抄録
Reliability of the pressure and tension axes in double couple mechanisms of deep earhquakes determined from P-wave initial motions is estimated for each subdivided region of the typical seismic zones in and near Japan. Our method of estimating the reliability is explained as follows: Based on the known results that mechanisms of earthquakes occurring in a small region are so similar to one another that we may define a representative mechanism in the region, we first imagine a hypothetical earthquake of the mechanism located at a point in each of appropriately divided regions. We next assign data of P-wave initial motion directions expected theoretically for the representative mechanism to 112 standard stations of the J.M.A. Network. The data are now regarded as observational data at these 112 stations. We can calculate in reverse the focal mechanism of hypothetical earthquake by the use of these fictitious data, where the representative mechanism from which the data are originally computed, of course, satisfies the data completely. However, if the pressure and tension axes are allowed to move to any direction on a focal sphere as far as the number of inconsistent stations is less than a prescribed small number including even zero, we can calculate permissible variations in directions of the pressure and tension axes, leading to an estimation of the reliability.
The main results are summarized as follows: A wide variation in orientation is permissible for both the pressure and tension axes in the seismic zones beneath the Japan Sea near U.S.S.R. and beneath the Pacific Ocean south off the coast of the Honshu (the main island of Japan), where distributions in azimuth of stations are not sufficiently good. In these regions both the axes are able to move on the focal sphere up to an angular distance of 50 to 70 degrees (90 degrees in some cases) in a particular direction, even though the number of inconsistent stations is taken to be only one. Determinations of both the pressure and tension axes are quite stable in the seismic zones beneath the Japan Sea coast near the central part of Honshu, where the azimuthal distribution of stations is satisfactory.
