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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.