Constraining lowermost mantle flow through observations and models of seismic anisotropy

地核-地幔边界（CMB）是地球内部最引人注目的物理边界。宇宙微波背景是岩石、对流地幔和液态铁外核之间的界面，后者的运动产生了地球磁场。CMB区域在组成、密度、粘度和温度上的主要差异意味着这个界面在控制地球内部的动力学和进化方面起着关键作用。具体来说，CMB代表了地幔对流的底部边界层，地幔对流是地球在地质时间内随着相对热的物质上升和相对冷的物质下沉而冷却的过程。一个尚未解决的主要问题是，CMB上方的地幔对流模式是什么样子的，以及这种模式如何与地幔其余部分的对流运动相互作用，以及它们在俯冲带等板块构造特征中的表面表现。这个项目的目标是利用对穿过最底地幔的地震波的观测来约束CMB上方的地幔流动模式。该项目涉及通过观测和建模研究地幔底部地震各向异性和流动模式的三年努力。由于形变与地震各向异性之间的因果关系，各向异性的表征和解释可以为地幔的流动模式提供重要的约束。虽然地震各向异性通常在上地幔进行研究，但从最下层的地幔各向异性中分离信号要困难得多；此外，最下层地幔矿物的应变与各向异性之间的关系仍存在较大的不确定性。尽管研究D '各向异性存在固有的挑战，但它作为一种破译地幔底部流动模式和理解驱动这些模式的过程的工具，具有非凡的前景。该项目解决了与最下层地幔结构和动力学相关的两个基本未解决的问题：1)D”层地震各向异性的几何形状是什么？2)最下层地幔的流动模式是什么？是什么物理过程驱动了这种流动？为了解决这些科学问题，研究者建议开展五项活动。首先，该团队将对S-ScS和SKS-SKKS相进行差分横波分裂观测，以限制在一系列射线传播方向上，在选定区域内地幔底部各向异性引起的分裂。其次，他们将对从D ‘ ’不连续面（PdP和sd）反射的相位进行阵列分析；这些相的极性受D′各向异性的影响，结合剪切波分裂测量可以更严格地约束各向异性几何。第三，他们将应用基于矿物物理学的正演建模框架，该框架使用单晶弹性来识别与地震观测相一致的合理的各向异性几何形状。第四，他们将利用这些各向异性的观测结果来检验地幔底部流动和弹性的全球模型的预测。最后，他们将整合项目各个阶段的结果，以检验由一组关于地幔底部流动驱动力的假设所做出的预测。这项工作的更广泛影响包括培养一名研究生、培养国际合作、建立一个面向公众的关于地球深部过程的网站，以及在科学出版物和公众教育和宣传演讲中传播研究结果。

The core-mantle boundary (CMB) is the most dramatic physical boundary within the Earth's interior. The CMB is the interface between the rocky, convecting mantle and the liquid iron outer core, whose motions give rise to the Earth's magnetic field. The major contrasts in composition, density, viscosity, and temperature across the CMB region mean that this interface plays a critical role in controlling the dynamics and evolution of the Earth's interior. Specifically, the CMB represents the bottom boundary layer for mantle convection, the process through which the Earth cools off over geologic time as relatively hot material rises and relatively cool material sinks. A major unsolved problem is what the pattern of mantle convection looks like just above the CMB, and how that pattern interacts with convective motions in the rest of the mantle and their surface expressions in plate tectonic features such as subduction zones. The goal of this project is to use observations of seismic waves that have passed through the lowermost mantle to constrain the pattern of mantle flow just above the CMB.This project involves a three-year effort to study seismic anisotropy and flow patterns at the base of the mantle via observations and modeling. Because of the causative link between deformation and seismic anisotropy, the characterization and interpretation of anisotropy can provide crucial constraints on flow patterns in the mantle. While seismic anisotropy is commonly studied in the upper mantle, it is much more difficult to isolate the signal from lowermost mantle anisotropy; furthermore, major uncertainties remain about the relationships between strain and anisotropy in lowermost mantle minerals. Despite the challenges inherent in studying D" anisotropy, however, it holds exceptional promise as a tool for deciphering patterns of flow at the base of the mantle and understanding the processes that drive these patterns. This project addresses two fundamental unsolved problems related to the structure and dynamics of the lowermost mantle: 1) What is the geometry of seismic anisotropy in the D" layer? and 2) What is the pattern of flow in the lowermost mantle, and what physical processes drive this flow? In order to address these science questions, the investigator proposes to carry out five activities. First, the team will carry out differential shear wave splitting observations of S-ScS and SKS-SKKS phases to constrain splitting due to anisotropy at the base of the mantle in selected regions over a range of ray propagation directions. Second, they will carry out array analysis of phases that have been reflected off the D" discontinuity (PdP and SdS); the polarities of these phases are affected by D" anisotropy and in combination with shear wave splitting measurements can more tightly constrain the anisotropic geometry. Third, they will apply a mineral physics-based forward modeling framework that uses single-crystal elasticity to identify plausible anisotropic geometries that are consistent with seismic observations. Fourth, they will use these observations of anisotropy to test the predictions of global models for flow and elasticity at the base of the mantle. Finally, they will integrate results from all phases of the project to test the predictions made by a set of hypotheses about the driving forces for flow at the base of the mantle. Broader impacts of this work include the training of a graduate student, the cultivation of international collaborations, the creation of a website on deep Earth processes aimed at the general public, and the dissemination of the results in both scientific publications and public education and outreach presentations.