262 T. Teraua and T. Mai'uzaWa 



the periods with durations of 4.5, 12, 33 days, and 3 months respect- 

 ively. Sunspot period has also been spoken of. In these respects, a 

 similar remark as given at the end of the last paragraph may be 

 applied. It may be added here that Terada^^^ drew attention to the 

 iact that an ajiparent or ' phantom ' periodicity may be brought forth 

 by a quite arbitrary sequence of events. 



(f ) Occurrence of fore-shocks. Examples of fore-shocks observed 

 before the main shocks have often been cited on the occasions of many 

 conspicuous earthquakes'^^ The data are, however, not yet sufficiently 

 abundant to allow of statistical treatment. The apparent existence 

 or non-existence of weak fore-shocks depends evidently on the sensibi- 

 lity of ihe instrument. In this respect, Shida's investigations^^^ now 

 in progress with an extremely sensitive seismograph is to be awaited 

 with much expectation. 



(g) Time distribution of after-shocks. Omori's hyperbolic formula 

 regarding the decay of the frequency of aftershocks is well, known^"'-'. He 

 showed that the same formula holds good even after the lapse of 

 eight years in the case of the Mino-Ovvari earthquake, though in some 

 cases it failed, as in the case of the Hukuoka earthquake of 1898. He 

 also noticed a quasi-periodic character of the deviation from the 

 formula. The physical interpretation of the formula has been attem- 

 pted by 0. En'ya^^^ and later elaborated by Kusakabe^"^ based on his 

 researches on the elastic properties of rocks. The latter inferred that 

 the rate of decrease of the frequency of after-shocks is determined by 

 the duration of time required for the accumulation of the stress before 

 the break down is effected. His conclusion has been applied by Saem. 

 Nakamuni,^^^ and K. Suda^^^ in the discussions of the time-frequency 

 relation with regard to the after-shocks of the Oomati earthquake and 



(1) T.S.B.K., 8 (1916), 492. 



(2) Oinori, Ho., 68A and B (1910) ; Bull., 2, No. 2 (1908). A. Imamura, Ho., 

 70 (1910); 77 (191?>); 82 (1915); 92 (1920). N. Yamasaki, Ho., 11 (1897). T. Iki, 

 Ho., 29 (1899). N. Fukuchi, Ho., 38 (1902). 



(3) Not yet published. 



(4j Ho., 2 (1894); 30 (1900); 32 (1900); S4 (1906); 94 (1921); Bull., 2, No- 2 

 (1908); Journ. Coll. Sci. Tok., 7 (1895) 112. See also Imamura, Ho., 70 (1910); 77 

 (1913); lOOA (1925); Seism. N., 6 (1924). 



(5) Ho., 35 (1901); 57 (1907); 61 (1908). 



(6) Pub., 17 (1904). 



(7) J.M.S. 38 (1919), 147. 



(8) Mem. Imp. Mar. Obs., 1, No. 4 (1924). 



