shocks.
Next to absolute number, the rapid decline in general frequency is the
most marked characteristic of after-shocks. Professor Omori has shown
that, excluding minor oscillations, it follows the law represented
geographically by the curves in Fig. 51, and algebraically by the
equation y = k / (h + x), where _y_ is the frequency at time _x_ and
_h_ and _k_ are constants for one and the same earthquake. By means of
this formula, it is possible to estimate roughly the interval of time
that must elapse before the seismic activity of the central district
resumes its normal value. For the Mino-Owari earthquake, this proves
to be about forty years, for the Kumamoto earthquake about seven or
eight years, and for the Kagoshima earthquake about three or four
years.
In a recent memoir on Italian after-shocks,[93] Dr. Cancani has urged
that other factors besides initial intensity determine the duration of
a seismic period, and prominently among these he places the depth of
the seismic focus. When the depth is very small, the duration of the
period is short, not much more than ten days; when the depth is
moderate, the duration may extend to three months; and, when great, it
may amount to several years.
The principal law that governs the distribution of after-shocks in
time may be regarded as well-established. It is otherwise with regard
to their distribution in space. This has been examined only in the
cases of the Japanese earthquake of 1891 and the Inverness earthquake
of 1901. So far as we can judge from the evidence which they furnish,
after-shocks appear to be most numerous within and near the central
portion of the seismic focus; though the area of maximum activity is
subject to continual oscillation. In this region, also, there is
evidence of a gradual decrease in the depths of the after-shock foci;
while, near the extremities of the epicentral area, there occur
districts of slightly greater frequency than elsewhere. With the lapse
of time, there seems therefore to be a constant extension, both
upwards and longitudinally, of the area over which the principal
fault-slip took place.
ORIGIN OF EARTHQUAKES.
In the introductory chapter, a brief sketch is given of the different
causes to which earthquakes are assigned. With those due to rock-falls
in subterranean channels, we need have little to do. The shocks are
invariably slight, and the part they play in the shaping of the
earth's crust is insignificant. Volcanic earthquakes possess a higher
degree of interest. They represent, no doubt, incipient or
unsuccessful attempts to produce an eruption. They may be the
forerunners of a great catastrophe.
Of far higher importance in the history of our globe is the third
class of earthquakes, including all those connected with the manifold
changes which the crust has undergone. In the slow annealing process,
to which it has been subjected from the earliest times, the crust has
been crumpled and fractured, elevated into the loftiest mountain
ranges or depressed below the level of the sea. Every sudden yielding
under stress is the cause of an earthquake. It is chiefly, perhaps
almost entirely, in the formation of faults that this yielding is
manifested. The initial fracturing may be the cause of one or many
shocks, but infinitely the larger number must be referred to the slow
growth of the fault, the intermittent slips, now in one part, now in
another, which, after the lapse of ages, culminate in a great
displacement. Of the length of time occupied in the formation of a
single fault, we can make no estimate in years. The anticlinal fault
of Charnwood Forest dates from a pre-carboniferous period. In 1893 it
had not ceased to grow.[94]
Still less can we conceive, however faintly, the number of elemental
slips that constitute the history of a single fault. We may think, if
we please, of the 143 tremors and earth-sounds noted at Comrie in
Perthshire during the last three months of 1839, of the 306
earthquakes felt in the Island of Zante during the year 1896, or the
1,746 shocks recorded at Gifu during thirty days in 1891; but we shall
be as far as ever from realising the vast number of steps involved in
the growth of a fault, let alone a mountain-chain.
Yet, all over the land-surface of the globe, the crust is intersected
by numberless faults, and hardly any portion is there in which some or
many of these faults are not growing. One country, indeed, such as
Great Britain, may have reached a condition of comparative stagnancy;
the fault-slips are few and slight, and earthquakes in consequence are
rare and generally inconspicuous. In another, like Eastern Japan and
the adjoining ocean-bed, the movements are frequent, occasionally
almost incessant, and few years pass without some great convulsion by
which cities are wrecked and hundreds of human lives are lost. At such
times, we magnify the rôle of earthquakes, and are in some danger of
forgetting that, in the formation of a mountain-chain or continent,
they serve no higher purpose than the creaking of a wheel in the
complex movements of a great machine.
FOOTNOTES:
[79] _Phil. Trans._, vol. li., pt. ii., 1761, pp. 625-626.
[80] _Journ. Sci. Coll. Imp. Univ._, Tokyo, vol. xi., 1899, pp.
194-195.
[81] _Journ. Coll. Sci. Imp. Univ._, Tokyo, vol. vii., pt. v., 1894,
pp. 1-4; _Ital. Sismol. Soc. Boll._, vol. ii., 1896, pp. 180-188.
[82] _Journ. Coll. Sci. Imp. Univ._, Tokyo, vol. xi., 1899, pp.
161-195.
[83] _Quart. Journ. Geol. Soc._, vol. lvi., 1900, pp. 1-7.
[84] There is no reason why the surface-undulations of the Indian
earthquake should not have produced a sensible shock even as far as
Italy. Taking their amplitude in that country at 508 mm. and their
period at 22 sec. (p. 283), the maximum acceleration would be about 40
mm. per sec., corresponding to the intensity 2 of the Rossi-Forel
scale. (_Amer. Journ. Sci._, vol. xxxv., 1888, p. 429.)
[85] _Nature_, vol. lii., 1895, pp. 631-633.
[86] Gerland's _Beiträge zur Geophysik_, vol. iii., pp. 485-518.
[87] _Phil. Trans._, 1900A, pp. 135-174.
[88] _Publ. of Earthq. Inves. Com. in For. Langs._ (Tokyo), No. 4,
1900, pp. 47-67.
[89] S.A. Papavasiliou, Paris, _Acad. Sci., Compt. Rend._, vol. cxix.,
1894, pp. 112-114, 380-381.
[90] _Geol. Mag._, vol. x., 1893, pp. 356-360.
[91] _Geol. Soc. Trans._, vol. v., 1840, pp. 618-619.
[92] The disturbed areas of these earthquakes contained, respectively,
221,000, 39,000, and 30,000 square miles.
[93] _Boll. Sismol. Soc. Ital._, vol. viii., 1902, pp. 17-48.
[94] _Roy. Soc. Proc._, vol. lvii., 1895, pp. 87-95.
INDEX.
Acceleration, maximum, of wave-motion in Japanese earthquake, 184, 185;
in Indian earthquake, 272
After-shocks, definition, 4;
frequency, 198, 256, 296, 344;
distribution in space, 200, 203, 298, 326, 345;
sound-phenomena, 207, 300;
connection with fault-scarps, 300;
outlining of epicentre by, 326;
origin of, 257;
of Neapolitan earthquake, 40;
of Ischian earthquakes, 56, 65;
of Andalusian earthquake, 97;
of Charleston earthquake, 133;
of Riviera earthquake, 167;
of Japanese earthquake, 198;
of Hereford earthquake, 240;
of Inverness earthquake, 256;
of Indian earthquake, 296;
of British earthquakes, 343;
of Italian earthquakes, 343;
of Japanese earthquakes, 344
Agamennone, G., 93, 94, 101, 319
Alluvium, displacement of, by Indian earthquake, 287
Amplitude of wave-motion, definition, 4;
in Neapolitan earthquake, 34;
in Japanese earthquake, 185;
in Indian earthquake, 270
Andalusian earthquake, preparation for, 75;
investigation of, 76;
damage caused by, 77;
isoseismal lines and disturbed area, 78;
the unfelt earthquake, 82;
position of epicentre, 84;
depth of focus, 85;
nature of shock, 87;
sound-phenomena, 91;
velocity of earth-waves, 92;
connection between geological structure and intensity of shock, 95;
fissures, 96;
landslips, 97;
effect on underground water, 97;
after-shocks, 97;
origin of, 99;
bibliography, 101
Animals, effects of earthquakes on, 143
Baluchistan earthquake of 1892, 288, 341
Baldacci, L., 70, 73
Baratta, M., 320
Barrois, C., 76
Bergeron, C., 76
Bertelli, T., 175
Bertrand, M., 76
Birds, effects of earthquakes on, 143
Bordwar, crust-fracture at, 309
Bréon, R., 76
Burton, W.K., 214
Cancani, A., 281, 282, 320, 343, 345
Castro, M.F. de, 76, 101
Charleston earthquake, investigation of, 102;
damage caused by, 103;
isoseismal lines and disturbed area, 104;
preparation for, 107;
nature of shock, 108;
double epicentre, 111;
origin of double shock, 120;
depth of foci, 122;
velocity of earth-waves, 126;
fissures, 130;
sand-craters, 130;
effects on human beings, 131;
feeling of nausea, 132;
after-shocks, 133;
origin of, 134;
bibliography, 137
Charlon, E., 175
Chedrang, fault-scarp at, 304
Clocks, untrustworthiness of time-records of stopped, 39, 94, 121, 127
Conder, J., 177, 213
Coseismal lines, 227, 324
Covelli, N., 67, 69
Damage caused by Neapolitan earthquake, 10, 24;
by Ischian earthquakes, 50, 56;
by Andalusian earthquake, 77;
by Charleston earthquake, 103;
by Riviera earthquake, 139;
by Japanese earthquake, 181;
by Hereford earthquake, 217;
by Inverness earthquake, 247
Darwin, H., 281
Daubrée, A., 73
Davison, C, 202-206, 208, 210, 213, 215-261, 295
Death-rate of Neapolitan earthquake, 24;
of Ischian earthquakes, 50, 56;
of Andalusian earthquake, 77;
of Charleston earthquake, 104;
of Riviera earthquake, 140;
of Japanese earthquake, 182
Denza, F., 155, 175
Depth of seismic focus, methods of determining, 25, 86, 122, 326;
of Neapolitan earthquake, 28;
of Ischian earthquakes, 54, 61;
of Andalusian earthquake, 86;
of Charleston earthquake, 122, 125;
of Riviera earthquake, 150;
of Japanese earthquakes, 328
Derby earthquake of 1903, 236
Direction of shock, 22, 33, 186, 225, 325
Disturbed area, definition of, 3;
of Neapolitan earthquake, 10;
of Ischian earthquakes, 51, 58;
of Andalusian earthquake, 80;
of Charleston earthquake, 107;
of Riviera earthquake, 145;
of Japanese earthquake, 183;
of Hereford earthquake, 219;
of Inverness earthquake, 249;
of Indian earthquake, 265;
connection between intensity of shock and, 323
Dolomieu, 11
Du Bois, F., 73
Dutton, C.E., 103-137
Dutton's method of determining depth of seismic focus, 122, 327
Earthquake-motion, nature of, 280, 282, 328, 330, 337;
propagation of, to great distances, 337
Earth-sound, definition of, 4
Edinburgh, record of Indian earthquake at, 281, 283, 285
Ellis, W., 83
Emergence, angle of, 13
Epicentre, definition of, 3;
methods of determining position of, 14, 52, 60, 324;
of Neapolitan earthquake, 22, 23;
of Ischian earthquakes, 53, 60, 67;
of Andalusian earthquake, 84;
of Charleston earthquake, 111;
of Riviera earthquake, 146;
of Hereford earthquake, 224;
of Inverness earthquake, 248;
of Indian earthquakes, 264, 276, 302
Epomeo, 45, 61, 71
Falb's method of determining depth of seismic focus, 86, 327
Fallen pillars, evidence of, 17, 19
Fault, originating, of Hereford earthquake, 219;
of Inverness earthquake, 249
Fault-scarp of Japanese earthquake, 189;
general appearance, 189;
length, 192;
throw, 193;
horizontal shift, 193;
course, 193;
swamp formed by it, 194
Fault-scarps of Indian earthquakes, 273, 304;
Chedrang fault, 304;
Samin fault, 308;
of Greek earthquake of 1894, 340, 341;
of Baluchistan earthquake of 1893, 341, 342;
formation and growth of, 342
Fault-slips, tectonic earthquakes due to, 5, 43, 100, 135, 174, 211,
219, 224, 241, 249, 255, 317, 346
Fishes, destruction of, by Riviera earthquake, 162
Fissures, caused by Andalusian earthquake, 96;
by Charleston earthquake, 130;
by Inverness earthquake, 247;
by Indian earthquake, 285
Focus, seismic, definition of, 3
Focus, seismic, depth of, methods of determining, 25, 86, 122, 326;
of Neapolitan earthquake, 28;
of Ischian earthquakes, 54;
of Andalusian earthquake, 86;
of Charleston earthquake, 122, 125;
of Riviera earthquake, 150;
of Japanese earthquakes, 328
Focus, dimensions of seismic, of Hereford earthquake, 224;
of Inverness earthquake, 250
Fore-shocks, 321;
of Neapolitan earthquake, 40;
of Ischian earthquake, 57;
of Andalusian earthquake, 76;
of Charleston earthquake, 107;
of Riviera earthquake, 142;
of Japanese earthquake, 201;
of Hereford earthquake, 239;
of Inverness earthquake, 246
Fouqué, F., 76, 84, 101
Fracture, crust-, at Bordwar, 309
Fractures in buildings, evidence of, 14, 15, 26
Fuchs, C.W.C., 102
Galli, I., 82
Geological structure and intensity of shock, connection between, 95,
106, 113, 115, 135, 164, 265
Gifu, records of Japanese after-shocks at, 183, 197
Gray, T., 295
Great Glen fault and Inverness earthquakes, connection between, 245
Greek earthquake of 1894, fault-scarp of, 340
Hayden, E., 103
Heath, T., 283, 320
Hereford earthquake, investigation of, 215;
preparation for, 215, 238;
isoseismal lines and disturbed area of, 216;
damage caused by, 217, 294;
position of originating fault, 219;
nature of shock, 220;
origin of double series of vibrations, 223;
position and dimensions of the two foci, 224;
direction of the shock, 225;
coseismal lines and velocity of earth-waves, 227;
sound-phenomena, 229;
isacoustic lines and sound-area, 234;
fore-shocks, 238;
after-shocks, 240;
origin of earthquake, 240;
bibliography, 261
Hills, changes in aspect of, after Indian earthquake, 311
Hirata, K., 327, 331
Human beings, effects of Charleston earthquake on, 131
Hypocentre, 3
Iberian peninsula, earthquakes of, 75
Indian earthquake, investigation of, 262;
isoseismal lines and disturbed area, 264;
nature of shock, 266;
visible earth-waves, 268;
elements of wave-motion, 270;
sound-phenomena, 274;
velocity of earth-waves, 275;
the unfelt earthquake, 280;
earth-fissures, 285;
displacements of alluvium, 287;
sand-vents, 288;
rise of river-beds, etc., 290;
landslips, 291;
rotation of pillars, 293;
after-shocks, 296;
structural changes in epicentral area, 301, 315;
structure of epicentral district, 302;
fault-scarps, 304;
crust-fractures, 309;
lakes and pools not due to faulting, 310;
changes in aspects of hills, 311;
revision of trigonometrical survey, 312;
origin of earthquake, 317;
bibliography, 319
Inverness earthquake, preparation for, 246;
damage caused by, 247;
fissure in ground, 247;
isoseismal lines and disturbed area, 247;
position of originating fault, 249;
nature of shock, 250;
sound-phenomena, 253;
origin of earthquake, 255;
after-shocks and their origin, 256;
sympathetic earthquakes, 259;
comparison with Japanese earthquake, 260;
bibliography, 261.
Investigation, Mallet's methods of, 12, 21
Isacoustic lines, 234;
of Hereford earthquake, 235;
of Derby earthquake, 236
Ischia, volcanic history of, 45, 70;
characteristics of eruptions, 49;
seismic history, 49
Ischian earthquake of 1881, investigation of, 50;
isoseismal lines and disturbed area, 51;
position of epicentre, 52;
depth of focus, 54;
nature of shock, 55;
after-shocks, 56;
origin of, 70;
bibliography, 73
Ischian earthquake of 1883, investigation of, 56;
preparation for, 57;
isoseismal lines and disturbed area, 58;
position of epicentre, 60;
depth of focus, 61;
nature of shock, 64;
landslips, 64;
after-shocks, 65;
origin of, 70;
bibliography, 73
Ischian earthquakes, characteristics of, 66; origin of, 70
Isoseismal lines, definition of, 3;
their use in determining position of epicentre, 219, 249, 325;
of Neapolitan earthquake, 9;
of Ischian earthquakes, 51, 58;
of Andalusian earthquake, 78;
of Charleston earthquake, 104;
of Riviera earthquake, 143;
of Japanese earthquake, 178, 182;
of Hereford earthquake, 216;
of Inverness earthquake, 247;
of Indian earthquake, 264
Issel, A., 139, 163, 164, 166, 175
Japanese earthquake of 1887, 18
Japanese earthquake of 1891, investigation of, 177;
structure of meizoseismal area, 179;
damage caused by, 181;
isoseismal lines and disturbed area, 182;
nature of shock, 184;
the great fault-scarp, 189;
minor shocks, 197;
distribution of after-shocks in time, 198;
preparation for, 201;
distribution of after-shocks in space, 203;
sound-phenomena of after-shocks, 207;
sympathetic earthquakes, 209;
origin, of, 211;
bibliography, 213
Japanese earthquake of 1894, 18, 329
Johnston-Lavis, H.J., 50-72, 327
Kilian, W., 76
Koto, B., 177, 180, 181, 184, 190-196, 209, 212, 213
Lakes formed by bending of river-bed during Indian earthquake, 310
Lakes formed by fault-scarp of Japanese earthquake, 194;
of Indian earthquake, 305
Landslips caused by Ischian earthquake, 64;
by Andalusian earthquake, 97;
by Indian earthquake, 291
Lévy, M., 76
Lisbon earthquake of 1755, 75, 82
McGee, W.J., 134
Macpherson, J., 101
Magnetographs, earthquakes recorded by, 82, 157, 160, 189, 277, 282
Mallet, R., 7-44, 85, 102, 124, 150, 294-296, 325
Mallet's method of determining depth of focus, 25, 327
Masato, H., 178, 213
Mascart, E., 159, 160
May Hill anticlinal and Hereford earthquake, connection between, 242
Meizoseismal area, definition of, 3;
of Andalusian earthquake, 99;
of Japanese earthquake, 179
Mercalli, G., 11, 57, 58, 60, 61, 63, 67, 70-73, 76, 80, 84, 85, 88,
90, 101, 138-175, 325, 327
Michell, J., 325
Milne, J., 35, 177, 181, 182, 186, 189, 200, 213, 281, 283
Minor shocks of Neapolitan earthquake, 40;
of Japanese earthquake, 197
Mountain ranges, effect of, on intensity of shock, 95, 106
Moureaux, T., 161
Nagaoka, H., 177, 214, 339
Nagoya, records of Japanese after-shocks at, 183, 197
Nature of shock, Neapolitan earthquake, 30;
Ischian earthquakes, 55, 64;
Andalusian earthquake, 87;
Charleston earthquake, 108;
Riviera earthquake, 150;
Japanese earthquake, 184;
Hereford earthquake, 220;
Inverness earthquake, 250;
Indian earthquake, 266
Nausea, feeling of, caused by Charleston earthquake, 132
Neapolitan earthquake, investigation of, 7, 12;
isoseismal lines and disturbed area, 9;
damage caused by, 10;
position of epicentre, 14;
depth of focus, 25;
nature of shock, 30;
sound-phenomena, 37;
velocity of earth-waves, 39;
minor shocks, 40;
origin, 41;
bibliography, 44
Ness, Loch, connection between Inverness earthquakes and formation of,
255, 257, 261
Nogués, A.F., 101
Oddone, E., 175
Offret, A., 76, 158, 159, 175
Oglialoro, A., 73
Oldham, R.D., 262-320, 337, 340
Omori, F., 19, 20, 177, 183-186, 188, 197-199, 207, 214, 262, 325, 327,
329, 331
Origin of earthquakes, 2, 5, 345;
of Neapolitan earthquake, 41;
of Ischian earthquakes, 70;
of Andalusian earthquake, 101;
of Charleston earthquake, 134;
of Riviera earthquakes, 174;
of Japanese earthquake, 211;
of Hereford earthquake, 240;
of Inverness earthquake, 255;
of Indian earthquake, 317
Overturned bodies, maximum acceleration deduced from, 184, 272
Palmieri, L., 57, 72, 73
Periodicity of Japanese after-shocks, 199
Perrey, A., 7
Potenza, evidence of damaged church at, 15, 26
Prediction of earthquakes, possible, 322
Preparation for earthquakes, 40, 57, 76, 107, 142, 201, 238, 246, 321
Rails, flexure of, by Charleston earthquake, 112;
by Japanese earthquake, 182;
by Indian earthquake, 288
Railway-tunnels, observations of Riviera earthquake in, 166
Rebeur-Paschwitz, E. von, 281
River-beds, rise of, caused by Indian earthquake, 290
Riviera earthquake, investigation, 138;
damage caused by, 139;
preparation for, 142;
isoseismal lines and disturbed area, 143;
position of epicentre, 146;
depth of principal focus, 149;
nature of shock, 150;
sound-phenomena, 156;
the unfelt earthquake, 157;
effects of earthquake at sea, 162;
destruction of fishes, 162;
seismic sea-waves, 163;
connection between geological structure and intensity of shock, 164;
observations in railway-tunnels, 166;
after-shocks, 167;
recent movements in the Riviera, 170;
seismic history of the Riviera, 171;
origin of, 171;
bibliography, 175
Rocca di Papa, record of Indian earthquake at, 281, 282, 285
Rossi, M.S. de, 57, 74, 82, 101, 175
Rossi-Forel scale of seismic intensity, 104, 216, 247
Rotation of pillars, caused by Hereford earthquake, 294;
by Indian earthquake, 293;
explanation of, 295
Rudzki, P., 336
Rumi, Prof., 169
Samin, fault-scarp at, 308
Sand-craters caused by Charleston earthquake, 130;
by Indian earthquake, 288
Schmidt, A., 336
Seismic sea-waves of Riviera earthquake, 142, 163
Seismic vertical, 12, 29, 62
Seismographic records of Riviera earthquake, 154;
of Japanese earthquake of 1894, 329
Sekiya, S., 18, 19
Serpieri, A., 74
Shillong, nature of Indian earthquake at, 266
Sloan, E., 103, 117-119, 134, 135
Sound-area, definition of, 3;
of Neapolitan earthquake, 38;
of Andalusian earthquake, 92;
of Hereford earthquake, 234;
of Inverness earthquake, 252;
of Indian earthquake, 275
Sound-phenomena, nature of sound, 38, 229, 252, 332;
inaudibility to some observers, 231, 274; its cause, 233;
isacoustic lines, 234-236;
variations in nature of sound throughout sound-area, 237;
time-relation of sound and shock, 238, 253;
origin of earthquake-sounds, 334;
sound-phenomena of Neapolitan earthquake, 37;
of Andalusian earthquake, 91;
of Charleston earthquake, 133;
of Riviera earthquake, 156;
of Japanese after-shocks, 207;
of Hereford earthquake, 229;
of Inverness earthquake, 252;
of Indian earthquake, 274
Structural changes, distribution of, in Indian earthquake, 315
Subsultory shock, 5
Sympathetic earthquakes of Japanese earthquake, 209;
of Inverness earthquake, 259
Tanakadate, A., 177, 214
Taramelli, T., 76, 84, 85, 88, 90, 101, 138, 150, 165, 175, 325
Tectonic earthquakes, 5
Thrust-plane, Indian earthquake due to movement along, 318
Time-curve of Indian earthquake, 278;
of principal epochs of earthquake-waves of distant origin, 338
Time-records, general inaccuracy of, 324
Time-relations of sound and shock in Hereford earthquake, 238;
in Inverness earthquake, 253
Trigonometrical survey, revised, of Khasi hills after Indian
earthquake, 312;
interpretation of results, 314
Twin earthquakes, origin of, 32, 89, 120, 153, 174, 223;
Neapolitan earthquake, 31;
Andalusian earthquake, 87;
Charleston earthquake, 108;
Riviera earthquake, 149, 150;
Hereford earthquake, 221
Undulatory shock, 5
Unfelt earth-waves, Andalusian earthquake, 82;
Riviera earthquake, 157;
Indian earthquake, 280
Uzielli, G., 143, 176
Velocity, maximum, of wave-motion, in Neapolitan earthquake, 35;
in Indian earthquake, 272
Velocity of earth-waves, methods of determining, 39, 93, 127, 229;
variation with depth, 336;
form of wave-paths, 336;
velocity of different phases, 339;
of Neapolitan earthquake, 39;
of Andalusian earthquake, 92;
of Charleston earthquake, 126;
of Japanese earthquakes, 188;
of Hereford earthquake, 229;
of Indian earthquake, 275, 279, 284
Visible earth-waves in Charleston earthquake, 110;
in Japanese earthquake, 186;
in Indian earthquake, 268
Volcanic earthquakes, 5, 70
Vorticose shock, 5
Water, effect of Andalusian earthquake on underground, 97
Waterfalls caused by fault-scarps of Indian earthquake, 305
Wave-path, 13
West, C.D., 272
Woolhope anticlinal and Hereford earthquake, connection between, 241
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| Page 336: 'per sec. per sec.' replaced with 'per sec.' |
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*** End of this LibraryBlog Digital Book "A Study of Recent Earthquakes" ***
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