Properties of the Stress Field in and around West China Derived from Earthquake Mechanism Solutions

The properties of the stress field in the West China region has been analyzed based on the 174 new solutions of focal mechanism of earthquakes with magnitudes (Mb) from 4.5 to 6.2 and the earthquake generating stress field from smoothed radiation patterns using more than 5000 initial motions of P waves of 169 earthquakes in the six concerned regions. Almost all events discussed in this paper were shallow ones. These data were obtained from the Bulletin of the International Seismological Center during the period from 1971 to 1980. The main results are as follows: In the West China region, the compressional axes mostly lie in N-S and NE-SW directions, and the tensional axes mostly lie in E-W and NW-SE directions. This general view of the distribution of the stresses over a wide area suggests that there is the transmission of tectonic forces from the collision between the Indian plate and the Eurasian plate along the Himalayan mountains through the Tibetan plateau. In the Himalayan mountains, along the margins of the Tibetan plateau and in the northwestern China regions, earthquakes caused by reverse faulting and strike-slip faulting are predominant. A lot of events with normal faults are concentrated in the western and central area of the Tibetan plateau with elevations higher than 5000m. It shows that the tensional forces in the WNW-ESE direction are predominant in this region. The directions of P-axes and T-axes are not so simple along the margins of microplates as are those in their inner part. Comparing various geological and geophysical data, the reason for the generation of such distribution of stress fields was disscused.