Variations in rupture speed, slip amplitude and slip direction during the 2008 Mw 7.9 Wenchuan Earthquake

Abstract

Theoretical and observational evidence implies that variable rupture velocity may be associated with high fault slip and high moment release rates during earthquakes. We investigate this relationship for the 2008 Mw 7.9 Wenchuan China, Earthquake, which appears to have experienced a highly variable moment release rate. We used an empirical Green's function (EGF) deconvolution analysis of teleseismic waveforms to retrieve the primary rupture characters for this event. Based on field observations and the deconvolved source—time function from thrust and strike–slip EGFs, we divided the ruptured fault into three segments, allowing us to determine the spatial slip distribution with different average rupture velocities on each segment. We deployed a grid search analysis, in which we integrated the teleseismic waveform inversion and forward modelling of the regional surface wave to determine the optimum rupture speed in each fault segment. Our result shows that the 2008 Wenchuan Earthquake had slip amplitude, direction and rupture velocity that were highly spatially variable. The earthquake initially ruptured with nearly pure thrust motion and a slow rupture velocity of 1.7 km s—1. Then, the rupture speed increased up to 3.1—3.3 km s—1 and produced the largest slip on the second segment, where two parallel faults ruptured simultaneously. Rupture velocity then slowed down to 2.5—2.9 km s—1 on the final segment, which underwent primarily strike–slip motion. In total, the fault extended 300 km with an average rupture speed in the range of 2.6—2.9 km s—1. The high rupture speed (close to the shear wave velocity) in the second segment may be related to the simultaneous rupture of the two parallel faults on well–established pre–existing structures. The spatial distribution of rupture velocity and slip are likely related to the strongly rotational and 3–D deformation in the eastern Tibetan Plateau margin, which leads to the heterogeneous stress field of the Longmen Shan region.