CSEDI: Testing Hypotheses for the Origin of Seismic Anisotropy at the Base of the Mantle

地震各向异性（即地震波以不同的速度向不同的方向传播）在60年代中期被发现，并很快被定性地解释为对流过程中晶体排列的结果。这一概念已被普遍接受。最近强烈的各向异性和不均匀性被记录在最低地幔金属和液体核心附近。神秘的D？该区域既是热边界层又是化学边界层，是许多有趣的地震观测所反映的复杂动态过程的场所。地幔底部地震各向异性的原因一直是许多猜测的来源，但其主要原因仍然不知道。 受已知发生在上地幔中的动力学过程的类比启发，通过调用与岩石圈板块到达地幔底部时的流动方向相关的变形过程来解释它是诱人和自然的，并且矿物是随着压力增加而发生相变的主体。主要观察速度更快的水平偏振剪切波比垂直偏振的（VshVsv）在D？表明矿物会变形并重新定向。为了更好地了解D？地震学家和矿物物理学家与地球动力学家合作开发的工具和观测应结合起来，具体而言，这项研究将依赖于五个模拟要素：1）下地幔条件下矿物微观变形机制的知识（特别是钙钛矿、后钙钛矿和镁铁矿），2）组分在压力和温度下的单晶弹性性质;（3）提供下地幔宏观应变场的大尺度动力学模型：（4）预测晶体在宏观变形场中重新取向的多晶塑性; 5）根据1-2-3-4，可以构建弹性各向异性及其3D变化的模型，并且给定适当的波传播代码，合成地震图可以针对适当的地震相位进行计算，并与实际观测结果进行比较。现在有精确的方法来计算合成地震图在一个三维各向异性的地球下降到体波频率。我们依靠两位主要研究人员在地震学和矿物物理学方面的不同专业知识。这项研究将提供建议，为今后的地震实验更好地表征各向异性D？并且所开发的方法将来也将提供给其他研究人员。

Seismic anisotropy (i.e. seismic waves travel in different directions at different speed) in the deeper earth was discovered in the mid-sixties and was soon interpreted in a qualitative way as a result of crystal alignment during convection. This concept became generally accepted. More recently strong anisotropy and heterogeneity was documented in the lowermost mantle adjacent to the metallic and liquid core. This enigmatic D? zone is both a thermal and chemical boundary layer which is the site of complex dynamic processes that are reflected in many intriguing seismic observations. The cause for seismic anisotropy at the base of the mantle has been the source of much speculation, but its dominant cause is still not known. Inspired by analogy with dynamic processes that are known to occur in the upper mantle, it is tempting and natural to explain it by invoking deformation processes associated with the direction of flow as lithospheric slabs reach the bottom of the mantle and minerals are subject of phase transformations with increasing pressure. The predominant observation of faster velocities for horizontally polarized shear waves than for vertically polarized ones (VshVsv) in D? suggests that minerals deform and reorient. In order to gain a better understanding of the origin of seismic anisotropy in D? tools and observations that have so far been developed independently by seismologists, and mineral physicists working with geodynamicists, should be combined.Specifically, this research will rely on five modelling ingredients: 1) Knowledge of microscopic deformation mechanisms of minerals at lower mantle conditions (specifically perovskite, postperovskite and magnesiowuestite), 2) single crystal elastic properties of the constituents at pressure and temperature; 3) plausible large scale dynamics models providing the macroscopic strain field in the lower and lowermost mantle; 4) polycrystal plasticity that predicts reorientation of crystals in a macroscopic deformation field; 5) from 1-2-3-4, a model of elastic anisotropy and its 3D variations can be constructed and, given an appropriate wave propagation code, synthetic seismograms can be computed for appropriate seismic phases and compared to actual observations. There are now precise ways to compute synthetic seismograms in a 3D anisotropic earth down to body wave frequencies.In order to obtain new insights into our understanding of deformation behavior at the base of Earth?s mantle, we rely on the different expertise in seismology and mineral physics of the two principal investigators. The research will provide suggestions for future seismic experiments targeted at better characterizing anisotropy in D? and the methodologies that are developed will become available to other researchers in the future.