New approaches to shear wave splitting tomography

地震波在地球上地幔中的传播速度取决于地震波传播的方向，这种现象被称为地震各向异性。当地幔岩石在地幔对流过程中被剪切或拉伸时，地震各向异性就产生了；因此，各向异性观测为地震学家提供了一种推断地幔对流模式的方法。虽然通过记录穿过地幔的地震波来测量地震各向异性相对简单，但破译各向异性结构的确切位置或深度是一个主要挑战。克服这一挑战的一个策略是采用一种称为断层摄影的技术，提供上地幔各向异性的图像或图片。就像医学断层扫描可以提供人体内部的图像一样，地震断层扫描可以获得地球深处内部结构的图像。PI将为博士生提供支持和指导。在研究期间开发的代码将通过GitHub公开提供。PI和研究生也将以演讲的形式向公众提供宣传。应用横波分裂层析成像技术是上地幔各向异性研究的一个长期目标，但成功实现该技术的已发表研究实例很少。面临的挑战有很多：反演问题是高度非线性的，反演的灵敏度核强烈依赖于初始模型；为了完整地描述各向异性，需要大量的参数；对于有噪声的数据，通常很难进行稳健的横波分裂测量。该项目将通过开发和应用有限频率SKS分裂强度层析成像框架来应对这些挑战，该框架实现了用于探测参数空间的模型空间搜索方法。调查人员提出了三项具体活动。活动#1包括完成和实施我们的分裂强度断层扫描方法的理论和计算框架。该团队正在将现有的有限频率灵敏度核理论扩展到一个新的参数化方案，该方案利用了矿物物理学对橄榄石弹性的限制。他们将完成一个基于马尔可夫链蒙特卡罗方法的模型空间搜索算法的实现，用于反演分裂强度观测。最后，研究人员将通过一系列合成各向异性模型和分裂强度数据集来测试他们的实现，旨在了解其成像方法的局限性、分辨率和参数权衡。活动#2将涉及对来自密集宽带地震网络的几个真实数据集实施横波分裂强度层析成像框架。活动#3将扩展他们的模型空间搜索方法，同时包括约束上地幔各向异性的其他类型的观测，包括各向异性接收函数和面波相速度测量。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。

The speed at which seismic waves travel in the Earth's upper mantle depends on the direction of wave propagation, a phenomenon known as seismic anisotropy. Seismic anisotropy arises when a mantle rock is sheared or stretched by the process of mantle convection; therefore, observations of anisotropy provide seismologists with a method of inferring mantle convection patterns. While measurements of seismic anisotropy from recordings of earthquake waves that have passed through the mantle are relatively straightforward, it is a major challenge to decipher the exact location or depth of the anisotropic structure. One strategy for overcoming this challenge is to move towards an approach that provides an image or picture of anisotropy in the upper mantle, using a technique known as tomography. Much in the same way that medical tomography can provide an image of the inside of a person's body, seismic tomography yields an image of the structure of the Earth's deep interior. The PI will provide support and mentoring for a PhD student. Codes being developed during the research will be openly available via GitHub. The PI and graduate student will also provide outreach in the form of presentations to the general public.The application of shear wave splitting tomography has been a long-standing goal in the study of upper mantle anisotropy, but there are only a few examples of published studies that have successfully implemented it. The challenges are many: the inverse problem is highly nonlinear and sensitivity kernels for the inversion depend strongly on the starting model; a large number of parameters are needed for a complete description of the anisotropy; robust shear wave splitting measurements are often difficult to make for noisy data. This project will meet those challenges by developing and applying a framework for finite-frequency SKS splitting intensity tomography that implements a model space search approach for probing parameter space. The investigators propose three specific activities. Activity #1 comprises the completion and implementation of a theoretical and computational framework for our splitting intensity tomography approach. The team is extending existing theory for finite-frequency sensitivity kernels to a new parameterization scheme that takes advantage of constraints on olivine elasticity from mineral physics. They will complete the implementation of a model space search algorithm, based on a Markov chain Monte Carlo approach, for inverting splitting intensity observations. Finally, the investigators will test their implementation with a series of synthetic anisotropic models and splitting intensity datasets, designed to understand the limitations, resolution, and parameter tradeoffs of their imaging approach. Activity #2 will involve the implementation of the shear wave splitting intensity tomography framework for several real-world data sets from dense broadband seismic networks. Activity #3 will extend their model space search approach to simultaneously include other types of observations that constrain upper mantle anisotropy, including anisotropic receiver functions and surface wave phase velocity measurements.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.