Abstract
The seismicity of shallow e a rthquakes has been investigated to a considerable extent; on the other hand little is known as to the seismicity of intermediate and deep-focus earthquakes. This is mainly because of the low sensitivity of the present network of seismological observation in Japan, which happens to be most suitable for the zones of intermediate and deep-focus earthquakes.
In the seismograms recorded at Matsushiro by a sensitive electromagnetic seismometer, however, many shocks which are small and probably deep have been found, and an example is presented in Chapter II. In order to detect those small deep shocks, a new method of estimating focal depth and epicentral distance by means of S-P time and apparent velocity, which can be observed by a multi-point observation of comparatively small dimensions, is proposed and discussed also in Chapter II, since the highly sensitive observation cannot he allowed in most of the present recording stations owing to high background noise. Thus, a continuous tripartite observation to find the apparent velocity and direction of seismic propagation was started at Matsushiro in June,1959.
Chapter III deals with the instruments employed in this observation. Particularly, the improved film recorder, on which the observational accuracy largely depends, is explained rather in detail.
The observation results and the seismicity of small but d eep shocks will be published in the next paper under the same title. Chapter I. Introduction
