220 F. OMORI. 



months, from Xovember, 1891, to April, 1893, we get the foUwing 

 equation, — 



16.9 



y- 



.2- + 0.897 {(1) 



in wliich x denotes time in months, the origin being November, 

 1S91, and y the mean daily activity during the month .r. Now x = l.b 

 represents the middle of the year 1892, and .t = 19.5 that of the year 

 189o. The corresponding values of y calculated from the above 

 equation are respectively 2.1 and 0.84, the actual mean daily activities 

 of after-shocks in the two years being respectively 2.4 and 0.84. 



If we put y = Tiî, equation (d) gives a; = 5 10 months or 42 years; 

 and if we put y = T(r, equation (c) gives ;r = 27000x ^ days or 37 years. 

 The meaning is that the seismic activity or frequency at Gifu due to 

 the residual effect of the great earthquake after al)out forty years from 

 the initial date may 1)e such that one " weak " or '' feeble " shock 

 occurs in each month. ]\Iakin2: gfreat allowance for error of calcula- 

 tion and quartering the above figures, we may conclude that at least 

 some ten years will elapse before the disturbed tract about Gifu can 

 practically regain its stability, that is before the activity or fre(juency 

 of after-shocks at that place reduces to the state of having one small 

 shock per montli. 



The above conclusion, though (^nly the result of rough approxima- 

 tions, seems a very likely one, when considered in reference to the 

 Kumamoto earthquake of July 28th, 1889, which was far smaller than 

 the Mino-Owari earthquake, and whose after-shocks are still occurring 

 at tlie present day, about 4|- years after. 



§ 12. The variation with time of the frequency or activity of 

 after-shocks of the Mino-Owari earthquake is comparatively simple, 

 and none of these shocks was of a magnitude comparable with that of 

 the initial earthquake itself. A few of the after-shocks, such as those of 



