ON THE CHARACTERISTIC OF DIRECTION OF THE EARTHQUAKE STRESS FIELD AROUND THE BEIJING AERA

The composite nodal-plane solutions of the earthquakes occurred in the area around Beijing during 1960 to 1977 are obtained on the basis of P wave onset data and also the confidence curves of the results. The directions of the maximum and minimum principal stresses are given as NE 65°—75° and NW 15°—25° respectively, whereas that of intermediate stress is nearly vertical. They appear to be uniform and stable in the entire area. The fault angle of the crustal rock is estimated not less then 27°. It is inferred that the earthquakes in the area are caused probably by the action of additional horizontal tension. No significant variation of the composite nodal-plane solutions was found before the great Tangshan earthquake.