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



quakes," in whicli the said formula was applied to the three 

 great earthquakes which had then recently occurred in Japan. 

 Lately, another formula in the form of a logarithmic function 

 has been obtained by ]\Ir. M. Enya,''' founded on three assump- 

 tions. The result of a laborious calculation is given by him to 

 show that the logarithmic formula is equally as good as the 

 hyperbolic. 



As to the space-distribution of after shocks, Professor Omori 

 proposed the empirical formula 



where a and b are constants, while r is the distance of an ob- 

 serving place from the seismic focus. As it is reasojiable to 

 regard the shocks as proceeding from the seismic focus, the iso- 

 frequency curves would take the form of a series of coneenti-ics 

 around the focus, if the earth were a homogeneous solid. The 

 existence of the so-called sympathetic shocks seems a mystery to 

 anyone who adheres to the above view and assumes, without any 

 reason, the surface intensity to be continuous. 



As a matter of fact, however, the earthcrust is made up of 

 rocks varying greatly in physical properties, each having its 

 own density and elasticity. To make the variation more discon- 

 tinuous, rocks of all geological ages have been mingled together- 

 as it wei-e, by a series of geological disturbances, and are scattered 

 about through the earthcrust. 



Consequently their space-distribution would never be ex- 

 pressed with any approximation to exactness by a formula wliieh 

 is a function of the distance alone. 



The first step in a theoretical investigation of the frequency 



* M. iMiya. The Piil). of the K. I. (!. (in Japanese) No. ;55, 1901. 



