4 ART. 1. — S. KUSAKABE : FREQUENCY OF 



after the accumulated stress has reached the necessary amount 

 at the given point, where the seismic focus is situated. Again, 

 if at the instant when the weak point gave way, all the strata, 

 being released from their overstrained state, were to come to 

 equiliblium at once and there were no residual strain which might 

 recover with the la})se of time, the earthquake at that instant 

 would be the only effect of the accumulated stress. 



According to the intrinsic meaning of the name " After- 

 shock," the nearest cause must be attributed to a residual dis- 

 turbance in the geotectonic condition after the original shock has 

 ceased. An earthquake not participating in this residual is not 

 an after-shock but an independent earthquake. To make clear 

 once for all my own standpoint, I must say, that the actual im- 

 jjerfectness of the elastic jjroperties of rocks which compose our , 

 planet appears to be the prime cause— all other causes being 

 secondary relative to it — of after-shocks or rallier of all earth- 

 quakes. Although the magazine of seismic energy is being con- 

 stantly and steadily replenished by the incessant recovery of the 

 rocks around the seismic centre from their overstrained yielding, 

 the effects are intermittent and manifest themselves as aftershocks. 

 As a matter of fact, the folding of rocks and other kind- 

 red phenomena pertaining to their manifold changes of shape are 

 found in great abundance within the earthcrust. It may be a 

 (question whether such phenomena once occurred with great 

 frequency in a short time under wholly plastic conditions while 

 now-a-days, the crust being permanently set, they occur nmch 

 less frequently ; or whether they are not the results of yieldings, 

 occurring from time to time, wrought by the continuous action 

 of stress, and always ready to recover tVoni the over-strained 

 state. 



