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

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。关于印度-欧亚碰撞带大陆变形样式的最基本方面有很多争论。一个学派认为西藏是一个厚的、弱的、流体状的地带，而另一些学派则认为该地区的活动构造是由主要断层包围的刚性微板块。另一种观点认为，低粘度通道在下地壳中起主要作用。为了解决这一争论，需要更好地了解青藏高原深层内部的流动强度或流变学。该项目的目的是严格确定西藏的流变特性？在最近的几次大地震之后，通过大地测量测量到的震后变形，我们可以看到美国的内部。卫星收集的合成孔径雷达干涉数据提供了有关地震后深层松弛过程引起的细微地表变形的宝贵信息。这些测量有效地探测了地震引起的应力变化引起的深部粘性流动的本构特性。这项工作还将提高对震后变形在触发更多地震中的作用的理解。关于西藏地壳深部粘性强度的定量信息将使构造演化和碰撞带的当前动力学模型得到改进。在对昆仑断裂带两次走滑地震后形变研究的基础上，本研究利用InSAR时间序列研究了最近8次中大型倾滑地震后的形变，探讨了西藏下地壳的流变特性。重要的是，倾滑地震后的余震、孔弹性反弹和粘性松弛引起的变形瞬态与走滑事件的变形瞬态差别更明显。这些事件大多是发生在青藏高原内部的正常断层破裂，而2008年5月毁灭性的汶川地震是发生在青藏高原东南边缘的逆冲事件。对于这一事件，InSAR数据由GPS测量补充。仔细分析变形瞬态的时空演变和综合的模型分析可以区分不同的应力松弛过程，并定义材料在深度的本构特性。高原周围地震的发生为评估整个地区流变结构的横向非均质性程度提供了一个独特的机会。我们还认真整合了广泛的互补的西藏地球物理和地质研究知识。除了有助于更好地了解西藏地区当前和未来的地震灾害外，总体上提高地震后机制的基本知识将有助于未来研究大地震后的应力变化，从而有助于地震灾害的变化。

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.