Collaborative Research: Full waveform inversion for P and S seismic structure beneath Tibet

地球演化的一个基本事件是两块大陆的碰撞。印亚碰撞是当今发生这种情况的首要例子。这次碰撞产生了青藏高原，这是世界上地势最高的地区，以及整个东亚地区的地震和火山活动造成的变形，影响了超过10亿人。尽管对这一主题进行了150多年的研究，青藏高原下地壳和地幔（岩石圈）的演化仍然是一个谜。令人惊讶的是，关于印度岩石圈在西藏下面的位置，人们几乎没有达成一致意见，因此，对于推动高原隆起和生长以及下面地幔演化的过程，人们几乎没有达成一致意见。本课题旨在建立西藏及周边地区地壳和地幔的高分辨率P、S波综合地震图像。目的是更好地了解大陆碰撞过程中地壳-地幔演化的动力学。这项工作将有助于了解过去的大陆碰撞，从而更好地了解大陆的地质和自然资源的发生。在过去的500年里，超过100万人在这个项目研究区域的地震中丧生。在这项工作中，详细的岩石圈结构成像将提高我们对这个巨大的大陆碰撞构造活动区地震活动性的理解。这项工作涉及与中国和印度科学家的国际合作，以及在美国和国际合作机构培训研究生和本科生。利用三分量区域宽带数据全波形反演，对西藏地壳和地幔进行采样，形成地震图像。研究人员还将在他们的模型开发中纳入远震走时测量和接收函数。利用青藏高原内外所有可用台站，充分利用过去致力于西藏研究的巨大资源，建立一个大型区域波形数据库。根据之前的经验，团队希望能够约束结构的尺寸在50公里左右，横向和纵向都是如此。他们的主要目标是绘制从印度北部到喜马拉雅山和青藏高原以下地区的印度大陆岩石圈（ICL）地图。研究人员希望能够对印度北部、西藏各地区及周边地区的地幔岩石圈厚度做出相对精确的估计。这将限制印度地幔岩石圈的增厚，以及从非常厚的印度克拉通岩石圈到较薄的印度大陆边缘岩石圈可能过渡的位置。进一步的目标将是观察是否可以在印度或西藏岩石圈底部拍摄到任何不稳定的图像，以及印度岩石圈是否以及在哪里下降到更深的地幔。克拉通岩石圈向深部地幔的沉降对克拉通地幔的持久性具有重要意义。有了精确的P和S图像，以及改进的过渡带不连续深度，该团队希望能够区分西藏地下更深地幔的化学和热异常。利用这一地区上地幔的完整三维图像，他们还应该能够看到碰撞的印度岩石圈是否在深度上被分割，以及沿着碰撞的走向表现出不同的行为。最后，他们希望对西藏岩石圈的行为进行约束，并确定是否存在欧亚岩石圈向南俯冲的证据，以及原始西藏岩石圈是否存在分层。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

A fundamental event in Earth evolution is when two continents collide. The India-Asia collision is the premier example of this happening today. The collision has produced the Tibetan Plateau, the region with the highest topography in the world, as well as deformation resulting in earthquakes and volcanic activity throughout eastern Asia affecting over a billion people. Despite more than 150 years of research on the topic, the evolution of the crust and mantle (lithosphere) beneath the Tibetan Plateau remains an enigma. There is surprisingly little agreement on the configuration of where Indian lithosphere is beneath Tibet and thus little agreement on the processes driving plateau uplift and growth as well as the evolution of the mantle beneath. This proposal is to develop a comprehensive high-resolution P and S wave seismic image of the crust and mantle beneath Tibet and surrounding regions. The goal is to better understand the dynamics of crust-mantle evolution during continental collision. This work will help in understanding past continental collisions and thus provide a better understanding of the geology of continents and the occurrence of natural resources. In the past 500 years more than one million people lost their lives in earthquakes within the study area of this project. The detailed lithospheric structures to be imaged in this work will improve our understanding of seismicity in this vast tectonically active zone of continental collision. This work involves international collaborations with scientists in China and India, as well as training of graduate and undergraduate students at collaborating institutions in the US and internationally.Seismic images will be developed through the application of full waveform inversion of three component regional broadband data that sample the crust and mantle beneath Tibet. The investigators will also incorporate teleseismic travel-time measurements and receiver functions in their model development. A large regional waveform database will be created using all available stations within and around the Tibetan plateau, taking full advantage of the enormous resources that have been dedicated to studies of Tibet in the past. Based on prior experience the team expects to be able to constrain structures with dimensions on the order of 50 km, both laterally as well as vertically. Their main goal is to map the Indian continental lithosphere (ICL) from northern India to regions below the Himalayas and the Tibetan Plateau. The investigators expect to be able to make relatively precise estimates of the thickness of the mantle lithosphere beneath northern India and various parts of Tibet and the surrounding regions. This should put constraints on thickening of the Indian mantle lithosphere as well the location of a possible transition from very thick Indian cratonic lithosphere to thinner Indian continental marginal lithosphere. Further goals will be to see if any instabilities can be imaged at the base of either the Indian or Tibetan lithosphere, and if and where Indian lithosphere is descending into the deeper mantle. Descent of cratonic lithosphere into the deeper mantle has implications for the permanence of cratonic mantle. With accurate P and S images, as well as improved transition zone discontinuity depths, the team hopes to distinguish chemical and thermal anomalies in the deeper mantle beneath Tibet. Using a full 3D image of the upper mantle in this region, they should also be able to see whether the colliding Indian lithosphere is segmented at depth and exhibits variable behavior along strike of the collision. Finally, they hope to place constraints on the behavior of the Tibetan lithosphere and determine whether there is any evidence for southward subduction of Eurasian lithosphere and whether any of the original Tibetan lithosphere been delaminated.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.