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

地球演化中的一个基本事件是两个大陆相撞。印度与亚洲的碰撞就是今天发生的最好的例子。碰撞产生了青藏高原，这是世界上地形最高的地区，也产生了变形，导致整个东亚地区发生地震和火山活动，影响到超过10亿人。尽管对这一主题的研究已经有150多年的历史，但青藏高原下面的地壳和地幔(岩石圈)的演化仍然是一个谜。令人惊讶的是，对于印度岩石圈在西藏下方的位置，人们几乎没有达成一致意见，因此，对于驱动高原隆升和增长的过程以及下面地幔的演化也几乎没有达成一致意见。这一建议是为了开发西藏及周边地区地壳和地幔的综合高分辨率P波和S波地震图像。其目的是为了更好地了解大陆碰撞过程中壳-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赋存状态。在过去的500年里，该项目研究区域内有100多万人在地震中丧生。这项工作中将要成像的详细的岩石圈结构将提高我们对这一大陆碰撞的巨大构造活动区中地震活动的理解。这项工作包括与中国和印度的科学家进行国际合作，以及在美国和国际合作机构培训研究生和本科生。地震图像将通过应用三分量区域宽带数据的全波形反演来开发，这些数据采样了西藏下面的地壳和地幔。研究人员还将在他们的模型开发中纳入远程地震旅行时间测量和接收器功能。将利用青藏高原内外所有可用的台站，充分利用过去专门用于西藏研究的巨大资源，建立一个大型区域波形数据库。根据以前的经验，该团队预计能够将尺寸限制在50公里左右的结构，无论是横向还是垂直。他们的主要目标是绘制从印度北部到喜马拉雅山脉和青藏高原以下地区的印度大陆岩石圈(ICL)图。调查人员希望能够对印度北部和西藏各地及其周边地区的地幔岩石圈厚度做出相对准确的估计。这应该会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向较薄的印度大陆边缘岩石圈可能过渡的位置。进一步的目标将是看看是否可以在印度或西藏岩石圈的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下降对克拉通地幔的永久性具有重要意义。通过准确的P和S图像，以及改进的过渡带不连续深度，研究小组希望区分西藏地幔深处的化学和热异常。利用该地区上地幔的全三维图像，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，并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施加限制，并确定是否有任何证据表明欧亚岩石圈向南俯冲，以及是否有任何原始的西藏岩石圈被剥离。这一裁决反映了NSF的法定使命，并通过使用基金会的智力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.