The stresses that led to the 2008 Mw 7.9 Wenchuan, China, earthquake: Constraints from coseismic slip, aftershocks, and topography

The 2008 Mw 7.9 Wenchuan, China earthquake occurred along the western margin of the Sichuan basin, an area characterized by extremely high topographic relief, and ruptured both the Beichuan and Pengguan faults of the Longmenshan fault zone (Lfz). Coseismic slip models derived from seismic and/or geodetic data indicate a complicated pattern of slip on fault segments with variable geometry. Roughly, all these models indicate that coseismic slip transitioned from predominantly thrust slip on moderately dipping fault segments in the southwest to dextral strike-slip along more steeply dipping fault segments in the northeast. Additionally, to the east of the southern segments of the Beichuan fault, the shallowly dipping Pengguan fault slipped mostly in thrust motion. Focal mechanisms of aftershocks are largely thrust in the southwest and strike-slip in the northeast, roughly consistent with the variation of coseismic slip rake in the mainshock. Finally, the topographic gradient across the Lfz generally decreases from the southwest to the northeast, although it is not clear how this relates to the coseismic slip in the Wenchuan earthquake. Using a probabilistic framework, we find that these coseismic slip models are consistent with a homogenous state of stress along the Lfz prior to the mainshock, where the changes in fault geometry were responsible for the change in slip rake during the earthquake. This is in contrast to the estimations of a heterogeneous state of stress inferred from aftershocks by Cai et al., (2011). However, if the coseismic stress changes are comparable to the magnitudes of background stress, the heterogeneity in the aftershock focal mechanisms may simply reflect the localized heterogeneous coseismic stress changes (CSC) due to the complex coseismic slip in the mainshock. We test this hypothesis by determining the state of stress prior to the mainshock using coseismic slip models as well as precisely determined aftershock focal mechanisms from Cai et al., (2011). Constraining the stress magnitude from the aftershock focal mechanisms is crucial in order to compare those inferred stresses to the mainshock CSC. Since stress magnitude is not well resolved from focal mechanisms alone, we include constraints based on both fault friction and stress drop, considering several scenarios within a probabilistic Bayesian analysis. The weight of the topographic features around the Lfz lead to heterogeneous shear stresses on the Beichuan and Pengguan faults that are comparable in magnitude to commonly inferred stress drops during earthquakes. These topographic shear stresses do not load the fault in the same direction as slip during the mainshock, and therefore may place additional constraints on the magnitudes of background stress within the Lfz.