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

青藏高原的基本地质结构已经争论了几十年。提出了两种主要的端元模型：[1]西藏的变形分布广泛，与地幔和下地壳岩石的韧性流动有关；[2)青藏高原是在刚性块体相互作用中形成的，变形集中在主要断裂上。有大量文献支持这两种情况，但都没有达成解决方案。为了解决这场争论，需要更好地了解青藏高原深处的力学性质或流变学。该项目的目的是根据最近发生的几次大地震后的大地测量震后变形，严格确定西藏内陆的流变性。这种测量有效地探测了由地震引起的应力变化引起的深层粘性流动的本构特性。这项工作的成果除了有助于更好地了解西藏形变的基本性质外，还直接关系到西藏地区当前和未来地震灾害的确定。震后变形是由大地震引起的区域应力变化引起的。大地测量的机械模拟能够分析地震后的瞬变，以便探索在下地壳和上地幔中产生的粘性流动的本构性质。本文主要研究了青藏高原近六年来(如2004年、2005年和2008年西藏西南部中坝地震和2008年西藏东部汶川地震)各种中-大倾滑地震的大地测量时间序列所提供的震后形变图像。项目组主要使用干涉合成孔径雷达(InSAR)来测量震后地表形变。此外，还使用了与北京Z.Shen.合作收集的2008年汶川地震的详细GPS时间序列，以限制模型探索。震后形变模式制约着西藏中下地壳(~15-80公里深度)的流变性。目标地震覆盖了高原的大片区域，因此有可能揭示地壳流变结构的横向非均质性。大多数地震都具有正常的断层机制，力矩震级大于6.5级，唯一的例外是2008年5月四川发生的毁灭性的7.9级汶川逆冲断层地震。具有倾滑破裂的事件有助于区分不同的震后深部应力松弛候选机制，因为对于正常断裂和逆冲事件，余滑和粘弹性松弛产生的地表变形场截然不同。该项目旨在通过大地测量的震后变形严格确定西藏内陆的流变特性，并仔细考虑先前广泛的地球物理、地质和实验研究得出的解释。

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.