Seismic Tomography Models for Alaska: Validation, Iteration, and Complex Anisotropy

地震学为研究地球内部结构提供了最好的机会。地球表面的地震仪记录当地和远处地震的地震波，这些记录可以用来获得地球内部的三维图像，从最深处的固体铁芯到地表。创建这些地震图像需要记录真实地震和模拟地震，计算地震波如何通过真实的三维地球结构模型传播。通过比较真实记录和模拟记录，有可能改进地球结构的模型。在这个项目中，这些地震波的模拟将用于研究阿拉斯加地下结构的先前建立的模型，以及基于最近记录的地震开发新的模型。这些结果将使人们更好地了解阿拉斯加地震和断层之间的关系，同时提供一种更准确的方法来确定由于地球表面沉积层的存在而预计某些地面运动相对强烈的地方。该项目将为其他科学家建立一般程序，以询问用其他方法和数据集制作的地球结构模型。该项目促进了免费和开放的软件开发和培训机会，并推进了与社会相关的两个主题——计算科学和弹性——对地震灾害评估、油气地震成像、材料科学和结构工程具有潜在的好处。地球内部结构的地震图像，也被称为层析模型，通常是由近似方法产生的，它们通常是相互定性比较的。这些图像和比较提出了关于图像准确性的基本问题，以及如何在地球的成分、热结构和动力学的背景下解释它们。这个项目解决了这个问题的三个方面。首先，它将使用地震波场模拟来生成所需的模拟地震图，以便与记录的地震图进行比较。这将使在成像问题中部署最精确的物理。其次，它将对参考数据集应用波场模拟，直接和公平地比较先前导出的层析模型。第三，它将使这些程序正规化，并通过举办一个虚拟研讨会向其他人提供这些程序，该研讨会将介绍如何从地球模型协作中访问层析模型，以及如何在开源软件包Specfem3D中执行地震波场模拟。这些努力的重点是阿拉斯加，自2017年以来，阿拉斯加的EarthScope地震台站覆盖范围非常大，并且由于阿留申-阿拉斯加俯冲带东部边缘的碰撞而表现出极端的地下构造复杂性。地震的可用性、台站覆盖的质量、地下结构的复杂性以及地下地球动力学的复杂性，都为将阿拉斯加作为验证和改进层析成像模型的更广泛努力的焦点提供了动力。这些努力将考虑以倾斜的横向各向同性的形式对地球结构的更复杂的表示，这种形式存在于矿物和岩石中（例如，页岩），但在更大的尺度上（例如地壳和上地幔）确定它是具有挑战性的。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。

Seismology provides the best opportunity to investigate the interior structure of Earth. Seismometers at the Earth's surface record seismic waves from local and distant earthquakes, and these recordings can be used to derive three-dimensional images of the Earth's interior, from its innermost solid iron core to the surface. Creating these seismic images requires recordings from real earthquakes and from simulated earthquakes that calculate how waves propagate through realistic three-dimensional models of Earth's structure. By comparing real recordings with simulated recordings, it is possible to improve the models of Earth's structure. In this project, these simulations of seismic waves will be used to investigate previously established models of the subsurface structure of Alaska, as well as to develop new models based on recently recorded earthquakes. The results will enable a better understanding of the relationship between earthquakes and faulting in Alaska, while providing a more accurate method for determining the places where certain ground motion is expected to be relatively strong due to the presence of sedimentary layers at the Earth's surface. The project will establish general procedures for other scientists to interrogate Earth structure models made by other methods and data sets. The project promotes free and open software development and training opportunities, and it advances two topics relevant to society—computational science and elasticity—with potential benefits to seismic hazard assessments, oil and gas seismic imaging, materials science, and structural engineering.Seismic images of Earth's interior structure, also known as tomographic models, are commonly produced by approximate methods, and they are typically qualitatively compared to one another. These images and comparisons raise fundamental questions regarding the accuracy of the images as well as how to interpret them in the context of the compositional and thermal structure and dynamics of the Earth. This project addresses three facets of this problem. First, it will use seismic wavefield simulations to generate the simulated seismograms needed for comparison with the recorded seismograms. This will enable the most accurate physics to be deployed within the imaging problem. Second, it will apply wavefield simulations to a reference data set to directly and fairly compare previously derived tomographic models. Third, it will formalize these procedures and offer them to others by hosting a virtual workshop featuring how to access tomographic models from the Earth Model Collaboration and how to perform seismic wavefield simulations in the open-source software package Specfem3D. The focus of these efforts in on Alaska, which has had exceptional coverage of EarthScope seismic stations since 2017 and exhibits extreme subsurface tectonic complexity caused by collision at the eastern margin of the Aleutian-Alaska subduction zone. The availability of earthquakes, the quality of station coverage, the complexity of subsurface structure, and the complexity of subsurface geodynamics all provide motivation for using Alaska as a focus for the broader effort of validating and improving tomographic models. The efforts will consider a more complex representation of Earth structure, in the form of tilted transverse isotropy, which is present in minerals and rocks (for example, shale) but which is challenging to determine at larger scales, such as the crust and uppermost mantle.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.