FEM simulation of interseismic and coseismic deformation associated with the 2008 Wenchuan Earthquake

We make use of two-step modeling to simulate inter- and coseismic deformation within one seismic cycle by means of visco-elastic finite element method (FEM). The first step is to perform inversion to find the optimum modeling parameters by fitting the observed interseismic deformation. Then the model continues to produce a major seismic event to simulate the Wenchuan earthquake. The model results show that, in order to fit the observed interseismic deformation, it needs a “soft” lower crust and upper mantle beneath the Eastern Tibet, and a very strong lithosphere of the Sichuan basin. The computed interseismic strain accumulation in the lower crust beneath the Eastern Tibet is much faster than that in the other regions. Especially, the elastic strain energy density rate (ESED) accumulates very rapidly in and around the Longmen Shan fault in the depth above 30km that may explain why the great Wenchuan earthquake occurs in the region of such a slow surface deformation rate. Simulations of coseismic processes reveal relationships among slip acceleration, normal stress and shear stress changes on surface of the seismogenic fault. Coseismic slip appears to initiate in the gently-dipping section of the fault, but primary slips (coseismic rupture) on both gently-dipping and steeply-dipping sections take place simultaneously. Minor fault slip accelerations decrease normal stress on fault surface to reduce frictional strength of fault, and in general increase shear stress on fault surface to enhance tectonic stress on the fault to drive the fault further slips. Both processes facilitate major slip or rupture on the listric reverse fault. Coseismic slip distribution also suggest that an important role of high-angle listric reverse faulting is to transfer overwhelming horizontal deformation of the Eastern Tibet into significant vertical displacement in the Longmen Shan during the 2008 Wenchuan earthquake. ► We model inter- and coseismic deformation associated to the Wenchuan earthquake. ► The inverted optimum model fits the observed data very well. ► High elastic strain energy rate is found in and around the Longmen Shan. ► Modeled coseismic normal and shear stress on the surface of the fault changes. ► These stress changes on the surface of the fault promote the Wenchuan earthquake.