On May 12th,2008,the M_w7.9 Wenchuan earthquake ruptured the Beichuan,Pengguan and Xiaoyudong faults simultaneously along the middle segment of the Longmenshan thrust belt at the eastern margin of the Tibetan plateau.Field investigations constrain the surface rupture pattern, length and offsets related to the Wenchuan earthquake.The Beichuan fault has a NE-trending rightlateral reverse rupture with a total length of 240 km.Reassessment yields a maximum vertical offset of 6.5±0.5 m and a maximum right-lateral offset of 4.9±0.5 m for its northern segment,which are the largest offsets found;the maximum vertical offset is 6.2±0.5 m for its southern segment.The Pengguan fault has a NE-trending pure reverse rupture about 72 km long with a maximum vertical offset of about 3.5 m.The Xiaoyudong fault has a NW-striking left-lateral reverse rupture about 7 km long between the Beichuan and Pengguan faults,with a maximum vertical offset of 3.4 m and left-lateral offset of 3.5 m.This pattern of multiple co-seismic surface ruptures is among the most complicated of recent great earthquakes and presents a much larger danger than if they ruptured individually.The rupture length is the longest for reverse faulting events ever reported.