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-欧亚大陆碰撞区大陆变形风格的最基本方面存在很多争议。一个学派认为西藏是一个增厚的、软弱的、类似流体的地带，而另一些学派则认为该地区的活动构造是由主要断层所包围的刚性微板块。另一种观点认为，低粘度通道在下地壳中的主要作用。为了解决这一争论，需要更好地了解西藏高原深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学特性？的内部从大地测量震后变形后，最近的一些大地震。卫星收集的合成孔径雷达干涉测量数据提供了关于地震后深度弛豫过程造成的细微地表变形的宝贵信息。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。这项工作还将提高对地震后变形在触发额外地震中的作用的理解。有关西藏地壳深部粘性强度的定量信息将有助于改进碰撞带构造演化和电流动力学模型。在昆仑断层两次走滑地震后变形的早期研究基础上，这项研究利用了干涉合成孔径雷达时间序列的震后变形后，最近八个中等到大倾角，滑动地震探测西藏下地壳的流变学性质。重要的是，由于后滑，孔隙弹性反弹和粘性松弛倾滑地震后的变形瞬变比他们做的走滑事件更明显的不同。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月汶川大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，全球定位系统测量补充了干涉合成孔径雷达数据。对变形瞬态的时空演化进行仔细分析和全面的模型分析可以分离不同的应力松弛过程，并确定材料在深度处的本构特性。高原周围地震的发生为评估整个地区流变结构的横向不均匀程度提供了独特的机会。我们还仔细地将西藏的地球物理学和地质学研究的广泛补充知识结合起来。除了导致更好地了解西藏地区目前和未来的地震危险性，提高震后机制的基本知识，一般将有助于未来的应力变化的研究，从而改变地震危险性，大地震后。

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.