DEVELOPMENT OF SEtSMOLOG Y IN JAPAN 265 



of the sea-level at, bi'foi-e or after the occurrence of the earthquake, 

 though the physical meaning of these facts is not clear. 



Recentlj' T. Isikawa^'-* found that the earthquake due to the 

 dislocation of a fault line is liable to occur when the barometric 

 gradient is changing its direction and tends to fall in with the 

 direction of the fault line, or when a conspicuous line of discont- 

 inuity of barometric pressure passes over the epicentral region. 



Among the relations which have been suspected to exist between 

 the frequency of earthquakes and meteorological phenomena, the most 

 remarkable is that found by Omori^-^ between the yearly number of 

 shocks in Tokyo and the precipitation in the Japan Sea coast of 

 Hokurikudo. The frequency is greater for the years with greater 

 amounts of rain and snow. T. Terada'^^\ from his studies on the 

 geographical distribution of precipitation, is inclined to believe that 

 it is the general barometric gradient in the central part of Japan 

 which determines both the earthquake frequency and the amount of 

 precipitation in the respective districts. 



The relation between the weather and earthquakes has also been 

 investigated by Omori^'^^, Saem. Nakamura^^^ and others. The results 

 are not quite decisive as might have been well expected. Nakamura 

 also remarked a possible relation between tiie mean diurnal range of 

 the magnetic declination of a year and the number of earthquakes in 

 the next year^''^ 



A probable relation between the occurrence of strong earthquakes 

 and the variation ot latitude has been pointed out by Omori*^^^, though 

 we must wait a longer time before the relation may be verified an 

 abundance of material. 



(i) Other problems regarding time distribution. Remarkable 

 examples in which earthquakes have occurred in a district in the 

 same hours of a day or on tlie same day of the same month have, 

 often been cited by Omori^*\ 



(1) J.M.S., [ii] 3 (1925), 255, e. 



(2) PIo., 68A (1910); I5ull., 1, No. 2 (1907j ; 2, No. 2 (1908). 



(3) Journ. Coll. Sci., 41, Art. 5 (1919). 



(4j IIo., 2 (1894); Bull., 2 (1908); J.M.S., 29 (1910), 87. 



(5) J.M.S., 40 (1921), 173. 



(6) T.S.B.K., 9 (1918), 319. 



(7) Ho., 49 (1905). 



(8) IJo., 57 (1906). 



