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

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

The fundamental geological structure of the Tibetan Plateau has been debated for decades. Two major end-member models have been proposed: [1] the deformation in Tibet is widely distributed and is associated with ductile flow of rocks in the mantle and the lower crust; [2] the Tibetan Plateau was formed during interactions among rigid blocks with localization of deformation along major faults. A large body of literature exists in support of either case without leading to a resolution. To resolve this debate, better knowledge of the mechanical properties or rheology of the deep interior of the Tibetan plateau is needed. The aim of this project is to rigorously determine the rheology of Tibet's interior from geodetically measured postseismic deformation following a number of recent major earthquakes. Such measurements effectively probe the constitutive properties of viscous flow at depth resulting from earthquake-induced stress changes. In addition to leading to a better understanding of the fundamental nature of deformation of Tibet, the results of this work are directly relevant for determinations of current and future seismic hazards in the Tibetan region. Postseismic deformation is caused by regional stress changes resulting from large earthquakes. Mechanical modeling of geodetic measurements enables analysis of postseismic transients in order to probe the constitutive properties of the resulting viscous flow in the lower crust and upper mantle. This study focuses on postseismic deformation patterns provided by geodetic time series of various moderate to large dip-slip earthquakes that have occurred on the Tibetan plateau during the last six years (e.g., 2004, 2005, and 2008 Zhongba earthquakes in southwestern Tibet and the 2008 Wenchuan earthquake in eastern Tibet). The project team primarily uses InSAR (Interferometry Synthetic Aperture Radar) to measure the postseismic surface deformation. In addition, detailed GPS time series for the 2008 Wenchuan earthquake collected by an ongoing collaboration with Z. Shen of Beijing is used to constrain the model exploration. The postseismic deformation patterns constrain the rheological properties of the Tibetan mid to lower crust (~15-80 km depth). The target earthquakes cover a large area of the plateau, and thus have the potential to reveal lateral heterogeneity in crustal rheological structure. Most of the events have a normal fault mechanism with a moment magnitude (Mw) larger than 6.5, the only exception being the devastating Mw 7.9 Wenchuan thrust faulting earthquake that occurred in Sichuan in May 2008. The events with dip-slip ruptures help discriminate between different candidate mechanisms for postseismic stress relaxation at depth, since for both normal faulting and thrust events, afterslip and viscoelastic relaxation give rise to very different surface deformation fields. The project aims to rigorously determine the rheological properties of Tibet's interior from geodetically measured postseismic deformation with careful consideration of interpretations derived from a wide range of prior geophysical, geological and experimental studies.