llß F. OMORI. 



of distiu'ljance is ii<jt .strictly verified between these tAVo e:irth(|uakes 

 and the Mino-Uwari earthquake, and tlie number of after-sliocks 

 for the last as recorded at Gifii seems to l)e comparatively too small. 

 It may be that the town of Gifu is not sufficiently near to the district 

 which is to be regarded as the proper centre of activity of after-shocks 

 of the Mino-Owari earthquake. The etfect that may l)e due to 

 difference of focal de})ths nuist also be kept in mind. 



Anyh(jw the frequencv ov activity of after-shocks increases with 

 the magnitude of the initial earthquake and therefore can to a 

 certain extent be regarded as a measure of tiie latter. Thus, for instance, 

 tlie Kumamoto earth(piake of July -8th, 1<S(S1), was foll(jwed, on 

 August ord, five days later, by a second strong shock. Acc(jrding to 

 the above criterion, the first shock was doul)le or trel)le as «""reat as the 

 second, the nundjers of their after-shocks beini>' in the ratio of 1)0 : 30 

 approximately. 



§ <S. To show the time relation of after-shocks, curves have 

 been drawn, (Figs. 1,5,(), ....). whose abscissae are equal intei'vals of 

 time, and ordinates the numbers or activities of after-shocks during 

 these intervals. The form of the curves drawn, in red, through tlie 

 ultim-ate mean positicjns will l)e seen to be very like that of a 

 I'ectangular hy])erbola. 



§ 9. The time rate of decrease of the frequency or acti\ity of 

 after-sh«jcks is at first very rapid, but afterwards becomes slow and 

 asymptotic. As far as the rate <.)f variati(m is concerned, the after- 

 shock curve may be regarded as ctmsisting of two nearly straight 

 ])ortions, during the first of which the rate is great, and during the 

 second of wliich it is small, the turning pc^int occm-ring so'jner with 

 smaller earthquakes. In the case of the after-shocks of the Mino- 

 Owari earth(piake this transition took place ten or twelve days after 

 the initial shock. 



