Love-wave propagation in oceanic upper mantle: constraints on radial anisotropy and implications for dynamics of the asthenosphere

导致地球表面类似板块行为的机制和控制板块运动的过程还没有完全被理解。这项研究使用地震产生的地震波，这些地震波是由部署在太平洋中部的海底地震仪记录的，用于探测该结构，特别是在板块底部附近。利用变形引起的矿物排列及其对地震信号的影响之间的已知关系，将评估板块与下伏地幔之间的耦合程度。一个刚性的构造板块与下面的地幔有何不同，这些物质在板块底部的运动是否一致，这些问题长期以来一直引起地球科学家的兴趣。它是理解板块构造的核心。该奖项支持的研究生将接受最前沿海洋地震数据分析方面的培训，并有机会使用独特的数据集。太平洋岩石圈强烈的方位地震各向异性与蛇绿岩中橄榄石排列的观测结果一致，并限制了海洋扩张中心动力学模型。相反，在太平洋软流层中观测到的高振幅径向各向异性提供了证据，证明板块下方存在高度变形和/或部分熔融层，可能使板块与下面的地幔分离。位于~70 Ma岩石圈的600x400 km海底地震仪（OBS）阵列提供了高质量的宽带地震数据，足以以分辨率（深度和横向）表征各向异性，这是全球分析无法实现的。瑞利波速度表明在岩石圈形成期间形成了极强的方位角各向异性，但这些数据表明下伏软流圈的方位角各向异性明显较弱。确定径向各向异性的相应深度分布需要对Love波进行详细的分析。利用一种新的波场分析方法，洛夫波基模和高模相速度将在OBS阵列上测量。结合现有的方位各向异性约束，由此得出的各向异性估计将使我们能够明确地测试流动诱导的橄榄石结构是否与观测结果一致，或者是否需要定向熔体来解释观测结果。

The mechanisms that enable plate-like behavior on the Earth's surface and the processes that control plate motion are not fully understood. This study uses earthquake-generated seismic waves that were recorded by seafloor seismometers deployed for year in the central Pacific to probe the structure, particularly near the base of the plate. Using known relationships between deformation-induced mineral alignment and its effect on seismic signature, the degree of coupling between the plate and the underlying mantle will be evaluated. The question of how a rigid tectonic plate differs from the underlying mantle and whether or not these materials move in unison at the base of the plate, or not, has long intrigued Earth scientists. It is at the heart of understanding plate tectonics. The graduate student supported by this award will receive training in forefront marine seismic data analysis and have the opportunity to work with a unique dataset.Strong azimuthal seismic anisotropy in the Pacific lithosphere is consistent with observations of olivine alignment found in ophiolites, and it constrains models of ocean spreading center dynamics. In contrast, high-amplitude radial anisotropy observed in the Pacific asthenosphere provides evidence for a highly deformed and/or partially molten layer beneath the plate that may decouple the plate from the underlying mantle. A 600x400 km ocean bottom seismometer (OBS) array, located on ~70 Ma lithosphere, provided high-quality broadband seismic data, sufficient to characterize anisotropy with resolution (in depth and laterally) that is unattainable from global analyses. Rayleigh-wave velocities indicate extremely strong azimuthal anisotropy developed during formation of the lithosphere, but notably weaker azimuthal anisotropy is indicated in these data for the underlying asthenosphere. Determining the corresponding depth distribution of radial anisotropy requires detailed analysis of Love waves. Using a novel analysis of the wavefield, Love wave fundamental- and higher-mode phase velocities will be measured across the OBS array. Combined with the existing azimuthal anisotropy constraints, the resulting estimates of anisotropy will allow us to explicitly test whether flow-induced olivine fabric is consistent with the observations, or whether oriented melt is required to explain the observations.