Surface rupture,thrusting and strike-slipping in the Wenchuan earthquake of Sichuan,China

The Ms.8.0 huge earthquake of May 12th,2008 occurred in Yingxiu-Beichuan fault of Longmen Mountains belongs to a thrusted-strike-slip type.By laying emphasis on the geomorphic features of the surface ruptures of the huge earthquake in Longmen Mountains on the eastern margin of Qinghai-Tibet Plateau,this paper confirms the vertical and horizontal offsets of the Yingxiu-Beichuan fault and the Pengxian-Guanxian fault by detailed field geomorphic surveys.The authors have measured surface ruptures and cracks,such as terrace offset,scarp,fault-controlled saddles,dextral shutter ridges,dextral channel offsets,graben,shatter belt,fluvial fans offset,highroad offsets,tectonic cracks,press ridges and slope trench in some key positions.The result indicate that the surface rupture of the Wenchuan earthquake stretches to NEE direction which strikes NE 30°~50°and trends NE with a dip angle 30°~40°.The surface rupture of the Yingxiu-Beichuan fault stretches 180~190 km from Yingxiu to NE.It belongs to a single side-multi-dots type characterized by some dextral strike-slip thrusts with steep dip angles of 1.60~6.00 m vertical offsets and 0.20~6.50 m horizontal offsets.And the surface rupture of the Pengxian-Guanxian fault can be observed between Cifengchang of Pengzhou and Hanwang of Mianzhu.It is 30~40 km long and has some dextral strike-slip thrusts with slight dip angles of 0.39~2.00 m vertical offsets and 0.20~0.70 m horizontal offsets.These evidences indicate that the surface ruptures are thrusts with dextral strike-slip and thrust displacements.The thrust displacement are equal to or slightly bigger than the dextral strike-slip displacements.According to the historical earthquakes and active tectonic geomorphic dating data,the authors think that there has been and wil be a strong earthquake every 1~3 ka at this fault.Based on this,the authors set a dynamical model to illustrate the possible links between surface processes and upward extrusion of lower crustal flow on the eastern margin of the Qinghai-Tibet Plateau and conclude that the vertical press and motions of lower crust materials result in the thrust motions of the Longmen faulted zone to east,the tectonic uplift of Longmen Mountains and the huge Wenchuan earthquake.