軟弱地層を伝わる爆破地震動

Abstract

The problem related to seismic waves generatad by small dynamite charged explo-sions in the field covered with alluvial deposits has been investigated based upon theexperiments carried out by the Exploration Group of Japan in 1956 and 1965.There are four different types of wave groups: namely, wave groups of I, II, III, and IV.The comparative investigations on the wave groups HI and IV were made boththeoretically and experimentally by several authors and it is concluded that they are dis-persive Rayleigh waves (Mn, M, 1) guided in the layered media. Consequently, in thepresent paper, characteristics of the wave groups I and II have been mainly treated.The wave group I is concluded to be composed of various kinds of converted wavesgenerated from the interfaces under the surface. While, in the wave group II, threedifferent kinds of waves are found; a kind of surface waves having the longest wave lengthamong the wave groups, a wavelet of direct P-wave and refracted SV-waves. Amongthese three kinds of waves, the main part of the wave group H having a surface wavetype is more interesting.From the investigation of the main part of the wave group II it is concluded thatthe characteristics of this type of waves are just equivalent to those of the normalmode waves in the liquid-liquid layers.On the other hand, the Poisson's ratios of our experimental field are almost 0. 5and observed seismic records from the explosions can be divided into two, that is, P-and S-zone before and after the train of refracted S-waves.The computations of the amplitudes of the converted SV-waves derived from theincident P-waves show that in the P-zone the converted SV-waves have a negligiblesmall amplitude compared with the P-waves when the Poisson's ratio of the medium isalmost 0. 5. Thus in the P-zone it resembles closely to the liquid state.