Regional characteristics of crustal stress field and tectonic motions in and around Chinese mainland

Focal mechanism solutions of 3115 middle and great crustal earthquakes(M≥4.6)have been analyzed in and around China between 1918 and 2006.The distribution of extensional and compressive axes was studied by statistic methods.The results suggest that crustal stress field and tectonic movement in and around China are attributable to motions relative to the Eurasian plate of the Indian,Pacific Ocean and Philippine Sea plates,and interactions along blocks of the mainland.The compressive stress due to the collision between the Eurasian and Indian plates dominates the movements from the Himalayas to Tianshan mountains in west China.Azimuths of the horizontal component of compressive stress P-axes likely lie between 20°and 40°,resulting in a stress field near about NE direction in the wide region west of the North-South Seismic Belt.Strong and great events with thrust faulting occurred frequently along the edges of the Tibetan plateau and in the Tianshan region.A lot of normal faulting type earthquakes concentrate in the central part of the Tibetan plateau with high elevations.It means that while north-south shortening occurs in the Tibet plateau and its surroundings,an extensional motion in near E-W direction,however,is revealed in the high elevation area of the plateau.Focal mechanism solutions in North China show that the events occurred under the compressive stress in near NE direction due to the subduction of the Pacific Ocean plate beneath Eurasian plate as well as an extensional stress of azimuth 170° from the Baikal extension.The boundary between North China and South China obtained from regional stress field is different from other tectonic results.The trend of boundary turns to southeast in and east of the Dabie region,passing through Wenzhou and the East China Sea,to the Ryukyu trench.The valley fault of Taiwan is the collision boundary between the Eurasian and Philippine Sea plates.The stress field due the northwestward motion of the Philippine Sea plate dominates the stress of lithosphere from the Taiwan region to the eastern part of the southern segment of the North-South Seismic Belt through South China.