DETEEMINATION OF EARTHQUAKE ORIGMNS. 211 



when seeking for the origin of the Iquique earthquake of 

 1877. The assumptions made in that particular instance 

 were, first, that the velocity of the disturbance through 

 the earth was known, and, secondly, that the velocity 

 with which a sea wave was propagated was also known. 



A method similar to the above was first suggested by 

 Hopkins. It depended on the differences of velocity with 

 which normal and transversal waves are propagated.^ 



Seebach's method. —To determine the true velocity of 

 an earthquake, the time of the first shock, and the depth 

 of the centre. 



Let the straight line 

 M, TTij, m^, m^ represent 

 the surface of the earth 

 shaken by an earthquake. 

 For small earthquakes, to 

 consider the surface of 

 the earth as a plane will 

 not lead to serious errors. 

 If an earthquake origi- 

 nates at C, then to reach 

 the surface at M it tra- 

 verses a distance h in *^^^- 

 the time t. To reach the surface at M, 

 distance h + x^ 

 propagation, 



32. 



, it traverses a 



in a time t^. If v equals the velocity of 

 then t=.'^, t^ = \±Jh^ 



I ft ~\~ iJUn 



&c. 



Seebach now says that if we have given the ^position 

 of M or epicentrum of the shock, and draw through it 

 rectangular axes like M m^ and M Tg, and lay down on 



* Report of British Association, 1847, p. 84. 



