A Study of Recent Earthquakes

CHAPTER X.. Conclusion 321

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.

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.

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