12 ART. 1— S. KUSAKABE : FREQUENCY OF 



In the above example, suppose the unit of time to be one 

 month, then the number of after-shocks during the first, second, 

 third, month etc., would be either 461, 298, 217, etc. or 494, 

 327, 244, etc. respectively, according as the time required by the 

 force to accumulate to the amount sufficient to cause the earth- 

 (juake was a hundred or a thousand months. 



We may remark that the solution of the above formula gives 

 a means of determining the length of time required to generate 

 that earthquake, and this must, at least, elapse before the region 

 to which it refers is again disturbed by a similar catastrophe. 



A few words may be inserted here in reference to the situa- 

 tion of the centre of after-shocks. It may appear, jyriina facie, 

 that all of the after-shocks must necessarily proceed from the 

 seismic focus of the original earthquake. But this is not neces- 

 sarily so, and it does not actually happen, that the centres of 

 the after-shocks and those of the original earthquakes coincide 

 with one another. 



From the above theory of yielding, howevei-, it comes to 

 Ije the common rule rather than an exception that they do not 

 generally coincide. The seismic focus of the original earthquake 

 is, of course, a region where the stratum giving way to the 

 internal stress, was crushed or dislocated, e.g., in the case of a 

 tectonic earthquake. All the after-shocks, however, m re the 

 result of recovery from the yielding so that they take place 

 most frequently where the rate of recovery is the greatest. No 

 doubt, a region once dislocated can never recover so as to cause 

 any after-shocks. Of the neic/hbouring 7'egions, that part which 

 co)isi8ts of rocks most capable of yielding and recovery is most likely 

 to become the centre of after-shocks. Hence, though the after- 

 shocks are the residual efects of the original earthquake, the 



