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

探索西藏的深层流变：近期倾滑地震的独特限制

• 批准号: 1014880
• 负责人: Roland Burgmann
• 金额: $ 20.36万
• 依托单位: University of California-Berkeley
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-10-01 至 2014-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1014880&HistoricalAwards=false
• 关键词: Probing; Deep; Rheology; Tibet; Unique

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.

青藏高原的基本地质结构已经争论了几十年。两种主要的端元模式被提出：[1]西藏变形分布广泛，与地幔和下地壳岩石的韧性流动有关; [2]青藏高原是刚性块体相互作用形成的，变形沿着大断裂局部化。大量的文献支持这两种情况，但没有导致解决方案。为了解决这一争论，需要更好地了解青藏高原深部的力学性质或流变学。该项目的目的是严格确定西藏内部的流变学从大地测量震后变形后，最近的一些大地震。这种测量有效地探测了地震引起的应力变化引起的粘性流的本构特性。这项工作的结果不仅有助于更好地了解西藏变形的基本性质，而且对确定西藏地区目前和未来的地震危险性也有直接的意义。震后变形是由大地震引起的区域应力变化引起的。大地测量的力学模型，使分析震后瞬变，以探讨由此产生的粘性流在下地壳和上地幔的本构特性。本研究的重点是地震后变形模式提供的大地测量时间序列的各种中大型倾滑地震发生在青藏高原在过去的六年（例如，2004年、2005年和2008年西藏西南部的仲巴地震和2008年西藏东部的汶川地震）。项目组主要采用干涉合成孔径雷达（Interferometry Synthetic Aperture Radar）测量震后地表形变。此外，与Z。北京的Shen用于约束模型探索。震后变形模式制约了西藏中下地壳（~15-80 km深度）的流变学性质。目标地震覆盖了高原的大面积区域，因此具有揭示地壳流变结构横向不均匀性的潜力。大多数地震都具有矩震级（Mw）大于6.5的正断层机制，唯一的例外是2008年5月发生在四川的破坏性Mw7.9汶川逆冲断层地震。倾滑破裂的事件有助于区分不同的候选机制震后应力松弛在深度，因为正常的断层和逆冲事件，后滑和粘弹性松弛引起非常不同的表面变形场。该项目的目的是严格确定西藏内部的流变特性，从大地测量的震后变形，并仔细考虑来自广泛的先前地球物理，地质和实验研究的解释。