Finite element modelling of co-seismic deformation of the Longmenshan thrust belt associated with 2008 M=8 Sichuan earthquake

Abstract

The devastating 2008 M=8 Sichuan Earthquake occurred in the Longmenshan (LMS) fault zone which comprises mainly the Wenchuan, Beichuan, and Pengguan Faults, between the Tibet and Yangtze Blocks of south-central China. Co-seismic fault dislocations ranging from 1m to 6m, and variations in gravity value up to 1.50 mGal were reported. We constructed finite element models to simulate the co-seismic deformation and post-seismic viscoelastic and poroelaestic behavior of the epicentral region to explain the observed crustal deformation and changes in gravity. Geometry of the models was modified from the published crustal sections of the traverse. Young's modulus, Poisson ratio, viscosity and void ratio were used as modelling parameters. Stress and pore pressure fields introduced by the rapid co-seismic dislocations along the main faults provide the initial conditions for simulations of the post-seismic processes. The modeling results suggest that fluid migration probably accounts for crustal deformation (by a few cm) within the first six months of the event, assuming a porous upper crust. Visco-elastic response of the upper mantle (by a few tens of cm) persisted for several years. The model is consistent with observed changes in gravity. Localities in the Tibetan Plateau and Yangtze Block having a reversal in vertical displacement several years after the earthquake possibly reveal viscoelastic relaxation of the lower crust beneath the LMS fault zone.