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中晶体取向(LPO)的结果。这一概念后来被普遍接受。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“带既是一个热边界层，又是一个化学边界层，其特征是复杂的动态过程，这在许多有趣的地震观测中都有所反映。最近在矿物物理学方面取得了很大进展，用来表征包括后钙钛矿相在内的最低地幔矿物的弹性和变形性质。地球动力学模拟的进步现在使我们能够跟踪地幔对流过程中的应变演变。因此，现在有了精确的方法来计算3D各向异性地球中的合成地震图，精确到体波频率。我们建议的研究是在两年前开始的前期工作的基础上进行的，重点是D“的LPO各向异性的正演模拟，目的是将地球动力学、地震学家和矿物物理学家开发的工具和观测结合起来，以便更好地了解D”的地震各向异性的起源，并确定哪些微观和宏观过程可能在起作用。在我们到目前为止的工作中，我们已经为此目的建立了一个多步骤的过程，它在一个逻辑链中结合了五个建模成分：(1)对于关于地幔动力学的特定假设，地球动力学模型提供了关于由单独的地幔物质包容纳的宏观应变变形的信息。(2)这种应变变形信息随后被用作数值模型的边界条件，用于计算多晶矿物集合体内的结果矿物结构(即LPO)。(3)根据矿物性质和优选取向确定的地震弹性常数被应用于整个感兴趣区域的众多矿物集合体，(4)随后通过步骤1-3获得的三维弹性各向异性模型进行正演地震模拟。(5)将所得模型和地震波形与已有的地震观测资料进行了对比。我们的初步结果表明，我们可以使用宏观观察来约束可能的成分和变形机制。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扩散的2D模型上。我们在这里建议将我们的地球动力学工具集扩展到3D情况，这将使我们能够在更现实的框架内探索地幔底部不同类型各向异性的预测分布，并将产生的地震波场与可用的宽带地震数据对峙。我们将进行实验和理论矿物物理计算。该项目促进国际地球动力学、矿物物理和地震学领域的跨学科合作和交叉教育，鼓励参与的学生和博士后助理通过分享不同的专业知识来学习解决复杂的科学问题。它还将为今后的地震实验提供建议，目的是更好地表征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".