Influence of the pre-existing Xiaoyudong salient in surface rupture distribution of the Mw 7.9 Wenchuan earthquake, China

Abstract

Field investigations show that the Wenchuan earthquake on the 12th of May 2008 ruptured two NW-dipping imbricate reverse faults along the Longmen Shan fault zone. The length of the Beichuan–Yingxiu Fault reaches nearly 240km. Southeast of this fault, a smaller displacement occurred along the Guanxian–Jiangyou Fault, which has a length of about 70km. A NW-striking left-lateral reverse fault, the Xiaoyudong Fault, was clearly observed between these two main surface ruptures. This co-seismic surface rupture pattern is one of the most complicated patterns of recent great earthquakes. In order to clarify how rupture progressed northward during the Wenchuan earthquake, we conducted a detailed investigation including rupture tracing, displacement measurement, fault kinematic estimation, and stress analysis of the Xiaoyudong area. Our results show that the Xiaoyudong Fault is not a simple tear fault. In contrast, it is an independent active fault located on the northern boundary of the Xiaoyudong salient. The Xiaoyudong salient is an independent topographic highland occupying a surface area of approximately 100km2. From a stratigraphic point of view, this highland is a little older than the adjacent area. The s-shape stream north of the Xiaoyudong salient indicates that the Xiaoyudong Fault is still active. The Xiaoyudong salient plays an important role in controlling fault propagation and stress delivery. When the Wenchuan earthquake occurred in the south of the Xiaoyudong salient, this salient partially impeded the northward propagating rupture and dispersed it into two branches to the north. The rupture distribution north of Xiaoyudong would be significantly different without this salient. The existence of the Xiaoyudong salient and the rupture pattern of the Wenchuan earthquake also prove that the multi-segment rupturing model is a better approximation than a single-segment model for estimating the maximum magnitude of the paleo-earthquake of the Longmen Shan fault zone.