AFTER-SHOCKS AND SPACF-DISTRTBUTION OF SEISMIC WAVES. 15 



wave coiKliu'tivity is IcMst for Cainozoie rocks, mikI iiu'rcasing 

 step by step from JMesozoic to PaluMjzoie, it becomes several 

 times greater foi* Arcbœan rocks. Hence, provided the frequency 

 of after-shocks ;it the seismic focus be given, the frequency at 

 any place having a given distance from the seismic focus increases 

 with the geological age of the rocks forming the path of the 

 wave between the focus and tlie place. 



In support of the above statement, I may cite the case of 

 the Mino-Owari earthquake. Seismologists have never enjoyed 

 so good an opportunity as that afforded them ])y the convulsion 

 in the Mino-Owari plain, of observing an enormons number of 

 after-shocks at a multitude of stations well distributed around 

 the seismic centre. After the catastrophe of October 28th, 

 1891, the after-shocks w^ere extraordinarily fiequent or almost 

 incessant : indeed, 1503 of them occurred during the remaining 

 two months of that year, and during the next year 867 were 

 recorded in the Meteorological Station of Gifu. The number of 

 observing stations for these seismic waves, on the other hand, 

 amounted to thirty-three in all, i.e. 14 in Mino, 9 in Owari and 

 10 in Mikawa Province. 



Four curves of iso-frequency carefully drawn — irrespective, 

 of course, of the geology of the regions — by Prof. F. Omori* 

 are shown, with the corresponding geological distributions of 

 rocks supplied by the author himself, in Fig. 1. The iso-frequ- 

 ency curve for 7^^=500 lies wholly within Quaternary rocks and 

 is in an elongated form extending nearly north and south between 

 Gifu and Nagoya. The central region of the after-shocks may 

 be in a similar form. The succeeding curves, however, so far 



* F. Omori. Loc. cit. 



