Probing the Deep Rheology of Tibet: Constraints from the 2008 Mw 7.9 Wenchuan, China Earthquake

The time-dependent surface deformation after a large earthquake reflects the response to the redistribution of stresses induced by the earthquake and can be used to probe the viscous strength of the lithosphere. Hence, understanding and modeling the postseismic processes can help determine the rheological structure of an area. The 2008 M_w 7.9 Wenchuan earthquake occurred at the eastern flank of the Tibetan Plateau, China, and its postseismic deformation gives us an opportunity to examine the long lasting question of whether the growth of the Tibetan Plateau is by brittle crustal thickening or by lower crustal flow. We process the InSAR data with more stable points in Longmenshan area, and set a threshold for each interferogram for phase unwrapping. The stable points selected based on this procedure are used for the time-series analysis spanning from mid 2008 to the end of 2009. Seventeen ALOS PALSAR and 22 ENVISAT interferograms reveal a pronounced postseismic uplift of up to 3 cm in the southern Longmenshan 1.5 years after the main shock, which decreases to zero along the northern Longmenshan. An analytical solution of viscoelastic relaxation based on a layered spherical Earth is applied for the calculation of the postseismic deformation. With a five-segment coseismic rupture geometry with different slip that extends to a depth of 20 km and runs along the 285 km Longmenshan fault zone oriented 229°, we find a best fitting forward model composed of a 20 km thick elastic upper crust and a Burgers body lower crust with a steady-state viscosity of 10¹⁹ Pa s and a transient viscosity of 4×10¹⁸ Pa s. This preliminary forward model can better fit the middle- to far-field InSAR data. The poor fit in the near-field is likely due to the over-simplification of the rheologic structure along the boundary of the Plateau and the adjoining Sichuan basin and because we did not consider the contributions of afterslip and poroelastic deformation in these preliminary models. Ongoing work focuses on the consideration of additional postseismic relaxation processes and on developing a more appropriate 3D rheological structure of the Plateau boundary region.