FOCAL MECHANISMS AND CRUSTAL STRESS FIELD IN NORTH CHINA

The focal mechanism data of 48 earthquakes of North China (30-41N, 105-124E) from 1937 to 1979 were analysed. They have been selected according to their reliability and spatial distribution of their epicenters. The results show that the com-pressional principal stresses P are mostly in the northeast-east direction and the tensile T in the north-northwest direction, both P and T are nearly horizontal; the null axes N are mostly rather steep; the two nodal planes strike along the north-northeast and northwest-west directions respectively, and the earthquakes of North China appear to be of the strike-slip type.The focal mechanisms of some small and moderate earthquakes appear to deviate from the above uniformity and being more disorderly, probably due to the effects of various local factors. Therefore, it seems that the focal mechanism of a single small or moderate earthquake will not represent the general feature of the stress field in the crust of the whole region.