DEVELOPMENT OF SEISMOLOGY tN JAPAN 291 



remote centre of atmospheric depression can affect tliG equilibrium of 

 the earth crust and give rise to pulsatory motions. Later, he"^ treated 

 an allied problem in connection with the effect of a barometric 

 gradient on earthquake frequenc}^, and showed that the rupture of the 

 crust is to be expected near the maximum of the gradient. As to the 

 pulsatory motions/-^ he also discussed the stationary vibration of one 

 end of a semi-intinite solid with two parallel edges. 



A similar problem of surface loading has been treated later by 

 K. Terazawa."^''^ The periodic disturbance of the level arising from the 

 load of neighbouring oceanic tides was discussed. The result arrived 

 at by him has recently been utilized by Suda^"*^ in his investigation 

 of the level changes of a well. 



0. Enya^^^ made an early attempt to explain the time distribution 

 of the number of after-shocks observed at the origin. His theory was 

 based on a hypothesis made on the manner in which the instability 

 of the crust determining the occurrence of the after-shocks, is removed 

 by these shocks He arrived at a logarithmic form of the formula for 

 the relation between the frequency and time. He also treated the 

 number of after-shocks observed at a })lace distant from the origin, 

 though the results obtained are somewhat complicated. 



S. Kusakabe*®' treated the same problem based on the results of 

 his experimental researches on the elastic properties of rocks. His 

 assumption was that the frequency of aftershocks is proportional to 

 the time rate of recovery from the state of residual stress to which 

 the crust is subjected. Both Omori's hyperbolic and Enya's logarithmic 

 law could be deduced as special forms of approximation. 



On the other hand, Nagaoka*^^^ pointed out a way of regarding 

 the decay of the frequency of after-shocks after the analogy of the 

 radioactive transformation and showed that the simple exponential law 

 of decay may be expected only in exceptionally simple cases. Terada^"*^ 

 drew attention to some analogy existing between the variation of the 



(1) T.S.B.K., 6 (1912), 208. 



(2) T.S.B.K., 3 (1906), 79 ; Pub., 22A (1908). 



(3) Jonrn. Coll. Sci. Tok., 37, Art. 7 (1916) ; Sci. Eep. Toh., [i] 7 (1918), 205 ; 

 Proc. Eoy. Soc. Lond., A 94 (1917), 13; Phil. Trans., A217 (1918), 35. 



(4) U.t.S., 5 (1925), 223. 



(5) Ho., 35 (1901). 



(6) Pub., 14 (1903). 



(7) T.S.B.K., 4 (1907), 66. 



(8) N.S.B.K., [iii] 1 (1919), 180. 



