Regional characteristics of the lithospheric stress field and tectonic motions in China and its adjacent areas

Abstract: The focal mechanism solutions of 3130 moderate and strong earthquakes occurred in continental China and its surroundings between 1918 and 2005 have been analyzed systematically, and according to their characteristics the tectonic divisions of the lithospheric stress field have been determined. The distribution of extensional and compressive axes was studied by statistics. The results suggest that the stress field of the lithosphere and tectonic movement in and around continental China are attributable to the motions of the Eurasian plate relative to the Indian, Pacific and Philippine Sea plates and interactions among blocks within the continental plate. The strong compressive stress due to collisions between the Eurasian and Indian plates dominated the movements from the Himalayas to Tianshan mountains in western China. Azimuths of the horizontal component of compressive stress P-axes likely lie between 20° and 40°, forming a nearly NE-directed stress field in the wide region west of the North-South Seismic Belt. Thrust type strong earthquakes occurred frequently along the edges of the Qinghai-Tibet Plateau and in the Tianshan region. Most normal fault type earthquakes are concentrated in the central Qinghai-Tibet Plateau with high altitudes. The horizontal component of the fault dislocation is oriented in an E-W direction. It implies that, simultaneously with the north-south strong compression and shortening in the Qinghai-Tibet Plateau and its surroundings, a nearly E-W-directed extensional motion apparently occurred in the high-altitude area of the central part of the plateau. Focal mechanism solutions in North China show that the earthquakes occurred under the combined action of the ENE compressive stress due to the subduction of the Pacific plate beneath the Eurasian plate and the NNW extensional stress in a unifying extensional stress field with a 170°azimuth in a vast region from Baikal Lake through North China to the Ryukyu Trench. The Longitudinal Valley fault of Taiwan is the boundary of collision and compression between the Eurasian and Philippine Sea plates. The stress field due to the northwestward motion of the Philippine Sea plate controls the stress field of the lithosphere from the Longitudinal Valley of Taiwan through the South China block to the eastern part of the southern segment of the North-South Seismic Belt. The results of the focal mechanism solutions also indicate that the North-South Seismic Belt that divides continental China into the eastern and western parts is the boundary of the influence scope between the Indian plate and eastern Philippine Sea and the Pacific plate within in the interior of continental China.