Probing the Deep Rheology of Tibet: Unique Constraints from Recent Dip-slip Earthquakes

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). There is much debate about the most fundamental aspects of the style of continental deformation in the India-Eurasia collision zone. One school of thought envisions Tibet to be a thickened, weak and fluid-like zone, while others consider the active tectonics in the region as that of rigid microplates bounded by major faults. An alternative view suggests a primary role of a low-viscosity channel in the lower crust. To resolve this debate, better knowledge of the flow strength or rheology of the deep interior of the Tibetan plateau is needed. The aim of this project is to rigorously determine the rheological properties of Tibet?s interior from geodetically measured post-earthquake deformation following a number of recent major earthquakes. Synthetic Aperture Radar interferometric data collected by satellites provide valuable information about subtle surface deformation resulting from postseismic relaxation processes at depth. Such measurements effectively probe the constitutive properties of viscous flow at depth resulting from earthquake-induced stress changes. This work will also improve understanding of the role of post-earthquake deformation in triggering of additional quakes. Quantitative information about the viscous strength of the deep Tibetan crust will allow for improved models of the tectonic evolution and current dynamics of the collision zone. Building on an earlier investigation of the deformation following two strike-slip earthquakes on the Kunlun fault, this research utilizes InSAR time series of postseismic deformation following eight recent moderate to large dip-slip earthquakes to probe the rheological properties of the Tibetan lower crust. Importantly, the deformation transients due to afterslip, poroelastic rebound and viscous relaxation following dip-slip earthquakes differ much more distinctly than they do for strike-slip events. Most of these events are normal faulting ruptures that occurred in the interior of the Tibet plateau, while the devastating May 2008 Wenchuan earthquake was a thrust event that occurred on the southeastern margin of the plateau. For this event, InSAR data is complemented by GPS measurements. Careful analysis of the spatio-temporal evolution of the deformation transients and comprehensive model analysis can separate different stress relaxation processes and define the constitutive properties of material at depth. The occurrence of the earthquakes around the plateau offers a unique opportunity to assess the degree of lateral heterogeneity in rheological structure across the region. We also carefully incorporate knowledge of a wide range of complementary geophysical and geological studies of Tibet. In addition to leading to a better understanding of current and future seismic hazards in the Tibetan region, improved fundamental knowledge of postseismic mechanisms in general will be helpful to future studies of changes in stress, and therefore change in seismic hazard, following large earthquakes.