The problem related to seismic waves generated by small dynamite 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, comparative investigations were made from theoretical and experimental view points 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 at the interfaces beneath 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, a wavelet of direct P waves and refracted SV waves. Among these the first one, corresponding to the main part of this wave group, is most interesting.
In order to make clear the mechanism of development of this type of waves, the observed data have been compared with the computed results of amplitudes and velocities (phase- and group-) of the normal mode waves in the liquid layers, and it is ascertained that they 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.
Through the computations of the amplitudes of the converted waves at the interfaces it was proved that in the P-zone the converted SV waves have a negligibly small amplitude compared with the P waves in the medium of which the 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 (wave groups III and IV) in this zone belong to the usual elastic waves in solid.
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.