DE VEL OP ME NT OF SETS MO L OGY IN JAPAN 259 



results obtained by their efforts, only a few will be summarized 

 below. 



(1) Time distribution of earthqvolie. 



(a) Annual variation of frequency. Annual or seasonal varia- 

 tion of the frequency of earthquakes has been extensively studied by 



Omori since the earliest days^^^ The variation shows quite different 

 and nearly opposite courses for destructive earthquakes and ordinary 

 or weaker ones^^\ For the data taken without regard to the locality, 

 the frequency shows a summer maximum for the former and a summer 

 minimum for the latter. The amplitude or range of variation is about 

 one half and one quarter respectively, of the average number, so that 

 the possibility of the result being due to accidental fluctuation can 

 safely be excluded. Besides, since the number of the destructive 

 earthquakes is only a small fraction of the total number of earth- 

 quakes, the chauce of obtaining the opposite variation for the two 

 classes uierely due to the arbitrary choice made in the discrimination 

 of the intensities of individual shocks, is out of the question. 



When different districts are treated separately, a remarkable fact 

 is revealed that there are two distinct types or groups of seasonal 

 variation, i.e. Group (A) in which the seasonal maximum falls in 

 spring and the minimum in summer or autumn, and Group (B) in 

 which the maximum is in summer and the minimum in winter or 

 autumn. The geographical distribution of these two types of district 

 shows some characteristics. Omori remarked that the difference of the 

 two groups consists in the difference of location of the seismic origins 

 which coutribute to the frequency in the districts. If we may be 

 allowed to use our own words, the districts belonging to Group (A) 

 are those localities which are ' sensitive ' to the earthquakes of inland 

 origin, while the districts of Group (B) are those mostly liable to be 

 disturbed by shocks of submarine origin. Thence, the opposite annual 

 course of variation for the two groups is attributed to the difference 

 of the annual variation of frequency for the origins situated in land 

 and sea. The physical agency giving rise to these modes of variation 

 was considered to be due to the effect of the variation of pressure 

 exerted upon the earth crust. The subject seem worth further study. 



(1) Ho. 2 (1894), 26 (1899), 30 (1900), 32 (1900), 54 (1906); Pub. 8 (1902); Bull. 

 2, No. 1. (1908); Journ. Coll. Sci. Tok., 11 (1899), 389. 



(2) Ho. 30 (1900) ; Pub. 8 (1902), 



