D E VEL OP ME NT OF SEISMOL OGY IN JAPAN 275 



shows tliat the Eayleigh wave requires a certain distance to he tra- 

 versed by the bodily waves before it is developed into a conspicuous 

 existence. Nakano^^^ also treated of the propagation of elastic waves 

 caused by different kinds of ideal sources of disturbance, consisting 

 of multiplets of displacements, and obtained a variety of possible 

 distributions of earthquake motions which could be cited for the 

 explanation of different actual examples. More recently, T. Matuzawa^-^ 

 discussed a similar mathematical problem and arrived at various 

 inferences of practical importance. According to their results, the case 

 of the Simabara earthquake, above cited, may be accounted for by 

 assuming a rotational centre of disturbance. 



At any rate, it seems beyond doubt that in most cases of near 

 earthquakes, the tirst part of the principal motions is not directly 

 associated with the Eayleigh wave. 



On the other hand, Saem. Nakamura^"'^ carried out the simult- 

 aneous observations of local earthquakes in the inside and the outside 

 of a railway tunnel. Though the results are not yet conclusive, they 

 seem not wholly unfavourable to the assumption of the existence of 

 the Eayleigh wave, so far as the ratio of the amplitudes of motions 

 in and outside the excavation is concerned. 



On the other hand, examples have been pointed out by Omori''^^ 

 and Kawazoe^"^ in which the preliminary phases were absent or very 

 faint in some stations, whereas in other stations comparable in distance 

 from the origin they appeared with the usual magnitudes, Terada'"^ 

 suggested a possible explanation by assuming a mosaic structure of 

 the superficial crust consisting of loosely connected blocks with more 

 or less independent freedom of vibration. Matuzawa, in his paper 

 just cited, showed that if the blocks were to be regarded 

 as embedded bodies in an infinite solid lying in the line of 

 propagation, tlie growth of their motion would be very gradual. 

 He showed that such anomaly may be plausibly explained by 

 assigning a proper mechanism of disturbance at the origin. Otiier 



(1) Seism. Bull, of C.M.O., 1 (1923) 92. 



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



(3) T.S.B.K., [iii] 7 (1925), 88, 127. See also K. Sekiya and F. Omori, Journ. 

 Coll. Sci., 4 (1891), 249. 



(4) Bull., 1, No. 3, (1907). 



(5) J.M.S., 38 (1919), 173. 



(6) Geogr. Eev., 1 (1925), 841. 



