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沿不同方向传播)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取向的结果。这一概念被普遍接受。最近，在与金属核和液态核相邻的最下部地幔中发现了强烈的各向异性和不均质性。这个神秘的D？该区域既是一个热边界层，也是一个化学边界层，它是许多有趣的地震观测中反映的复杂动态过程的地点。地幔底部地震各向异性的原因一直是许多猜测的来源，但其主要原因仍不清楚。受到与已知发生在上地幔的动力学过程的类比的启发，通过引用与流动方向相关的变形过程来解释它是诱人的和自然的，因为岩石圈板块到达地幔底部，矿物随着压力的增加而发生相变。结果表明，在D？表明矿物会变形并重新定向。为了更好地了解D？地震各向异性的成因。到目前为止，地震学家和矿物物理学家与地球动力学专家合作开发的工具和观测应该结合起来。具体地说，这项研究将依赖于五个模拟成分：1)矿物在下地幔条件下(特别是钙钛矿、后钙钛矿和镁闪锌矿)的微观变形机制的知识，2)组分在压力和温度下的单晶弹性性质；3)可信的大尺度动力学模型，提供地幔下部和最下部的宏观应变场；4)多晶塑性，预测晶体在宏观变形场中的重定向；5)从1-2-3-4出发，可以建立弹性各向异性及其三维变化的模型，给出适当的波传播代码，就可以计算出适当震相的合成地震记录，并与实际观测结果进行比较。现在已经有了精确的方法来计算三维各向异性地球中的合成地震记录，精确到体波频率。为了对我们对地球底部变形行为的理解有新的见解，我们依赖于两位主要研究者在地震学和矿物物理方面的不同专业知识。这项研究将为今后的地震实验提供建议，以更好地表征D？开发的方法论将在未来供其他研究人员使用。

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.