MEAN STRESS FIELD IN TANGSHAN AFTERSHOCK AREA OBTAINED FROM FOCAL MECHANISM DATA BY FITTING SLIP DIRECTIONS

Alternatively taking each of the two nodal planes of focal mechanism solutions as the possible fault plane,the mean directions of principal stress axes and the relative magnitudes of intermediate principal stress in 3 sub-regions of the Tangshan aftershock area are estimated from 29 focal mechanism solutions by minimizing the angle between the calculated shear stress and the observed slip vector on every fault plane. The result indicates that the mean principal compressional stress axis in the Tangshan aftershock area is nearly horizontal,taking approximately the E-W direction. Comparing with the situation before the great 1976 Tangshan earthquake,this axis may have horizontally rotated about 30?clockwise. The stress axis directions inferred from the mean P,B and T axes of the composite fault plane solutions of aftershocks in the same 3 sub-regions are generally in agreement with the above result. A numerical test shows that,in some special case,there may appear a systematic deviation between the directions of the mean P,B and T axes and that of real principal stress axes in the crust.