The Spatial and Temporal Variation of Modern Tectonic Stress Field in North China before and after the 1976 Tangshan Earthquake

By using 126 earthquake focal mechanism solutions (M S≥4.7) during the period of 1963～1998, modern tectonic stress field in North China is inverted by means of the step by step convergence. The inversion results indicate that the tectonic stress field in the research region is clearly variational in space and time: (1) The middling principal stress axis σ 2 is basically vertical. The maximum and minimum principal stress axes σ 1 and σ 2 are nearly horizontal, but the azimuths of σ 1 and σ 3 are inconsistent in different districts and periods. (2) Before the Tangshan earthquake in 1976, the three principal stress axes are uniform. The azimuth of maximum principal stress axis σ 1 is 68° (striking in a NEE-SWW direction). (3) After the Tangshan earthquake, the maximum principal stress axis σ 1 and minimum principal stress axis σ 3 have variations in different districts. In the northern area of North China and on the eastern side of the Tancheng-Lujiang fault zone, the maximum principal stress axis σ 1 is also striking in a NEE-SWW direction. Its azimuth is 68°. It is the same as that before the Tangshan earthquake. In the southern area of North China, the maximum principal stress axis σ 1 is striking in a E-W direction and its azimuth is 87°.