ON THE DIRECTION OF THE MAXIMUM COMPRESSIVE PRINCIPAL STRESS BEFORE AND AFTER THE 1976 SONGPAN-PINGWU EARTHQUAKE(M=7.2) OF THE SICHUAN PROVINCE

The average nodal plane solutions of the earthquakes occurred in southern Gansu and northern Sichuan Provinces from 1966 to 1976 are obtained by use of P-wave onset data. On the basis of these solutions, it can be inferred that the direction of the maximum compressive principal stress during the process of development of the earthquake is nearly east-west before the Songpan-Pingwu Earthquake. Whereas that obtained from the data of the Songpan seismological station, it appears to have been rotated by an angle of about 90? However the direction of the maximum compressive principal stress in releasing as obtained from data of aftershocks is also nearly east-west.