290 T. Tekapa and T. Matfzawa 



tlie expressions for tlie tails following the princij)al disturbances were 

 deduced, which appear as the effects of damping and a certain reaction 

 fU, for example the reaction of an underlying layer. 



Interesting remarks on the dispersion of seismic waves^^^ have also 

 heen made by the same investigator. Mountains, mountain ranges, 

 small islands, lakes, plains, etc. were considered as bodies embedded 

 in the transmitting medium and their effects on the incident plane 

 waves were discussed, after the method of treating the anomalous 

 dispersion of light. According to his argument, the principal portion 

 of earthquake motion is explicable as waves satisfying a differential 



equation of the type = c^ , i.e. longitudinal or transverse 



waves. On ihe other hand, the preliminary portion could be associat- 

 ed with waves of the tlexiu-al type. The period corresponding to the 

 anomalous dispersion was supposed to be 14 seconds or so in Tokyo. 

 Next, as a natural sequence, the j)roblem of group velocities'^^ to be 

 expected in distant earthquakes, was taken up. The group velocity 

 of a fiexural vibration is approximately twice the wave velocity. 

 If the liquid interior be assumed, gravitational waves are also possible. 

 Thus the group velocity of sea waves is one half of the wave velocity. 

 A tentative explanation has been proposed tor the presence of the 

 iirst and second preliminary tremor. 



K. Aichi'^^ treated the case of transverse waves on the surface of 

 heterogeneous material and showed the existence of dispersion in a 

 special case. Eecently, K. Suda<^^^ discussed the diffraction of seismic 

 waves, developing Stokes's theory on the diffraction of light and 

 attempted to explain the directions of earthquake motion. "^^^ T. 

 Matuzawa'"^ in his recent paper to be quoted later, touched on the 

 problem of the motion of bodies embedded in the medium through 

 which elastic waves are propagated. 



As a secondary cause of earthquakes, the effects of surface loading 

 over a circular area were considered by Nagaoka/"^ Even a very 



(1) T.S.B.K., 3 (1906), 44. 



(2) T.S.B.K., 3 (1906), 52. 



(3) T.S.B.K., [Hi] 4 (1922), 137. 



(4) J.M.S., 41 (1922), 426. 



(5) U.t.S., 5 (1925), SO. 



(6) Jap. J. Aptr. Geophys., 4 (1926), 1. 



(7) T.S.K.K., 3 (1906), 75; Pub., 22A (1908). 



