Fast calculation of near-fault static stress change and its implications for the hypo-central estimation of moderate to large aftershocks : A case study of the 2008 Wenchuan Mw7. 9 earthquake

It is widely accepted that earthquake rupture occurs when the shear stress accumulated on the fault exceeds the fault frictional strength. The fault slip motion is accompanied with a partial shear strain release, and the shear stress change can be resolved from final slip distribution or co-seismic slip on the causative fault. We use the Fourier transform method (FTM) to fast calculate the shear stress changes on and near faults after the 2008 M(w)7. 9 Wenchuan, China, earthquake due to a heterogeneous slip distribution on the main fault and try to understand the relationship between stress change and aftershock distribution. For a single fault plane, the shear stress change on the main fault can be calculated efficiently by using the FTM, and the calculation of off-fault stress change is based on an approximate decay relation proposed by Helmstetter and Shaw. For a 2D anti-plane strain problem, we compare our numerical model with the analytical model given by Burridge and Halliday. The results show that the FTM algorithm is easy to be implemented and effectively avoid the singular values appearing in conventional stress calculation. Based on the finite fault slip model inverted from tele-seismic data for the Wenchuan event, we obtain the stress changes on and off the fault and create a 3D view of the aftershock distribution around the main fault together with the spatial stress change pattern resulted from heterogeneous fault slip. The result of 3D view of the aftershock distribution shows that the triggered events or aftershocks are mostly concentrated in the regions with a positive stress change, and the value of the maximum positive stress change caused by the co-seismic slip is roughly equal to the localized largest stress drop (stress change is negative). We calculate the seismicity rate against time following the Wenchuan main shock based on the temporal distribution of stress changes. The results my help us better understanding the aftershock duration and seismicity reduce caused by the stress drop. We emphasize that the fast and effective computations of stress changes on and off the fault immediately following a main shock can give us an insight into the source physics behind earthquake nucleation which provides us the plausible indicators of future potential areas of the moderate to strong aftershocks. In addition, the spatial distribution of stress change is strongly dependent on the finite fault slip model. So we discuss the influence of non-unique inversion of slip distribution on the stress change calculation.