Investigating the Pervasiveness of Complex Seismic Anisotropy and Its Origin Beneath Continents

过去50年来的大量地球科学调查令人信服地证明，地球表面被分成七大块和许多小块，称为构造板块。板块之间的相对运动产生了地震、火山、雄伟的山脉和深海盆地。然而，驱动板块运动的物理和化学过程仍然知之甚少。增进我们对这些过程的了解不仅对于了解地球如何运作至关重要，而且对于实现可靠地预测和减轻地震和火山喷发等自然灾害的最终目标也是至关重要的。剪切波分裂分析是研究板块运动的有效工具之一，它是基于地震产生的剪切波在板块运动形成的各向异性区域传播时会分裂成两个波的观测结果。以往的横波分裂研究大多是在只有一层各向异性的假设下进行的。最近地震台站和记录的地震数量的急剧增加表明，实际情况比这种单层假设要复杂得多。该项目将开发和测试一套复杂的工具，在全球范围内系统地研究复杂的各向异性，目的是对与板块动力学和板块运动有关的一些假说提供约束。该项目还将支持对一名研究生、两名本科生和一名暑期实习生的培训。剪切波分裂(SWS)已越来越多地被用于量化地震方位各向异性，并了解其形成的地球动力学过程。主要是由于地球上大多数站可获得的数据量有限，以及由此产生的对入射XKS(包括SKS、SKKS和PKS)射线的方位覆盖较差，绝大多数现有的SWS研究，包括由PI进行的大多数研究，假设各向异性的最简单形式，即具有水平对称轴的单层。另一方面，一些SWS研究已经确定了观测到的分裂参数作为入射XKS射线方位角的函数的系统变化。这种相关性是复杂各向异性(例如，具有倾斜对称轴的各向异性和/或具有水平轴的两个或多个层的各向异性)的最重要的诊断。通过利用最近宽带地震数据的数量和质量的急剧增加，该项目将利用大陆上的长期运行的地震台来解决复杂各向异性的普遍性这一重要问题。该项目还打算通过确定与层相关的分裂参数来表征复杂的各向异性。由此得到的复杂各向异性的空间分布将成为地球动力学建模、地震层析成像和矿物物理领域中各种地球科学研究的有价值的约束和输入参数。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Numerous geoscientific investigations over the past 50 years have convincingly demonstrated that the Earth's surface is divided into seven large and many small blocks known as tectonic plates. The relative movement of the plates against each other produces earthquakes, volcanoes, majestic mountains, and deep ocean basins. The physical and chemical processes that drive plate motion, however, are still poorly understood. Improving our understanding of such processes is essential not only for understanding how the Earth works, but also for achieving the ultimate goal of reliably predicting and mitigating natural hazard such as earthquakes and volcanic eruptions. One of the effective tools to investigate plate motion is shear wave splitting analysis, which is based on the observation that when a shear wave originated from an earthquake travels through an anisotropic area formed by plate motion, it will split into two waves. Previous shear wave splitting studies were mostly conducted under the assumption that there is only one layer of anisotropy. The recent dramatic increase in the number of seismic stations and recorded earthquakes suggests that the actual situation is more complicated than this single layer assumption. This project will develop and test a set of sophisticated tools to systematically investigate complex anisotropy on a global scale, for the purpose of providing constraints on a number of hypotheses related to plate dynamics and plate motion. This project will also support training of a graduate student, two undergraduate students, and a summer intern.Shear wave splitting (SWS) has been increasingly used to quantify seismic azimuthal anisotropy and to understand the geodynamic processes responsible for its formation. Mostly due to the limited amount of data available at most of the stations on Earth and the resulting poor azimuthal coverage of the incoming XKS (including SKS, SKKS, and PKS) rays, the vast majority of existing SWS studies, including most of the studies conducted by the PIs, assumed the simplest form of anisotropy, i.e., a single layer with a horizontal axis of symmetry. On the other hand, some SWS studies have identified systematic variations of the observed splitting parameters as a function of the azimuth of the incoming XKS rays. This dependence is the most important diagnostic of complex anisotropy (e.g., anisotropy with a dipping axis of symmetry and/or with two or more layers with a horizontal axis). By taking advantage of the recent dramatic increase in the quantity and quality of broadband seismic data, this project will address the important question of the pervasiveness of complex anisotropy using long-running seismic stations on the continents. The project also intends to characterize the complex anisotropy by determining the splitting parameters associated with the layers. The resulting spatial distribution of complex anisotropy will be a valuable constraint and input parameter for various geoscientific studies in the areas of geodynamic modeling, seismic tomography, and mineral physics.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.