Lithospheric rheology of the Tibetan Plateau: an overview of estimates from postseismic deformation

Abstract:

Lithospheric rheology is critical to geodynamic modelling of various tectonic features and processes (e.g., formation and evolution of plateaus). Stress perturbation due to earthquakes causes viscoelastic relaxation of weak materials close to seismogenic layer. Therefore, modelling time-dependent postseismic deformation following large earthquakes enables us to constrain the lithospheric rheology. Here, I present some recent postseismic studies, together with an overview of recent advances in modelling postseismic deformation following large earthquakes on the Tibetan Plateau. These large-to-major earthquakes have various mechanisms, and the geodetic observations span from months to decades following these events. It is clear that short-term postseismic deformation following several large normal fault earthquakes can be well explained by afterslip, and contribution from viscoelastic relaxation is not detectable. In these cases, a lower bound of lower crustal and/or upper mantle viscosity is estimated to be ~10¹⁸ Pa.s. Decadal postseismic deformation following the 1997 Mw7.6 Manyi strike-slip earthquake in north-western Tibet suggests combined contributions from viscoelastic relaxation and afterslip. Late-stage postseismic deformation following the 1951/1952 Bengco earthquakes (Mw ~7.6) in central Tibet can be reproduced by relaxation of a thickened viscoelastic lower crust with viscosity of 6×10¹⁹ Pa.s. Recent major earthquakes near the plateau margins provide opportunities to explore the lateral heterogeneity in rheology, and the main findings are discussed here. Finally, I summarise geodetically-determined lithospheric rheology for the whole plateau, explore the spatial characteristics of viscosity distribution, and discuss the difference between these decadal-scale viscosity and long-term rheology. This presentation aims to provide some thoughts on estimating lithospheric rheology from postseismic studies of earthquakes on the seismically active Tibetan Plateau in an era of massive geodetic data availability.