DE VET OP ME NT OP SETS MO I. OG Y TN JAPAN 289 



His considerations were, however, based on the characteristic elastic 

 properties of rocks on which he made extensive experimental studies, 

 developing Nagaoka's^'^ research on the same matter. He found that 

 the elastic constants of rocks become apparently smaller when the 

 range of applied force is larger. Tliis fact was theoreticall}^ treated 

 as the result of elastic yielding, for which he obtained the law as a 

 logarithmic function of time, both from theoretical and experimental 

 gi'ounds. The elastic constants turn out smaller for larger deformation, 

 and hence the small quick vibrations outrace the larger slow motions. 

 The latter is after all the principal portion and the former the 

 preliminary tremors of an earthquake. 



By the way, Kusakabe's^-^ experiment on the effect of heat on the 

 elasticity of rocks has revealed a remarkable fact that a plutonic rock 

 such as granite shows a considerable plasticity at the comparatively 

 low temperature of 400°-500°C. This property is quoted by him in 

 association with the liability of this kind of rock to form veins or 

 intrusions into subterranean fissures. In the applications of his theory, 

 he introduced the idea of seismic-wave-conductivity,*^^^ as already 

 mentioned, with which he made a general discussion on the relations 

 between the geological structures and the intensity of the frequency of 

 earthquakes. 



On the other hand, different valuable suggestions on the nature 

 ol earthquake motions as well as many allied problems, were proposed 

 by Nagaoka. In his discussion on the rigidity of the earth,^*^^ he 

 noticed that the velocity of elastic waves deduced from seismic obser- 

 vations agrees with that estimated from the Chandler period. In 

 another paper,-'^' he remarked that the deviation from Hooke's law 

 may result in the formation of overtones and combination tones also 

 in the case of seismic disturbances. 



The formation of tails in. distant earthquakes was explained by 

 Nagaoka*^"' as a result of discussions on damped progressive waves. 

 Starting from the equation for damped plane waves in its generalized 



form d- U ^- U d fJ 



= c' - 2/9 f-U, 



d t- d x^' d ^ 



(1) Pub., 4 (19001 



(2) T.S.B.K., 3 (1906), 110. 



(3) T.S.B.K., 2 (1905), 395, 447; 3 (1906), 88. 



(4) T.S.R.K., 2 (1905), 353. 



(5) T.S.B.K., 2 (1905), 443; Omori's discussion, 460; reply, 46^ 



(6) T.S.B.K., 3 (1906), 17. 



