Abstract
A method for computing focal mechanisms by using polarities and amplitudes of the P wave first motions was designed. In the first step, some suitable solutions were picked up by using only polarities. A method based on the ‘Fourier method’ designed by AOKI (1986) for fast computation was adopted. In the second step, the most suitable solution was selected by using amplitudes. In order to avoid the path effect on the attenuation of the P wave amplitude, an earthquake cluster occurring in a very small region was analyzed and the amplitude correction for each station was made.This method was applied to the analysis of the aftershock sequence following the earthquake with M4.9 which occurred in Kyoto-Osaka border region on May 28, 1987. As a result of this analysis, some characteristic features of the distribution of focal mechanism types were found. Most of focal mechanisms of aftershocks showed the same type as that of the main shock, that is, east-westward P axis and thrust type focal mechanism. But those of the aftershocks which located at the edge of the aftershock area or slightly distant from the fault plane of the main shock almost showed strike-slip type or intermediate type focal mechanism. Such focal mechanisms may be related to the stress field disturbed by the dislocation of the main shock.It is conculuded that this method is very effective to calculate focal mechanisms for a group of earthquakes whose polarity data are scarce with sufficient accuracy.
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Schedule a callMore From: Zisin (Journal of the Seismological Society of Japan. 2nd ser.)
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