ON SEISMIC WAVES GENERATED BY SMALL EXPLOSIONS [II]

Abstract

Field experiments on seismic wave generation by means of explosions has been carried out by the Exploration Group of Japan and clarified that four different types of wave groups (I, II, III and IV) are generally appeared. Except the wave group II satisfactory conclusions have been reached by many authors. In the previous paper characteristics of the wave group II has been mainly investigated and a proposal in connection with the generation mechanism of this wave group has been done, saying that the waves having the longest wave length in this group must be a kind of surface waves similar to the normal mode waves in the liquidliquid layers. The present paper is one of the continuations to the subject. At first computation and comparison of amplitudes, velocities of normal mode waves in the liquid layers to the observation have been performed and ascertained that the characteristics of the main part of the wave group II have many points of similarity to the normal mode waves in the liquid medium. When the Poisson's ratios of the experimental fields are nearly equal to 0.5 the observed seismic records may seem to be divided into two, that is, P- and S-zones before and after the head part of the critically refracted SV waves. Secondly through the computations of the amplitudes of the converted waves at the interface it was proved that in the P-zone the converted SV waves have negligible small amplitudes compared with the P waves in the medium of which Poisson's ratio is almost 0.5. Thus in the P-zone the circumstances resemble to the liquid conditions. The surface waves under consideration can be looked upon as the normal mode waves mentioned above. While, in the S-zone both P and SV waves are coexisting. The surface waves in this zone (wave groups III and IV) belong to the usual elastic waves in solids. In connection with the propagation phenomena of the seismic waves the medium, especially in the physical condition that the Poisson's ratio is near to 0.5, can have two different properties such as fluidity and elasticity.