Tectonic stress field of China inferred from a large number of small earthquakes

Mean principal tress axes were inferred for China using 9621 P wave first motion polarity readings from 5054 small earthquakes (1 ≤ ML < 5). The area studied was divided into 76 subregions. The mean P (compressive), B (intermediate), and T (relatively tensional) axes corresponding to composite focal mechanism solutions of multiple earthquakes for each subregion were determined by a grid test to all possible orientations of the P, B, and T axes with a step of 5° or 10°. In order to get a relatively homogeneous sampling in space we have avoided using the readings from spatially and temporally clustered earthquakes. We have rechecked most of the polarity readings by inspection of original seismograms. The focal mechanism solutions of large (M≥6) individual earthquakes are also presented for comparison. The results indicate the existence of an overall radial pattern of the present-day maximum horizontal stress orientations throughout the continental area of China. This pattern is thought to be closely related to indentor models of continental collision between the Indian Ocean and the Eurasian plate.