Coseismic Slip Model of the 2008 Wenchuan Earthquake Derived From Joint Inversion of InSAR, GPS and Field Data

We derived a coseismic slip model for the M7.9 2008 Wenchuan earthquake based on radar line of sight displacements from ALOS interferograms, GPS vectors, and geologic field data. The surface rupture of the Wenchuan earthquake extended over 270 km from the epicenter toward the northeast along Longmen Shan thrust belt, on the steep margin of the Tibetan plateau. Available InSAR data provided a nearly complete coverage of the surface deformation along both ascending (fine beam mode) and descending orbits (ScanSAR to ScanSAR mode). The earthquake was modeled using 4 subfaults with variable geometry and dip to capture the simultaneous rupture of both the Beichuan and the Pengguan fault. We inverted the InSAR and GPS data using a non-negative least squares (NNLS) algorithm incorporating the scarp height data as an additional inequality constraint. The inversion scheme considered the first-order effect of topography by perturbing the depth of modeled slip patches according to a local elevation. Our model misfits show that the InSAR and GPS data are highly compatible; the combined inversion yields a 93% variance reduction. The inferred subsurface rupture length is 304 km, which is consistent with the 316 km long distribution of aftershocks. The best-fit model has fault planes that rotate from shallow dip in the south (35 deg) to nearly vertical dip toward the north (70 deg). This result is in a general agreement with the relocated aftershocks. The inferred total geodetic moment of 6.79x10E20 Nm (equivalent to earthquake moment magnitude of 7.8) is nearly equally partitioned into strike-slip (3.61x10E20 Nm) and dip-slip (4.96x10E20 Nm) components. Our rupture model is complex with variations in both depth and rake along two major fault strands. In the southern segment of the Beichuan fault, the slip is mostly thrust (<10 m) and occurred principally in the upper 10 km of the crust; the rupture progressively transformed to right-lateral strike-slip as it propagated northeast (with maximim offsets of 4 to 7 m). A region of maximum slip on the Pengguan fault is limited to a very shallow depth (3.5 km) and is mostly dip-slip (< 9 m). Our coseismic rupture model of the 2008 Wenchuan earthquake, which is mainly based on space geodetic observations, suggests that most of the moment release was limited to the shallow part of the crust (depth less than10 km). We didn’t find any “shallow slip deficit” in the slip-depth distribution of this mixed-mechanism earthquake. Aftershocks were primarily distributed below the section of the fault that ruptured coseismically. This coseismic rupture model will be used in forward models of postseismic deformation to provide insights on mechanisms of postseismic relaxation, the effective rheology of the lower crust and upper mantle, stress transfer, earthquake triggering and future seismic hazard in the area.