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

地震学提供了研究地球内部结构的最佳机会。地球表面的地震仪记录来自本地和远距离地震的地震波，这些记录可用于获取地球内部（从最里面的固体铁芯到表面）的三维图像。创建这些地震图像需要记录真实地震和模拟地震，以计算波如何通过地球结构的真实三维模型传播。通过将真实记录与模拟记录进行比较，可以改进地球结构的模型。在该项目中，这些地震波模拟将用于研究先前建立的阿拉斯加地下结构模型，并根据最近记录的地震开发新模型。这些结果将有助于更好地了解阿拉斯加地震与断层活动之间的关系，同时提供更准确的方法来确定由于地球表面存在沉积层而导致某些地面运动预计相对较强的位置。该项目将为其他科学家建立通用程序，以询问通过其他方法和数据集制作的地球结构模型。该项目促进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、石油和天然气地震成像、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，也称为断层扫描模型，通常通过近似方法生成，并且通常对彼此进行定性比较。这些图像和比较提出了有关图像准确性以及如何在地球的成分、热结构和动力学背景下解释它们的基本问题。该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与记录的地震图进行比较所需的模拟地震图。这将使最准确的物理学能够在成像问题中得到应用。其次，它将波场模拟应用于参考数据集，以直接、公平地比较先前导出的断层扫描模型。第三，它将正式化这些程序，并通过举办虚拟研讨会将其提供给其他人，该研讨会的主题是如何访问地球模型协作组织的层析成像模型以及如何在开源软件包 Specfem3D 中执行地震波场模拟。这些工作的重点是阿拉斯加，自 2017 年以来，EarthScope 地震台的覆盖范围非常广泛，并且表现出阿留申-阿拉斯加俯冲带东缘碰撞造成的极端地下构造复杂性。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使用阿拉斯加作为验证和改进断层扫描模型的更广泛努力的焦点提供了动力。这些工作将考虑以倾斜横向各向同性的形式更复杂地表示地球结构，这种结构存在于矿物和岩石（例如页岩）中，但在更大的尺度（例如地壳和最上地幔）上确定它具有挑战性。该奖项反映了 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。

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.