GEOLOGICAL BACKGROUND OF LONGMEN SHAN SEISMIC BELT AND SURFACE RUPTURES IN WENCHUAN EARTHQUAKE

Longmen Shan is located the special joint between the Qinghai-Tibetan Plateau in the west and the Yangtze craton in the east. Consisting of a series of parallel imbricated thrusts, it develops, from the west to the east, the Maoxian-Wenchuan, Yingxiu-Beichuan and Pengxian-Guanxian faults.The Longmen Shan fault belt is a dangerous earthquake belt and the three main faults have occurred Ms. 7 earthquakes. The Yingxiu-Beichuan fault would be one of the most important causes of the earthquake hazard. According to the Qingshiping trench in the Pengxian-Guanxian fault, we believe that in this fault, the latest strong earthquake took place in 930 40 a. BP.Then we can preliminarily determine the recurrence interval of strong earthquakes should be at least about 1000 years. We think that the Longmen Shan fault zone and its inner faults belong not only to a low-frequency seismic activity region and also to a potential powerful earthquake region. Wenchuan Earthquake is a thrust with strike-slip type. The surface ruptures are located in Yingxiu-Beichuan fault zone and Pengxian-Guanxian fault zone. The surface ruptures of the Yingxiu-Beichuan fault can be divided into two high-value and two low-value zones. The two high-value zones are located in the southern part of Hongkou, Yingxiu and the northern part of the Leigu, Beichuan- Dengjiaba area. The two low-value zones are located in mid-south part of the Baishuihe-Chaping area and Huangjiaba-Shikanzi, Pingwu areas. These four sections are divided by Xiaoyudong fault, Leigu fault and Dengjiaba fault. The two high-value areas were caused by the Xiaoyudong and Leigu faults. The scratches of the surface ruptures reveal the thrust movement occurred early and the strike-slip movement occurred lately during the earthquake. The earthquake is characterized by thrust and dextral strike-slip movement, which does not agree with both the crustal thickening model and the lateral extrusion model in the eastern margin of the Qinghai-Tibetan Plateau.  Based on the geological background, tectonic setting, stratigraphy and lithology, the active tectonics of Longmen Shan and surface ruptures of the Wenchuan Earthquake, a dynamical model to illustrate possible links between surface processes and upward extrusion of lower crustal flow channel at the eastern margin of the Tibetan plateau have been proposed. The result is the material in lower crust in the Longmen Shan moving as nearly-vertical extrusion and uplift, resulting in the surface rate of tectonic movement differing according to depth rate as well as the occurrence of large shallow Wenchuan Earthquake. According to the hazards caused by Wenchuan Earthquake, this paper provides some suggestions on the reconstruction. 