270 T. Terada and T. Matuzawa 



meaning and the degree of accuracy have often been subjects of much 

 discussion, especially among the adherents of the physical school of 

 seismology. The improvement of the formula has often been attempt- 

 ed, for exam])les by K. Hasegawa^'^ and Saem. Nakamura"^^^. The 

 latter derived a parabolic formula instead of the linear one, for the 

 cases of near earthquakes, from a theoretical consideration consisting 

 of an extension of Wiechert's theory on the seismic wave propagation. 

 Nevertheless, Omori's formula has survived witli all its proper practical 

 merits. 



For finding the position of the epicentre by means of Omori's 

 formula, many practical methods have been devised, for example by 

 T. Usiyama^^^ and M. Kawazoe^''^ The latter pointed out the difficulty 

 of identifying the introduction of the S-phase and proposed to take 

 the direction of motion into account for this purpose. He also made 

 a statistical study of the ratio of the durations of the P- and the S- 

 phases. 



Omori's formula has also been utilized for the determination of 

 the depth of the seismic focus, on the assumption that the formula 

 preserves its validity for any distance up to the origin. The method 

 has been discussed by Hasegawa^^^ K. Yamazawa'^*'^ M. Kawazoe^'^^ H. 

 Maruoka^®^, etc. The dei)th obtained by these methods for different 

 earthquakes varies within the wide range of 10 to 60 km. R. Hirano*^^^ 

 made an elaborate use of the above mentioned formula for the case of 

 the great Kwanto Earthquake and obtained 40 km. On the other 

 hand, Saem. Nakamura^^"^ obtained, from his application of Wiechert- 

 Geiger's theory iii the case of near earthquake, a depth so great as 

 160 km., which is much at variance with the above values. The 

 latter investigator gave also another method for estimating the focal 

 depth from the intensity distribution, assuming after Shida, that the 



(1) J.M.S., 36 (1917), 359, 



(2) J.M.S., 36 (1917), 425; T.S.B.K., 9 (1918), 224; [iii] 3 (1921), 116. 



(3) J.M.S., 41 (1922), 114. 



(4) J.M.S., 35 (1916), ]85; 38 (1919), 173; 39 (1920), 228. 



(5) J.M.S., 35 (1916), 385. 



(6) J.M.S., 30 (1911), 99. 



(7) J.M.S., 36 (1917) 332. 



(8) J.M.S., 30 (1911), No. 10, 16. 



(9) J.M.S., [ii] 2 (1924), 112. 

 (10) J.M.S., 37 (1918) e. 10, 43. 



