Wen-Shan Chen(陳文山),Yong-Kang Ranand XiweiXu

1)Department of Geosciences,Taiwan University,Taipei 10617 2)Institute of Geology,CEA,Beijing 100029

The May 12,2009 MW8.0 Wenchuan earthquake brought nearly 300 km in length surface ruptures along the frontal Longmenshan orogenic belt where occurred the largest natural disaster in western Sichuan,China.The orogenic belt consists of a series of subparallel thrust faults of Wenchuan-Maoxian,Beichuan and Pengguan faults,which form an east-verging fold-thrust belt.Based on the historical earthquake record during the past 2000 years,the western Sichuan has been a non-seismically active area(larger earthquake).Basic information on the paleoseismicty of active faults are virtually lacking,and therefore,we need urgently to collect data about the recurrence,slip rate,and amount of displacement per event that characterize the Holocene behavior of the Beichuan and Pengguan faults in the frontal Long men shanorogenic belt.In this study,we selected two excavated sites of Yingxiu and Beilu trenches across the Beichuan and Pengguan faults,respectively.Discriminating between structural and depositional relationships in the excavations,the Yingxiu site across the Beichuan fault can be identified with three surface-rupturing earthquake events during the past 5.9 k·a,the Beilu site across the Penggan fault is identified with two events during the past 14.3 k·a. We estimate slip rate of the Beichuan and Pengguan faults which have the long-term vertical slip rate of 1.3 mm/yr and 0.28 mm/yr,respectively.