Diurnal Periodicity of Earthquakes in Japan. 915 



The following results may be deduced from this Table : — 



(i.) In the early after-shocks, the maximum epoch of the 

 diurnal period occurs near midnight, or twelve hours later 

 than for ordinary earthquakes. The epoch of the semi- 

 diurnal period is at first variable. 



(ii.) In the later after-shocks — after a month or more at 

 Nemuro and after about two years at Grif u — the epoch of the 

 diurnal period returns to the neighbourhood of noon, and 

 that of the semi-diurnal period at Gifu to about 8 or 9. 



(iii.) In the early after-shocks, the amplitudes of both 

 diurnal and semi-diurnal periods are about double those 

 in the later after-shocks. 



Origin of Seismic Periodicity. 



(i.) Annual Period. — The maximum epoch of the annual 

 barometric period occurs almost invariably in December. 

 Prof. Omori lias shown that the resultant pressure on the 

 ocean-bed is subject to an annual variation, owing to the 

 presence of a high-pressure system in the neighbourhood 

 of the Aleutian Islands *. The maximum epoch of the 

 annual period occurs in October at Misaki, and in September 

 at Ayukawa, Otaru, Iwasaki, Wajima, and Hamada. The 

 annual periodicity of earthquakes may be due largely to that 

 of barometric pressure, but this is probably not its only 

 cause. The summer maximum in earthquake-frequency at 

 the north-eastern stations may be due in part, as Prof. 

 Omori suggests, to the variation in pressure on the sea-bed. 

 In this part of the country, many earthquakes are of sub- 

 marine origin. 



(ii.) Semi-Annual Period. — The first maximum epoch of 

 the semi-annual period occurs from March to May, usually 

 in March or April. That of the mean pressure on the 

 ocean-bed occurs in April, May, or June. As the maximum 

 epoch of the semi-annual period, whether for land or sea 

 earthquakes or both, usually occurs from March to May, it is 

 probable that the semi-annual periodicity of earthquakes is 

 due to that of the barometric pressure, the variation in total 

 pressure on the sea-bed being due to the same cause. 



(iii.) Diurnal Period. — The maximum epoch of the diurnal 

 period of barometric pressure at Tokyo occurs atabout H a.m., 

 that of wind-velocity at Tokyo atabout 0.45 p.m., and that o( 

 the barometric gradient between Nogano and Tokyo at about 

 0.15 P.M. The maximum epoch of the diurnal period o( 



* Publ. Earthq. Inv. Com., No. 18, pp. 23-26 (1904). Bull. Earthq. 

 Inv. Com., vol. ii. pp. 35-50 (1908). 



