J.Bruce H.Shyu(徐澔德),L ing-Ho Chung,Xi-wei Xu,Kuang-Yin Laiand Yue-Gau Chen

1)Department of Geosciences,Taiwan University,Taipei 10617 2)Institute of Geology,China Earthquake Administration,Beijing

The disastrous Wenchuan,Sichuan,earthquake(MS=8.0)on 12 May 2009 rup tured several major thrust faults of the Longmenshan fold-and-thrust belt,along the w estern margin of the Sichuan Basin.This earthquake produced significant damages of the area,with more than 60,000 of casualties and countless economic losses,and wasone of the largest natural disasters ever occurred in the Sichuan Province in history.The earthquake was accompanied by several surface ruptures,including the central rupture(Beichuan fault),the frontal rupture(Pengguan fault),and a shorter Xiaoyudong rup turebetween the central andfrontal ruptures.

The frontal rupture,which is part of the Pengguan fault,had p redom inantly thrust offset.The total length of the frontal rupture is up to 90 km,with a maximum vertical offset betw een 3 and 4 m.A lthough it is called the frontal rup ture,the structure is not the frontal fault of the system.South of Hanwang Tow n,the Pengguan fault follow s amountain frontparallel valley,whereas the actual mountain front structure did not rupture during this earthquake.Immediately north of Han wang Town,on the other hand,the mountain front app roximately follow s the fault,but the rupture terminated not very far to the north.Uplifted alluvial surfaces,however,indicate that there is still a blind frontal thrust underneath the western margin of the basin.Along the Jing River,widespread and continuous river terracesp rovide the opportunity to characterize the late Quaternary slip ratesof the faults.Furthermo re,unique drainage patterns south of the Xiaoyudong rup ture suggest that the area behaves distinctively,and the Xiaoyudong rupture maybe an independent fault with previous rupture events.