Determination of horizontal shortening and amount of reverse-faulting from trenching across the surface rupture of the 2008 MW 7.9 Wenchuan earthquake, China

Abstract

The 2008 Wenchuan earthquake ( M W 7.9) ruptured two large thrust faults within the Longmenshan fault zone (along the eastern margin of the Tibetan Plateau): the Beichuan-Yingxiu Fault and the Guanxian-Jiangyou Fault. Previously, coseismic displacements, including vertical and horizontal slip along the surface rupture, have been investigated in detail, along with far-field horizontal shortening, estimated using GPS measurements. However, the distribution of near-field horizontal shortening across the surface rupture is relatively ambiguous because there are few markers in the earthquake zone and very few appropriate landforms for surveying. Palinspastic restoration of paleoseismic trench logs, as an alternative approach, may compensate for these deficiencies and reflect true deformation types and shortening across the surface rupture. We present several methods for estimating shortening based on the genesis of earthquake scarps. The results demonstrate that the shortening ranges from 2.5 m to 0.6 m for localities along the Guanxian-Jiangyou Fault with the highest values at the southwestern end. Along the Beichuan-Yingxiu Fault, near-field shortening ranges from 1.3 m to 3.4 m with two maxima toward the NE and SW ends of the fault, near the towns of Shenxigou and Leigu — these locations correlate with the two most heavily damaged regions. Horizontal shortening on the Guanxian-Jiangyou Fault is generally less than that across the Beichuan-Yingxiu Fault. Based on the distribution of near-field horizontal shortening from trenching across surface rupturing, the maximum near-field horizontal shortening across the surface rupture for the Wenchuan earthquake is ∼ 5.3 m.