Topographic stresses and slip heterogeneity in the 2008 Wenchuan earthquake: Constraints on regional stress, fault friction, and pore pressure

Abstract:

Coseismic slip in the 2008 Wenchuan, China earthquake was highly heterogeneous in both magnitude and rake. The slip was predominantly thrust slip on shallow dipping fault segments near the epicenter, rotating to strike-slip on more steeply dipping fault segments to the northeast. The earthquake ruptured along the front of the Longmenshan, one of the steepest and highest mountain ranges on Earth. Fault segments that slipped in a reverse sense roughly correspond to regions of larger topographic gradients than segments that slipped in strike-slip. The weight of the mountains result in highly variable fault shear stresses on order of 10 MPa and normal stresses up to 80 MPa. The heterogeneity of coseismic slip might indicate a heterogeneity in pre-earthquake stresses, possibly influenced by the heterogeneity of topographic relief. We constrain the accumulated stress that led to the Wenchuan earthquake in a probabilistic manner using geodetically and seismically constrained coseismic slip models, aftershock focal mechanisms, and topographic stresses. We find that coseismic slip models are consistent with a homogeneous pre-earthquake stress orientation along the Longmenshan, although those models are unable to constrain the stress magnitude. We also find that the majority of the aftershocks are broadly consistent with the stress inferred from the mainshock, and that significant rotation of the stress during the Wenchuan earthquake is not required to explain the disparate mechanisms of the aftershocks. More importantly, we find that topographic shear stresses are generally opposed to the direction of fault slip, and thus that the Wenchuan earthquake worked against topographic loading. Given that tectonic stresses needed to overcome topographic stresses for the Wenchuan earthquake to have occurred, allows us to determine a minimum bound on the magnitude of accumulated tectonic stress, in addition to fault friction and pore-pressure. While we find that heterogeneities in topographic fault shear stresses likely did not lead to heterogeneities in the earthquake slip, a negative correlation between slip magnitude and topographic normal stress magnitude suggests that normal stresses may influence fault arrest.