Abstract
It is well known that the ScS wave appears very distinctly in the seismograms of deep-focus earthquakes recorded near the epicenter. Contrary to our expectation, we have scarcely observed the sScS wave. The amplitude decrease of the seismic wave due to the reflection at the surface is first examined by means of the observational data, but it is proved that the scattering of waves caused by the reflection can not so effective as to make the sScS wave so weak. Thus the author has tried to interprete this phenomenon, “defect of S wave”, as an effect of a certain layer in the earth's crust. In some parts of the crust, which are of course shallower than the depth of focus, -possibly these exist just near the focus or in a very shallow place-there must be a special surface where the seismic waves are subjected to scattering according to the angle of incidence. For, the particles of the earth's crust are considered to be allowable to give relative tangential movements between the just upper and lower sides of this surface.
In the second part of this paper some calculations are done concerning this problem. The boundary conditions which have to be satisfied at this surface are assumed as follows:-
Thus the amount of the scattering of elastic waves due to the existence of such a surface is calculated for two cases when the incident waves are distorsional and dilatational respectively.
