Regional stresses inferred from coseismic slip models of the 2008 Mw 7.9 Wenchuan, China, earthquake

During the 12 May 2008 Mw 7.9 Wenchuan earthquake, coseismic slip propagated from the southwest to the northeast along the Longmenshan fault zone on the border of the Sichuan basin and eastern Tibet. Seismologic and geodetic coseismic slip models indicate that the Beichuan fault slipped primarily in thrust motion on moderately dipping fault segments in the southwest, and as dominantly right lateral strike slip motion on more steeply dipping segments in the northeast. Additionally, the shallowly dipping Pengguan fault, located to the east of the Beichuan fault, slipped as mostly pure thrust. In this study, we explore the connection between sense of slip and fault geometry, and investigate whether models of coseismic slip yield information about the pre-earthquake background stress in this region. Specifically, we test whether coseismic slip models are consistent with a homogeneous regional stress, or if a heterogeneous stress field is required to explain the change in slip rake along strike. We assume that coseismic slip is parallel to the direction of the maximum shear stress on each of the fault planes and consider several published coseismic slip models with differing fault geometries. Using a Bayesian probabilistic methodology, we find that the coseismic slip of the Wenchuan earthquake is consistent with a homogeneous state of stress prior to the earthquake. The inferred most compressive stress direction is near horizontal and east-west trending while the intermediary compressive stress is also near horizontal, but trending north-south. These stress directions are roughly consistent with proposed mechanical models of fault loading in the western Sichuan basin. In contrast to the homogeneous pre-earthquake stress, aftershocks indicate a heterogeneous post-earthquake stress that is dominated by the coseismic stress changes in the Wenchuan earthquake. To constrain the relative magnitudes of the principal stresses we consider both static and dynamic frictional stability.