Study On The Active Tectonics In Longmen Mountain Area And Surface Ruputure Of Wenchuan Earthquake

The Wenchuan Ms8.0 earthquake of May 12th, 2008, is the largest magnitude intraplate earthquake recorded by instruments in the world, which occurred in Longmen Shan. This earthquake changed the landform, and caused the deformation of geomorphologic system and fluvial system. It is the most accurate, complete and initial record for the active tectonics of Longmen Shan.In the frist day after the Wenchuan earthquake, we carried out field survey for Longmen Shan area, relying on National Natural Science Foundation Emergency Project"Study on Surface Ruptures and Deformation of Wenchuan Earthquake", and measured 70 sets of datas to surface ruputure caused by Wenchuan earthquake, collecting 200 sets of datas. So it will offer a good studying foundation for the active tectonics of Longmen Shan and surface ruputure of Wenchuan earthquake.This paper will rely on National Natural Science Foundation Emergency Project"Study on Surface Ruptures and Deformation of Wenchuan Earthquake"and China Geological Survey Plan Project"Study on the basic Geology of seismic Activity Belt in the eastern Margin of Tibetan Plateau", titled as"Study on the active Tectonics of Longmen Shan and Surface Ruputure of Wenchuan Earthquake". Based on the previous research achievements of geological background and field survey in the Longmen Shan area, taking the active tectonics of Longmen Shan and surface ruputure of Wenchuan earthquake as research objects, using the geological phenomena revealed surface ruputure of Wenchuan earthquake, this paper which is a proving for the two projects, will study on the distributing feature and plane fabric pattern of Wenchuan earthquake surface ruputure, spatial variation characteristic of displacement, seismogenic model, recurrence intervals of strong earthquakes and so on. The main research results are as follows: (1) Based on accurate measurement for the surface ruputure of Wenchuan earthquake, This paper mainly analyzes the distributing feature, spatial variation characteristic of displacement, types of deformation of Wenchuan earthquake surface ruputure and so on. research shows that there are four mainly surface ruputures of Wenchuan earthquake in Longmen Shan area, including Yingxiu-Beichuan surface ruputure, Pengxian-Guanxian surface ruputure, Xiaoyudong surface ruputure and Leigu surface ruputure. Yingxiu-Beichuan surface ruputure and Pengxian-Guanxian surface ruputure show thrust flexural belt, striking to NE45-55°, along their active faults, with high dip angle ranged form 50-70°. Yingxiu-Beichuan surface ruputure turns out to be characterized by thrust and dextral strike-slip, extending about 245 km. Pengxian-Guanxian surface ruputure shows thrust without evident slip, extending about 70-80 km. In addition, Xiaoyudong surface ruputure and Leigu surface ruputure are two tear faults in Xiaoyudong and Leigu area, having characteristics of fragile with approximately N-S striking. The two surface ruptures pattern shows that Xiaoyudong tear fault and Leigu tear fault formed by differential movement of thrusting of Longmen Shan thrust zone during the Wenchuan Earthquake, striking to N-S direction which approximately is vertical to the strike of Yingxiu-Beichuan fault and Penxian-Guanxian fault and dipped at high angles with thrust and left lateral. So the Xiaoyudong fault and Leigu fault are two tear faults, having clear characteristics with adjusting the imbalance shorten between two blocks in the Xiaoyudong and Leigu area.(2) Having measured by GPS and EDM, this four surface ruputures cut varieties of terrain units. The spatial variation characteristic of vertical and horizontal offset along this surface ruputures shows that Yingxiu-Beichuan surface ruputure turns out to be characterized by obvious segmented features, in the middle and south segments from Yingxiu to Leigu town, thrusting with little dextral strike-slip occurred, while in the north segment from Beichuan to Qingchuan, with thrust and dextral strike-slip along this segment. Pengxian-Guanxian surface ruputure shows thrusting without evident dextarel slip, while turning out to be a litile component of dextarel slip in the northern and eastern segment. There are two high displacement and two low displacement zone along the Yingxiu-Beichuan surface ruputure from SW-NE. The two high displacement zone are located in the Bajiaomiao village of Dujiangyan and Shaba village of Beichuan. And this spatial variation characteristic of displacement in Yinxiu-Beichuan fault is matched with the result of the double-listric finite-fault model. (3) According to the typical outcrop (N31°8.719', E103°41.513') of Yingxiu-Beichuan fault scratch in the Bajiaomiao village where is 20km far away from Dujiangyan city, we can reflect the surface rupture process of Wenchuan Earthquake. Based on the attitude variance of the two kinds of scratch in the outcrop, this result suggests that the motion characater of the causative fault is thrust and dextral strike slip, which is in agreement with the results of surface rupture process inversed by Wenchuan earthquake focal mechanism solution.(4) There are two high displacement and two low displacement area along the Yingxiu-Beichuan surface ruputure from SW-NE. And the segment boundaries beiween high displacement and low displacement zone maybe related to the Xiaoyudong tear fault and Leigu tear fault. The Baishui River-Chaping low displacement zone is corresponding to the Pengxian-Guanxian surface ruputure in the plane fabric pattern.(5) Based on the plane fabric pattern and deformation style of the surface ruptures of Wenchuan earthquake, it can be divided into four patterns and five types, including parallel thrust fault, echelon thrust fault linked by a tear fault, branching fault, the rift valley on the top and peripheral thrust along the frontal margin. And the profile pattern of the rupture is consisting of two imbricated thrust fault in the depth.(6) On the basis of studying the denotative meaning of surface ruputure of Wenchuan earthquake, we analyze the tectonic shortening and uplifting caused by Wenchuan earthquake, discussing on the seismogenic model of Wenchuan earthquake.(7) Based on studying previous research achievements in active tectonic of Longmen Shan, measuring the offset data of surface rupture of Wenchuan earthquake, and making activity segmentations to Yingxiu-Beichuan fault. This paper will use calculation methods of earthquake magnitude to assess magnitudes of the coming earthquakes in Yingxiu-Beichuan fault, and estimate the recurrence intervals of strong earthquakes for Longmen Shan. The results show the estimations of magnitudes are 7.15±0.35 in Xuankou-Xiaoyudong segmentation, 7.36±0.33 in Xiaoyudong-Leigu segmentation, 7.30±0.39 in Leigu-Shikan segmentation, 7.62±0.41 in Yingxiu-Beichuan for the coming earthquakes, and the recurrence interval of strong earthquake for Ms 8.0 is 2142a, for Ms 7.5 is 1059a, for Ms 7.0 is 519a in Longmen Shan.