DE VEL OP ME NT OF SEISMOL OGY IN JAPAN 27 1 



seismic ray emitted in tlie liorizontal direction from the origin is most 

 intense. 



Eecently, S. Kunitomi^^^ found from his own examination of the 

 reliable records of a number of earthquakes that the velocity of the 

 longitudinal waves is decidedly greater in the direction perpendicular 

 to the arc of the Japanese Islands than along the arc when the origin 

 lies on the Pacific side of the land, the velocities being respectively 

 9.6 and ^.^ km/sec. The difference is not so marked in the case of 

 Japan Sea earthquakes. The velocity of the surface waves show, in 

 the meanwhile, no sensible difference with the direction of propagation. 

 His subsequent investigations in a number of local shocks have verified 

 the above inference. The explanation of the remarkable fact has been 

 sought in some aeolotropy of the rocks forming the backbone of the 

 land. On this latter point an alternative explanation has been pro- 

 posed by T. Matuzawa^^^, from his theoretical discussion of the mode 

 of propagation of waves in crystalline niedia. 



The existence of the so-called Mohorovicic wave has drawn but 

 little attention in this country, till recently K. Wadati^^^ fell upon 

 this kind ef waves independently in the course of his studies on the 

 Tazima Earthquake of 1925. His results turned out to be a beautiful 

 confirmation of Mohorovicic's view. Wadati found that the P-phase 

 was overtaken by P at a distance of about 120 km. This distance 

 has also marked a very conspicuous boundary, within which the first 

 motion is directed inwards, i.e. towards the origin, with a conspicuous 

 amplitude, while outside this boundary the motion is directed outwards 

 and is decidedly small. According to his calculation, the focal depth 

 of the said earthquake was 32 km., while the depth of the layer of 

 discontinuity turned out to be 42 km. Kunitomi^''^ showed that the 

 difference of the velocities in different directions as above cited 

 holds good for the P-wave, i.e., for the upper layer of the earth crust, 

 as well as for the lower layer, but to a less marked degree in the 

 latter. 



K. Suda^^^ made also some detailed investigations on the Mohorovicic 

 waves and found that these waves appear more frequently on the 



(1) J.M.S., [iil 3 (1925), 55, 176, 281, e. 



(2) Jap. J. Astr. Geophys., 4 (1926), 1. 



(3) J.MS., [ii] 3 (1925), 201. 



(4) J.M.S., [ii] 3 (1925), 281, e. 



(5) U.t.S., 5 (1925), 182, 202. 



