ON THE EARTHQUAKE AND VOLCANIC PHENOMENA OF JAPAN. 173 



three kinds of movements, each of which has a different velocity of 

 propagation. On the surface of the earth there is an undulatory motion 

 which from the researches of Lord Rayleigh we might expect to travel at a 

 rate slightly slower than a distortional wave, but as pointed out by 

 Lord Kelvin it is probable that this rate is accelerated by the influence of 

 gravity. What we should expect and what we find are therefore fairly in 

 accordance. From a centrum to various points upon the surface of the 

 earth we should expect truly elastic waves to be propagated, the velocities 

 of which would vary along paths of varying depths. At great depths, as 

 for example along a straight or curvilinear path between Japan and 

 Europe, the velocity of propagation might be higher than that of a con- 

 densational rarefactional wave in glass, and exceedingly high velocities 

 have apparently been observed. Lastly, in an epifocal area there may be 

 instantaneous disturbance or an apparent high velocity due to bodily 

 displacement within an earthquake core and the transmission of elastic 

 and quasi- elastic vibrations, or to the combination of such phenomena. 



{d) The Faths followed hy Earthquake Motion. 



What has next to be considered are the paths by which an earthquake 

 originating at a centrum reaches various points upon the sui-face of the 

 globe. 



Three hypotheses present themselves. Motion may reach various 

 points on the earth's surface along the rectilinear wave paths of Hopkins 

 or Seebach, by curvilinear paths as suggested by Dr. A. Schmidt, or lastly 

 by either of these paths, after which from an epifocal area the radius of 

 which is not likely to exceed the focal depth, there is a transmission on the 

 surface of elastic gravitational waves. 



Before discussing the merits of these hypotheses, it may be well first to 

 consider the case or cases to which they are applicable. 



Because we have no evidence of a disturbance being simultaneously 

 felt at a number of places on the surface of our globe and at their anti- 

 podes, and for other reasons, we may exclude the idea of a disturbance 

 having originated near to the centre of our sphere. Nor can it be admitted 

 that a disturbance originated at half or quarter such a depth, for if it did 

 so, then in an epicentral area, possibly 400 miles in diameter, apparent 

 velocities should have been observed which not only would be enormously 

 high, but would be at least five times greater than those observed between 

 more distant stations. 



From what we know respecting the causes of earthquakes, it is a 

 reasonable supposition to imagine that their origins are confined to the 

 crumpling of a superficial layer of the material constituting the crust 

 of our globe, which according to the Rev. O. Fisher and other investi- 

 gators, in all probability does not exceed thirty miles in thickness. The 

 enormous faulting which has accompanied certain disturbances shows that 

 at least a portion of the initial impulse was delivered actually at the 

 surface. About the depth to which such faults have descended or the 

 mean depth from which an earthquake has originated, we have no certain 

 information. Confining our considerations to disturbances which have 

 originated at depths which are extremely small relatively to the radius of 

 our earth, we may now turn to the hypotheses respecting earthquake 

 radiation. 



