ON THE AFTER-SHOCKS OF EARTHQUAKES. I35 



Mino-Ow:!ri, and one of the Kagoshima cnrtliqnake) tliree took place 

 ill JannaTv and one in September. I believe that periodicity ])lay.s a 

 very important part in the frequency of e:u'thcj^uakes, and its attentive 

 study may be of help in the prediction of changes in seismic activity 

 •md other events. 



I shall here confine myself to merely stating the ficts. Theore- 

 tical speculations on this interesting branch of tlie earth's physics are 

 reserved f^r a future occasion, Avlien more materials respecting after- 

 shocks shall have been obtained. 



§ 29. Aijaiii of the Seismic Frequencii in Töl'uü, etc. 



As seen above, after-shocks indicate many periods in the variation 

 of the seismic frequency whose lengths range between a few hours and 

 one year. It is of C(v.irse possible that there should exist periods of still 

 lonii'er duration. We shall here ao'ain consider the seismic record 

 taken by instruments in Tokyo during eighteen vea.rs from ISTl! to 

 1.S93.' The curve of monthly e:u'th([uake numbers, a portion of 

 "which is shewn in Fig. 22, indicates very clearly the existence of the 

 annual period. There seem to exist also fliictuati(jns of a few 

 UKUiths' duration, of which, for instance, T can count more or less 

 distinctly— 



10 during .']!) montlis Ijetween Jainiary, 1<S7<), and April, 1870; 



15 ,, 50 ,, ,, September, 18(S2, and Xovemljer, 1(S85; 



and 15 ,, 50 ,, ,, November, 1885, and Janu:u-y, 18'.)lj 



The average lengtli of the period is 3.5 months. 



Tiie curve of yearly earthquake numliors at Tokyo (Fig. 23) 

 shews fluctuations, the maxima of which occurred in the years 1880, 

 1884-5, and 188!', and the minima in the years 187.S, 1883, 

 and 188(), there being besides these a slight maximum in 1891. The 

 average length of the period is -l 3'ears. 



1 Seiî Tal)leXVI[. 



