CSEDI collaborative research: a multidisciplinary approach to investigate the origin of anisotropy at the base of the mantle

地震各向异性（即地震波以不同的速度在不同的方向上传播）在60年代中期被发现，并很快被定性解释为对流（LPO）过程中晶体排列的结果。这个概念后来被普遍接受。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。这个神秘的D”区既是热边界层又是化学边界层，其特征是复杂的动态过程，这些过程反映在许多有趣的地震观测中。近年来，矿物物理学研究取得了很大进展，对地幔底部矿物（包括后钙钛矿相）的弹性和变形性质进行了表征。地球动力学模拟的进展使我们能够跟踪地幔对流过程中的应变演化。因此，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地球下降到体波频率。我们提出的研究是在2年前开始的初步工作的基础上进行的，重点是D”中LPO各向异性的正演模拟，目标是结合地球动力学家、地震学家和矿物物理学家开发的工具和观测结果，以便更好地了解D”中地震各向异性的起源，并确定可能起作用的微观和宏观过程。在我们迄今的工作中，我们已经为此目的建立了一个多步骤的程序，它结合了逻辑链中的五个建模成分：（1）对于一个特定的假设，地幔动力学模型提供的信息，宏观应变变形所容纳的个别包地幔材料。 (2)然后，该应变变形信息被用作计算多晶矿物聚集体内的所得矿物学纹理（即LPO）的数值模型的边界条件。(3)将根据矿物性质和优选取向确定的地震弹性常数应用于整个感兴趣区域的许多矿物聚集体，（4）随后通过从步骤1-3获得的3D弹性各向异性模型进行正演地震建模。(5)所得模型和地震波形进行比较，现有的地震观测。我们的初步结果表明，我们可以使用宏观观测来约束合理的成分和变形机制。到目前为止，我们的工作主要集中在俯冲板块到达核幔边界（CMB）及其随后沿CMB沿着扩展的二维模型。在这里，我们建议将我们的一套地球动力学工具扩展到3D的情况下，这将使我们能够探索预测分布的不同种类的各向异性在地幔的基础上，在一个更现实的框架，并面对由此产生的地震波场可用的宽带地震数据。我们将进行实验和理论矿物物理计算。该项目促进PI、参与学生和博士后同事在地球动力学、矿物物理学和地震学领域的跨学科工作和交叉教育，通过分享不同的专业知识，促进学习解决复杂的科学问题。它还将为未来的地震实验提供建议，旨在更好地表征D”的各向异性。

Seismic anisotropy (i.e. seismic waves travel in different directions at different speeds) 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 (LPO). This concept since 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 characterized complex dynamic processes that are reflected in many intriguing seismic observations. Much progress was recently achieved in mineral physics, to characterize elastic and deformation properties of lowermost mantle minerals including the post-perovskite phase. Advances in geodynamic modeling now allow us to track the strain evolution during mantle convection. As a result, there are now precise ways to compute synthetic seismograms in a 3D anisotropic earth down to body wave frequencies. Our proposed study follows on preliminary work started 2 years ago, and is focused on the forward modeling of LPO anisotropy in D", with the goal of combining tools and observations developed by geodynamicists, seismologists and mineral physicists, in order to gain better understanding of the origin of seismic anisotropy in D", and determine which microscopic and macroscopic processes may be at play. In our work to-date, we have set up a multi-step procedure for this purpose, which combines five modeling ingredients in a logical chain: (1) For a particular hypothesis regarding mantle dynamics, geodynamical models provide information on the macroscopic strain deformation accommodated by individual packets of mantle material. (2) This strain deformation information is then used as boundary conditions for numerical models that calculate the resulting mineralogical texture (i.e. LPO) within a polycrystalline mineral aggregate. (3) Seismic elastic constants, determined from mineral properties and preferred orientations, are applied to numerous mineral aggregates throughout the region of interest, (4) followed by forward seismic modeling through the 3D elastic anisotropic model acquired from steps 1-3. (5) Resulting models and seismic waveforms are compared to available seismic observations. Our initial results illustrate that we can use macroscopic observations to constrain plausible constituents and deformation mechanisms. So far, our work has focused on 2D models of a subducting slab reaching the core-mantle boundary (CMB) and its subsequent spreading along the CMB. We here propose to extend our set of geodynamic tools to the 3D case, which will allow us to explore the predicted distribution of different kinds of anisotropy at the base of the mantle in a more realistic framework and confront the resulting seismic wavefields to available broadband seismic data. We will perform experiments and theoretical mineral physics computations. This project promotes interdisciplinary work and cross-education in the fields of geodynamics, mineral physics and seismology among the PI's, participating students and postdoctoral associates, stimulating learning to solve complex scientific problems by sharing different expertise. It will also provide suggestions for future seismic experiments targeted at better characterizing anisotropy in D".