Testing the hypothesis of pervasive two-layer azimuthal anisotropy beneath North America

剪切波分裂参数（快速极化方向和分裂时间）已越来越多地被各种地球科学家用于了解地球深部的结构和动力学。观测到的横波分裂是来自冰冻在冷岩石圈内的化石各向异性，还是由下伏软流圈的现今地幔流产生的，或者是两者的结合，仍然是一个有争议的话题。虽然地球科学界一致认为剪切波分裂测量是了解地球深部结构和动力学的基础，但到目前为止，仍然缺乏一个连贯的、全面的分裂参数数据库。在过去的几年中，IRIS（地震学联合研究机构）DMC（数据管理中心）的数据档案中增加了大量高质量的数据。此外，在过去的几年里，网络中增加了大量的宽带永久站，许多短周期站在同一时期被更新为宽带站。最后，正在进行的USArray中的站点正在提供大量的新数据。最近获得的数据集显著提高了分裂参数的空间覆盖，特别是在北美大陆的稳定部分。PI和她的研究小组正在测量所有宽带站的剪切波分裂参数，使用他们开发的可靠测量和主观排序剪切波分裂测量的稳健程序。由于该项目，目前正在获得大约10,000个定义良好的站-事件对。提出的研究的第一个目标是通过使用从IRIS DMC可访问的数据集生成单个（非站点平均）分裂参数的数据库。第二个组成部分是使用分裂参数的综合相干数据库来验证Marone和Romanowicz[2007]的假设，他们通过联合反演表面波形和剪切波分裂观测，提出了北美中部和东部地区普遍存在的两层各向异性模型。这项工作的预期结果将被广泛的地球科学家用来解决关于地球深部结构和动力学的一系列基本问题。例如，如果普遍的两层各向异性假设通过了本研究将产生的相干剪切波分裂数据库的测试，地球动力学建模者将在他们的计算中部署两层模型，矿物物理学家将在岩石圈和软流圈解耦的假设下进行实验来研究水的相变和影响。

Shear-wave splitting parameters (fast polarization direction and splitting time) have been increasingly used by a wide variety of geoscientists for the understanding of the structure and dynamics of the Earth's deep interior. Whether the observed shear-wave splitting is originated from fossil anisotropy frozen within the cool lithosphere, is produced by present-day mantle flow in the underlying asthenosphere, or is from combination of the two is still a debated topic. While it is a consensus in the earth science community that shear-wave splitting measurements are fundamental for the understanding of the structure and dynamics of the Earth's deep interior, so far there is still a lack of a coherent comprehensive database for the splitting parameters. Over the past several years a significant amount of high-quality data have been added to the data archives at the IRIS (Incorporated Research Institutions for Seismology) DMC (Data Management Center). In addition, a lot more broadband permanent stations have been added to the network over the past several years, and many short-period stations have been updated into broadband stations over the same period of time. Finally, stations in the on-going USArray are providing a large amount new data. The recently available data sets significantly improve the spatial coverage of splitting parameters, especially in the stable part of the North American continent. The PI and her research group are measuring shear-wave splitting parameters at all the broadband stations using a robust procedure that they developed to reliably measure and subjectively rank shear-wave splitting measurements. About 10,000 well-defined station-event pairs are being obtained as a result of the project. The first goal of the proposed study is to generate a database of individual (not station averaged) splitting parameters by using datasets accessible from the IRIS DMC. The second component is to use the comprehensive coherent database of splitting parameters to test the hypothesis of Marone and Romanowicz [2007], who, by joint inversion of surface waveforms and shear-wave splitting observations, suggested a pervasive two-layer anisotropic model beneath central and eastern North America.Expected results from the proposed work will be used by a broad spectrum of geoscientists to address a wide range of fundamental questions regarding the structure and dynamics of the Earth's deep interior. For instance, if the hypothesis of pervasive two-layer anisotropy passed the test using the coherent shear-wave splitting database to be produced by the proposed study, geodynamic modelers will deploy the two-layer model in their calculations, and mineral physicists will perform experiments to study phase change and effects of water under the assumption that the lithosphere and the asthenosphere are decoupled.