THE DEPTH OE AN EAETHQUAKE CENTEUM. 217 



For example, let it be assumed that the first impulse 

 of an earthquake is due to the sudden formation of a 

 fissure, rent open from its centre, and that the waves leave 

 the walls at all points normal to its surface. Then, as 

 Mallet points out, it is evident that the disturbance will 

 spread out in ellipsoidal waves, the greatest axis of which 

 will be perpendicular to the plane of the fissure. 



By taking a number of cases of fissures lying in various 

 directions and drawing the ellipsoidal waves which would 

 result from an elastic pressure, like that of steam suddenly 

 admitted into such cavities, the differences in effect 

 which would be simultaneously produced by these waves 

 on reaching the surface can be readily understood. The 

 following example of an investigation on this subject will 

 serve as an example to illustrate the general nature of the 

 many other cases which might be taken. 



Let a disturbance simultaneously originate from all 

 points of the fissure//. This will spread outwards in 

 ellipsoidal shells to 

 the surface of the 

 earth e e. The 

 major axis of these 

 ellipsoidal shells 

 will be the direc- 

 tion of greatest 

 effect. In the di- 

 rection c d the 

 waves will plunge 

 into the earth, and 

 places to the right 

 side of the fissure 

 will, to use an expression due to Stokes, when speak- 

 ing of analogous phenomena connected with sound, be 

 in earthquake shadoiu. The same expression has been 



Fig. 33. 



