Anisotropic Mantle: Advancing Models of Regional Flow and Associated Seismic Signature

地幔内部的流动是热量和物质在地球深处和我们居住的地表之间传递的关键过程。确定这种流动模式的为数不多的方法之一是测量地震各向异性——这表明波在一个方向上的传播速度比另一个方向快。各向异性可能与地幔流动有关，因为构成地幔的矿物质在受到剪切力的作用时往往会缓慢排列，而它们排列的方向通常比交叉方向具有更快的地震波速度。在这个项目中，之前的实验室岩石变形结果将与区域流动的计算机模拟相结合，在区域流动中，构造板块的运动产生了使地幔岩石变形的力。需要解决的新问题是，矿物排列是否会显著地使地幔变硬或变弱，以及这是否会影响整体流动模式，从而可能改变岩浆产生等交换机制。沿着地球表面传播的地震波很适合记录各向异性随深度的变化。该项目的第二个方面是确定最佳的表面波数据分析方法，以检测复杂的各向异性模式，如预计在构造板块边界附近发展。一套数值模型将阐明流动诱导的变形对上地幔黏性结构的影响，以及它对流动发展模式的影响程度，以及相关的地震特征。本研究的地球动力学部分将提供的主要进展是在上地幔流动模型中明确包含各向异性流变学。一个多尺度（纳米-100公里），连接的数值过程是必要的，更新现有的程序是努力的一个组成部分。流动模型将强调远离海洋扩张中心的演化，但对流动结构与地震各向异性模式/震级之间关系的见解将与其他环境相关，如夏威夷羽流和俯冲带。这项研究的新地震学模型将记录表面波色散如何依赖于（弹性）各向异性的分布。这在以前的地幔流动和纹理的关联模型中还没有被研究过，但它是一个关键方面，特别是考虑到现在可用的地震阵列数据。在板块边界和板块内热点约1000公里范围内，预期各向异性结构的复杂性要求进行全套体波和面波分析，以便以尽可能少的非唯一性来识别结构。同时，在垂直和横向结构连续变化的环境中，需要对地震波的潜在偏差和分辨率限制进行定量评估。通过提供一套强大的正演模型，我们将为地震数据分析中应用的逆方法奠定基础。

Flow within the mantle is the key process by which heat and material are transferred between the deep Earth and the surface we live on. One of the few ways of determining the pattern of this flow is to measure seismic anisotropy- an indication that propagation of waves occurs faster in one direction than another. Anisotropy can be linked to mantle flow because minerals that make up the mantle tend to slowly align when subjected to shear forces and the direction they align typically has faster seismic wave speeds compared to the cross direction. In this project, prior laboratory rock deformation results will be combined with computer simulations of regional flow, where motion of tectonic plates generates the forces to deform mantle rocks. The new question to be addressed is whether alignment of minerals can significantly stiffen, or weaken, the mantle and whether this may influence the overall pattern of flow, thus potentially altering exchange mechanisms such as production of magma. Seismic waves that travel along Earths surface are well suited for documenting the variation in anisotropy with depth. A secondary aspect of the project is to determine optimum surface wave data analysis methods for detecting complex patterns of anisotropy such as are expected to develop near tectonic plate boundaries.A suite of numerical models will elucidate the effects that flow-induced texturing can have on the viscosity structure of the upper mantle and the extent to which it influences the pattern of flow that develops, as well as the associated seismic signatures. The main advance that the geodynamical part of this study will provide is explicit inclusion of anisotropic rheology in models of upper mantle flow. A multi-scale (nanometer-100's km), linked numerical procedure is required and updates to existing programs are an integral part of the effort. The flow modeling will emphasize evolution away from an oceanic spreading center, but insights on the relationships between flow structure and the patterns/magnitudes of seismic anisotropy will be relevant to other settings such as the Hawaiian plume and subduction zones. The new seismological modeling for this study will document how surface wave dispersion depends on the distribution of (elastic) anisotropy. This has not been studied previously for linked models of mantle flow and texturing but it is a key aspect, especially in light of seismic array data that are now becoming available. The complexity of expected anisotropic structure within ~1000 km of plate boundaries and intraplate hotspots require that the full suite of body and surface wave analyses be performed in order to discern structure with the minimum possible non-uniqueness. At the same time, a quantitative assessment of the potential bias and limits of resolution is needed for seismic waves in settings where both vertical and lateral structure vary continuously. By providing a strong suite of forward models, we will set the stage for inverse approaches that can be applied during seismic data analysis.