Constraining lowermost mantle anisotropy with body waves: a synthetic modelling study
Constraining lowermost mantle anisotropy with body waves: a synthetic modelling study
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用体波约束最低地幔各向异性：综合建模研究
DOI：
10.1093/gji/ggz049
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发表时间：
2019
影响因子：
2.8
通讯作者：
Creasy N
中科院分区：
文献类型：
--
作者：
Creasy N
不同的机制已经被提出来解释地幔底部的地震各向异性，包括各种矿物（硼镁石，后钙钛矿和铁方镁石）的晶体学优选取向和弹性不同的材料，如部分熔体的形状优选取向。对D”各向异性机制的研究通常会产生模糊的结果，因为地震观测很少（如果有的话）唯一地限制一种机制或方向，并且通常依赖于重要的假设来推断深部地幔中的流动模式。剪切波分裂和极性的SdS和PdP反射的D”不连续性的观察是我们探测D”各向异性的最佳工具之一，但是，目前可用的数据集不能约束一个独特的情况下，提出的矿物物理学文献。在这项工作中，我们通过正向建模方法确定需要哪些体波相位组合（例如SKS、SKKS和ScS）来唯一地约束D '各向异性的机制。我们测试九个模型的基础上提供的矿物物理研究的单晶和多晶弹性张量。通过求解Christoffel方程，我们的模型预测了SKS、SKKS和ScS相的快剪切波分裂方向，以及D”界面外P波和S波反射的极性。我们运行测试，使用随机选择的合成数据集的基础上给定的起始模型，控制测量的总数，方位角分布，地震相位的类型。对于每个合成数据集，我们搜索所有可能的弹性张量和方向，以确定哪些与合成数据一致。总的来说，我们发现很难用一种测量技术（SKS、SKKS、ScS或反射极性）来唯一地约束具有典型数量的地震各向异性测量（基于当前可用的研究）的各向异性机制。然而，包括SKS、SKKS和ScS测量或剪切波分裂和反射极性测量的组合的数据集增加了唯一地约束起始模型及其方向的概率。基于这些发现，我们确定了特定地区（即北美，西北太平洋和澳大利亚）的最低地幔有足够的射线路径覆盖的测量技术的组合。
Different mechanisms have been proposed as explanations for seismic anisotropy at the base of the mantle, including crystallographic preferred orientation of various minerals (bridgmanite, post-perovskite and ferropericlase) and shape preferred orientation of elastically distinct materials such as partial melt. Investigations of the mechanism for D" anisotropy usually yield ambiguous results, as seismic observations rarely (if ever) uniquely constrain a mechanism or orientation and usually rely on significant assumptions to infer flow patterns in the deep mantle. Observations of shear wave splitting and polarities of SdS and PdP reflections off the D" discontinuity are among our best tools for probing D" anisotropy; however, currently available data sets cannot constrain one unique scenario among those suggested by the mineral physics literature. In this work, we determine via a forward modelling approach what combinations of body wave phases (e.g. SKS, SKKS and ScS) are required to uniquely constrain a mechanism for D" anisotropy. We test nine models based on single-crystal and polycrystalline elastic tensors provided by mineral physics studies. Our modelling predicts fast shear wave splitting directions for SKS, SKKS and ScS phases, as well as polarities ofP-andS-wave reflections off the D" interface, for a range of propagation directions, via solution of the Christoffel equation. We run tests using randomly selected synthetic data sets based on a given starting model, controlling the total number of measurements, the azimuthal distribution, and the type of seismic phases. For each synthetic data set, we search over all possible elastic tensors and orientations to determine which are consistent with the synthetic data. Overall, we find it difficult to uniquely constrain the mechanism for anisotropy with a typical number of seismic anisotropy measurements (based on currently available studies) with only one measurement technique (SKS, SKKS, ScS or reflection polarities). However, data sets that include SKS, SKKS and ScS measurements or a combination of shear wave splitting and reflection polarity measurements increase the probability of uniquely constraining the starting model and its orientation. Based on these findings, we identify specific regions (i.e. North America, northwestern Pacific and Australia) of the lowermost mantle with sufficient ray path coverage for a combination of measurement techniques.
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