The influence of topography and crustal heterogeneity on coseismic deformation of the 2008 Mw7.9 Wenchuan Earthquake, China

Abstract

The Longmen Shan Thrust Belt, which lies at the convergence of the eastern Tibetan Plateau and Sichuan Basin, is characterized by a topographic relief of 4000 m over a distance of 100 km. While the rock underlying the Sichuan Basin is strong, the crust underlying the Tibetan Plateau is weak. Four models that simulate deformation caused by distributions of dislocations along a geometrically complex fault system are developed to investigate how the contrast in elevational and crustal heterogeneity influences coseismic deformation of the 2008 Mw7.9 Wenchuan earthquake. The models are configured with an assembly precise ramp-décollement structures and planar fault geometries. The coseismic slip distribution is estimated by inverting three component GPS observations with Green's functions calculated for each model configuration. By comparing the slip distribution and residuals among the displacement predictions from these four models and measurements of GPS, the crustal heterogeneity model robustly explained most of the observed GPS horizontal and vertical coseismic displacements. Results revealed four composite dislocation asperities distributed in the shallow crust and a slip of 4–8 m on the sub-surface décollement fault with a depth of ~20 km. The resulting geodetic moment is 8.10 × 1020 Nm, which is in agreement with the seismological estimates. The deep slip predictions obtained with the ramp-décollement fault geometry indicate that the eastern Tibetan Plateau was thrust over the Sichuan Basin rather than the plateau being uplifted owing to the underlying weak crust.