EAR-PF: Shear Wave Splitting based on 3D Seismic Wave Simulations: Forward to Inverse Modeling of Upper Mantle and D" Anisotropy

Neala Creasy博士已获得美国国家科学基金会EAR博士后奖学金，在科罗拉多矿业学院（CSM）开展研究和教育计划。她将利用地震产生的地震波，研究地幔在高压和高温下是如何变形的。解释这些地震波以及它们如何与地幔中的这些变形和矿物学过程直接相关是困难的，部分原因是由于在高压和高温下进行矿物物理实验所固有的必要假设和基本限制。她将计算一个三维的合成波形，真实的地球来探索地震观测如何与地幔变形相关。虽然一些先前的研究已经探索了一些用于简化这些观测的假设的可行性，但还有许多其他方面需要充分探索，以充分了解地球的复杂性。了解地球如何通过地幔对流而变形是很重要的，因为这种变形以火山活动和地震的形式控制着板块构造的地表表现。这项研究将有助于阐明如何利用地震波来充分发挥其潜力，以了解地球地幔的当前过程的一般认识。她的教育计划包括担任CSM研究生和本科生的研究导师，创建教育材料（例如，信息图表，教材）和虚拟现实（VR）设置，以激发年轻科学家追求基础科学，并通过IRIS（地震学本科实习计划的联合研究机构）和丹佛科学博物馆等组织在当地社区继续开展推广工作。限制地球地震各向异性的模式和性质有助于揭示矿物物理、地幔对流和地震学之间的关系。岩石圈中各向异性的来源如冻结各向异性、过渡带和D′使横波分裂测量复杂化，导致横波分裂可能不同于板块运动。如果我们能更好地了解来自岩石圈的地震各向异性，我们也可以更好地约束D”各向异性，这需要在一定程度上对上地幔进行校正。射线理论是常用的，在一定范围内是合适的，但并不是所有的含义都被探讨过。射线理论是一种无限频率近似，其有效性取决于波的周期、非均质尺度、传播路径的长度和多次到达的叠加，这使得解释地震各向异性观测更加困难。数值方法和高性能计算的进步为在进行地震观测研究时利用真实的三维地球结构考虑波传播的全部物理特性提供了新的机会。在这项工作中，Creasy博士将通过3D全球波传播求解器SPECFEM3D_GLOBE进行数值模拟，探索横波分裂的假设，并梳理出不同模型和地球各向异性观测之间的差异。这项工作将有助于提高对地震各向异性观测与地幔形变模型的关系以及地壳、上地幔和地壳各向异性的来源的理解。这些见解将有助于阐明区域（如北美和澳大利亚）和全球尺度上不同地震各向异性观测技术之间的差异。这项工作还将有助于在全球全波形反演中使用横波分裂，解决在反演过程中描述地幔体波各向异性的适当参数化问题。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。

Dr. Neala Creasy has been granted an NSF EAR postdoctoral fellowship to carry out research and educational plans at Colorado School of Mines (CSM). She will investigate how the Earth's mantle deforms under high pressures and temperatures by making use of the seismic waves produced by earthquakes. Interpreting these seismic waves and how they directly relate to these deformation and mineralogical processes within the mantle is difficult, in part due to necessary assumptions and fundamental limitations inherent to mineral physics experiments at high pressures and temperatures. She will calculate synthetic waveforms for a 3D, realistic Earth to explore how seismic observations relate to mantle deformation. While some prior research has explored the viability of some of the assumptions used to simplify these observations, there are many additional aspects that need to be fully explored to fully understand the complexity of the Earth. Understanding how the Earth deforms via mantle convection is important because this deformation controls the surface expression of plate tectonics, in the form of volcanic activity and earthquakes. This research will help clarify the general understanding of how to use seismic waves to their full potential in understanding current processes in Earth's mantle. Her educational plan involves acting as a research mentor for graduate and undergraduate students at CSM, creating educational material (e.g., infographics, teaching materials) and a Virtual Reality (VR) setup to excite young scientists to pursue basic science, and continued outreach efforts in the local community through organizations including IRIS (Incorporated Research Institutions of Seismology undergraduate internship program) and the Denver Museum of Science. Constraining the pattern and properties of seismic anisotropy in the Earth can help reveal relationships between mineral physics, mantle convection, and seismology. Sources of anisotropy in the lithosphere as frozen-in anisotropy, transition zone, and D" complicate shear wave splitting measurements, resulting in shear wave splitting that can differ from plate motion. If we better understand seismic anisotropy sourced in the lithosphere, we could also better constrain D" anisotropy, which requires correcting for the upper mantle to some extent. Ray theory is commonly used and is appropriate within certain limits, but not all implications have been explored. Ray theory is an infinite frequency approximation and its validity depends on the period of waves, the scale of heterogeneities, the length of its propagation path, and the superposition of multiple arrivals, making interpreting seismic anisotropy observations more difficult. Numerical methods and advances in high-performance computing offer new opportunities to take the full physics of wave propagation into account using realistic 3D Earth structures while conducting seismological observational studies. In this work, Dr. Creasy will explore the assumptions made in shear wave splitting as well as tease out discrepancies between different models and observations of anisotropy within the Earth, by conducting numerical simulations via 3D global wave propagation solver SPECFEM3D_GLOBE. This work will help improve the understanding of how seismic anisotropy observations are related to models of deformation in the Earth's mantle and the sources of anisotropy in the various regions of the crust, upper mantle, and D". These insights will help illuminate discrepancies between different seismic anisotropy observational techniques on regional (e.g. North America and Australia) and global scales. This work will also assist the development of using shear wave splitting in global full waveform inversion addressing appropriate parametrization to describe body-wave anisotropy in the mantle during the inversion process.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.