132 



I'. OMORI. 



Tlie Miiio-O\v;iri and the Ka<^()sliim;i earthquakes are not yet 

 sufficiently old to give positive results respecting the annual fluctuation. 

 § 2G. We have, in §§ 20, 21, and 22, found various periods of 

 the after-shock frequency whose lengths vary from a few hours to 

 several months. Tlie five periods of about 4 hours, S or Ü hours, 4^ 

 days, 12 days, and 33 days, seem to occur constantly in after-shocks 

 of différent earthquakes, lîesides these, there may exist also a con- 

 stant period of some three months, (see § 20). iîut the longest period 

 is different in the cases of the Mino-Owari and the Ivumamoto earth- 

 quakes, the average length being about G months in the former and 

 8 months in the latter. 



To see whether these periods and the diurnal atid annual fluctua- 

 tions stated in §§ 24 and 25 can be identified in the frequency of 

 ordinarji earth({uakes, we shall next consider the seismometrical 

 observations in Tokyo and over the whole of Japan. 

 § 27. (it) Seismometrical Ohscrraiioiis in Tökijö. 

 The hourly distribution of IKÎS earthipiakes rec.mled instru- 

 mentally at Tokyo during sixteen years, from INTG to 1S<)1, (Table 

 X\'III),' is shewn in Fig. li>, (4). l^he diurnal fiuctuation presents 

 tJirce mai-inia, which occur resj-ectively between Î) and 10 a.m., between 

 3 and 4 p.m., and between <S and !> p.m.; and the titrée minima, which 

 occur respectively between 2 and 3 a.m., between 11 a.m. and noon, 

 and 1)etween (> and 7 p.m. The intervals between successive maxima 

 are (i. 5, and 13 hours, and those between successive minima 9, 7, and 

 S hours. The mean curve, drawn in red, seems to indicate a 

 very slight diurnal variation, of which the maximum occurs in the 

 evening and the mininunn in the early morning. 



The monthly distribution of 1300' earthquakes instrumentally 



1 Tables XVIIl ami XIX are t iken, hj periuission, from the Earthquake Report or the 

 Central Meteorological Observatory. 



2 See the uote to Table XVII. 



