EW-trending extensional motions in central Tibetan Plateau

In the present paper, the authors analyzed the earthquake mechanism solutions (M4.6～8.5) during 1933～2003 for the study of the extensional tectonic movement at the surface of the Tibetan Plateau and its relative dynamics. The results show that a lot of normal fault type earthquakes are concentrated in the central part of the Tibetan Plateau with the elevation greater than 4?000 m. Many earthquakes are pure normal fault events. Their focal mechanism solutions show that the strike directions of faults are almost in the N-S direction. The dislocation slip vectors of the normal fault type events have large components in the E-W direction, suggesting the extensional motions in nearly E-W direction in the high elevation area of the plateau. The tensional forces in the E-W or WNW-ESE directions are predominant in this region, and may result in the above extensional tectonic motions. The dynamics for extensional motions can be attributed to the gravitational collapse of the rising plateau and the stress of its surrounding areas. Around the Tibetan Plateau, earthquakes caused by reverse and strike-slip faulting are predominant on the north and west margins (except for the east margin), and the Himalayan Mountains. Horizontal stress field in nearly NE or NNE direction dominates the tectonic motions along all margins. The tectonic forces from the relative movement between the Indo-Australian and Eurasian plates cause P-axes to line up in the NE-SW direction in the western China.