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FIGURE

46. Fissures Opened, and Wrecked Railway Bridge, MinoOwari Quake

47. Village almost Engulfed, Mino-Owari. The Land is Reported Here to have Slipped about Twenty Metres.

48. Wooden House Wrecked by Charleston Quake, near Summerville, S. C.

49. Wooden House Wrecked by Charleston Quake, near Summerville, S. C.

50. Locomotive Derailed while Running near Epicentre of Charleston Quake. This and the Preceding Disasters, from Figs. 42 to 49, Are all the Results of Epifocal Waves

51. Omori's Typical Long-Distance Seismogram

52. Diagram

53 Seebach's Method of Estimating Depth of Centrum of

Coseismals

54. Diagram

55, 56, 57. Diagram

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58. Aug. Schmidt's Inference of Refraction

204

59. Dr. Agamennone's Speed Diagram of Five Zante Earthquakes.

208

60. Von Rebeur-Paschwitz's First Long-Distance Seismograms at Strassburg and Nicolaieff

215

61. Dr. Oldham's Time-Distance Relations

221

62. Dr. Davison's Method of Representing Frequency

249

63. Dr. Rudolph's Contours of Ocean-Bed in the St. Paul's Rocks and Equatorial Seismic Districts

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EARTHQUAKES

CHAPTER I

NATURE AND DEFINITIONS

The Two Lines of Inquiry-Typical Notion of an Earthquake-The Centrum -The Epicentrum-Intensity-Isoseismals-Meizoseismic Area-Coseismals-General Notion of an Earthquake Vibration-The Spherical Shell-A Case of Radiant Energy-General Statement of the Cause of an Earthquake, which is Anything that Calls Suddenly into Action the Elasticity of the Earth-Brief Description of the Action Taking Place during a Great Earthquake

HE subject of earthquakes naturally divides itself into

THE

two lines of inquiry. The first relates to what occurs upon the surface of the ground, the other to what takes place below it. The surface actions, being seen and felt, are matters of direct observation, while the subterranean action is altogether a matter of inference. But while pursuing either line of inquiry we can never be unmindful of the other, or else we should cease to view the phenomenon in its entirety. We are compelled to regard an earthquake as having its origin or cause beneath the surface, while the movements that are seen and felt are regarded as effects transmitted to the surface from the origin.

The typical idea of an earthquake as a whole involves the concept of a central point below the surface, from which the vibratory impulses originate, or are propagated in all directions, like sound-waves, or waves of light. This point of origin is called the centrum. It is ideal to a large extent and never strictly real, though it seems essential to consistent reasoning. Yet the real facts are believed to constitute approximations to the idea sufficiently near to serve the general purpose of the truth without material error.

It is clear that the centrum cannot be a literal point; it must be a locus occupying a space with three finite dimensions. Its shape may be anything. In truth there are many earthquakes which we have reason to think have been caused by those shearing movements of the rocks which produce what geology terms faults. Others are known which appear to have been caused by the sudden dropping, or fall, of a considerable tract of the earth's surface with its underlying rocks, of great but unknown thickness. In such cases large areas of the surface, more than a hundred square miles in extent, suddenly sank, causing the earth to tremble many hundreds of miles away.

But though such movements seen at first to have little connection with the notion of a centrum, further reflection will indicate a certain consistency with it. For size is relative and the depth of the centrum is not fixed. A tract of three or four hundred square miles is but a small spot in one of tens of thousands of square miles, and, being always centrally situated with respect to the shaken area, may be regarded as the centrum. Nor is this consistency seriously affected by the fact that the sunken block has for its upper

surface the surface of the ground, for its vertical dimension is doubtless considerable and the portion whose impact generates the tremors is far below.

The idea of a centrum, therefore, may be taken with some qualifications, none of which materially affect its validity. It is a locus but not a point. It may be at any depth, of any size, and of any shape. A wave, or series of waves, is generated upon the still surface of a pond, whether we throw into it a crooked stick at any angle, or whether we drop into it vertically a perfectly spherical pebble. Though there will be differences in the minor details of the resulting waves, the effects are generically the same. So, too, with the generation of earthquake waves. Indefiniteness and irregularity of locus of origin is of consequence only in the details of the resulting waves, not in their general nature.

Wave-motion is such a complex subject and the waves which actually occur in nature are so varied in kind and so vast in number that it is a sufficient burden upon the mind and attention to follow out the action of a single wave of the simplest kind with as much elimination of complexities as possible. Limitations of speech and expression, therefore, compel us to treat of the centrum as if it were a literal point from which a single simple wave, or series of successive harmonic waves, emanated. Otherwise we should be lost in the complexities of the subject. The centrum, therefore, becomes an ideal point and not a real one.

A point on the surface of the ground, vertically over the centrum, is termed the epicentrum. This, too, is an ideal point and one that can never be determined with exactitude,

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