Characteristics of the regional stress field and tectonic movement on the Qinghai-Tibet Plateau and in its surrounding areas

Abstract：The authors systematically analyzed the focal mechanism solutions of 905 earthquakes （M4.6-8.5） that occurred in and around the Qinghai－Tibet Plateau between 1931 and 2005 and studied the characteristics of the regional stress field and tectonic movement of the lithosphere below the Qinghai-Tibet Plateau. The results show that the NNE or NE horizontal compressional stress due to the northward movement of the Indian plate predominates the tectonic stress fields in and around the study region. The horizontal components of the principal compressional stress P axes are oriented in a nearly NE-SW direction, forming a wide NE-SW compressional stress field in a wide areal extent from the Himalayas to south of Baikal Lake, including western China. Especially in the surrounding areas of the Qinghai－Tibet Plateau, earthquakes caused by reverse and strike-slip faulting are predominant in the Himalayan Mountain front on the southern Qinghai－Tibet Plateau and on the northern and western margins of the plateau, except the eastern margin. It implies that a strong horizontal compressional stress exists around the plateau. The characteristics of the stress field show that the direct cause of the uplift of the Qinghai－Tibet Plateau is due to the compressional stress field formed by the northward movement of the Indian plate and India-Eurasia collision. Many normal fault type earthquakes are concentrated in areas with elevations of >4000 m above sea level on the central Qinghai-Tibet Plateau while the surrounding areas of the Qinghai－Tibet Plateau are subjected to the control of the strong compressional stress field. Of these, many earthquakes are pure normal fault type ones. Their focal mechanism solutions show that the nearly E-W or WNW-ESE horizontal tensional stress controls the lithospheric stress field in the region. The normal fault type earthquake faults mostly strike N-S and the horizontal component of the displacement vectors of the faults is largely oriented in a nearly E-W direction. This indicates that a nearly E-W extensional movement exists in areas with a high altitude in the central part of the plateau and that the extensional movement is the result of the action of the tensional stress field. The dynamics for the extensional movement may be attributed to the gravitational collapse caused by the increasing load of the continuously rising plateau and the regional stress of its surroundings. The study of the regional characteristics of the compressional and extensional stress fields and their tectonic movement on the Qinghai－Tibet Plateau has very great significance for understanding the geodynamic mechanisms of the formation and development of the Qinghai-Tibet Plateau.