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

The 12 May 2008, Mw 7.9 Wenchuan earthquake occurred on the Beichuan fault in the southwest to northeast trending Longmenshan fault zone (LMSfz), marking the border of the Sichuan basin and eastern Tibet. During the Wenchuan earthquake, the Beichuan fault slipped primarily in thrust motion on moderately dipping fault segments in the southwest, and as largely 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 thrust slip. 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 the region of the Wenchuan earthquake. 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 the fault, and consider several published coseismic slip models with differing fault geometries. Using a Bayesian probabilistic estimation, we find that the coseismic slip of the Wenchuan earthquake is consistent with a constant orientation of principal stresses along the strike of the LMSfz. This suggests a homogeneous state of stress prior to the earthquake, although our analysis only provides weak constraints on the relative magnitudes of the principal stresses. The inferred most compressive stress direction is sub-horizontal and approximately east-west trending. The intermediate compressive stress is sub-horizontal and north-south trending, and is most likely about 30% the magnitude of the most compressive stress. The least compressive stress is near-vertical.