The Wenchuan earthquake occurrence background in deep structure and dynamics of lithosphere

Abstract

Because the western region of China is pushed and pressed to the north by the Indian plate,the Qinghai-Tibet Plateau is extensively deformed,and strong earthquakes occur frequently on the active faults in the plateau interior and on its margin.This is a most active earthquake area in the intracontinent.The M=8.0 Wenchuan earthquake occurs in the main central fault zone of the Longmen Mountains belt tending towards NE.The main central fault zone separates the Songpan-Ganzi block and the Yangtze block on the east margin of the Qinghai-Tibet Plateau.This paper uses the results of the surface wave tomographic image,the passive seismological experiment profile and the explosive seismic profile across the Longmen Mountains to research the lithosphere structure and dynamic characteristics of the Songpan-Ganzi block and the Yangtze block.It is discovered that there is a significant difference in the lithosphere structure and the nature between the two blocks.The lithosphere of the Yangtze block has characteristics of high speed,firmness and stable state,but that of the Songpan-Ganzi block has characteristics of low speed,brittleness and easy break.Generally,a low velocity layer exists in the middle crust of the Songpan-Ganzi block.It is the slipping layer to push the upper and middle crust upthrust movement to form the Longmen Mountains belt.The depth of the main shock is 14 km,and the hypocenter is just located in the Yingxiu-Beichuan central fault zone.