ON SEISMIC WAVES GENERATED BY SMALL EXPLOSIONS

Abstract

The problem related to seismic waves generated by small dynamite charged explosions has been investigated based upon the experiment carried out by the Exploration Group of Japan at Shirane City, Niigata Pref. in 1965. There are four different types of wave groups; namely, wave groups of I, II, III and IV. On the wave groups III and IV, the comparative investigations were made both theoretical and experimental by several authors. Consequently, in the present 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 waves generated from the interfaces under the surface. While, in the wave group II, three different kinds of waves are found; a kind of surface waves having the longest wave length among the wave groups, a wavelet of direct P wave and refracted SV waves. Among these three kinds of waves, the main part of the wave group II having a surface wave type is more interest. Its wave form and characters are equivalent for the normal mode wave in the liquid-liquid layers. Namely, its phase velocity is lying between the propagation velocities of the P waves in the surface layer and half space, its amplitude distribution in depth shows the same form as the surface wave having a loop on the surface, one node in the layer and its amplitude decreases with depth. Moreover, Poisson's ratio of our experimental field is almost 0.5, then it resembles closely to the liquid state. Therefore, it is quite possible that among the wave groups generated by the explosions, there is a suface wave having a type similar to the normal mode wave in liquid-liquid layers.