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

在地球表面形成板块状行为的机制和控制板块运动的过程还没有完全被理解。这项研究使用部署在中太平洋多年的海底地震仪记录的地震产生的地震波来探测该构造，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特征的影响之间的已知关系，将评估板块和下伏地幔之间的耦合程度。坚硬的构造板块与下伏的地幔有何不同，以及这些物质在板块底部是否一致运动，这一问题长期以来一直吸引着地球科学家的兴趣。它是理解板块构造的核心。该奖项资助的研究生将接受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向地震各向异性与在蛇绿岩中发现的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，在太平洋软流圈中观察到的高幅度径向各向异性提供了板块下方高度变形和/或部分熔融的层的证据，该层可能使板块与下伏地幔分离。位于约70 Ma岩石圈上的600×400千米海底地震仪(OBS)阵列提供了高质量的宽带地震数据，足以描述分辨率(深度和横向)的各向异性，这是全球分析无法达到的。瑞雷波速度表明在岩石圈形成过程中形成了极强的方位各向异性，但在这些数据中，软流圈的方位各向异性明显较弱。要确定相应的径向各向异性深度分布，需要对Love波进行详细的分析。使用一种新的波场分析，将在整个OBS阵列上测量Love波的基模和高模相速度。结合现有的方位各向异性约束，由此得到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组构是否与观测一致，或者是否需要定向熔融来解释观测。

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.