Scattering beneath Western Pacific subduction zones: evidence for oceanic crust in the mid-mantle
Scattering beneath Western Pacific subduction zones: evidence for oceanic crust in the mid-mantle
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DOI：
10.1093/gji/ggu043
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发表时间：
2014-06-01
影响因子：
2.8
通讯作者：
Rost, S.
中科院分区：
文献类型：
--
作者：
Bentham, H. L. M.;Rost, S.
地幔中的小尺度异质性可以为了解地球内部的动力学和组成提供重要的见解。在这里，我们分析了作为 PP 前兆发现的地震能量，这些能量分散在与上地幔和中地幔俯冲带相关的小尺度异质性中。我们使用震中距离90A度-110A度范围内的浅源地震（深度小于100公里）的数据，并使用阵列方法来研究PP到达之前的100 s窗口。我们的分析重点是从源和接收器之间的大圆路径到达的能量。我们根据相似加权波束功率谱自动选择相干到达，从而最大化弱幅度到达的选择。假设单个 P 到 P 散射并使用阵列处理中的方向性信息，我们通过一维速度模型的射线追踪来定位散射原点。利用阿拉斯加小孔径艾尔森阵列 (ILAR) 的数据，我们能够对与西太平洋俯冲带不均匀性相关的结构进行成像。我们在当今日本、伊豆-小笠原、马里亚纳和西菲律宾俯冲带周围地区发现了类似 300 个小规模异质性的证据。大多数检测到的异质性位于地壳和上地幔中，但 6% 的散射体位于 600 公里以上的深处。过渡带中的散射体与层析图像中快速特征的边缘以及从板片地震活动得出的俯冲板片轮廓有很好的相关性。我们在伊豆-小笠宁/马里亚纳俯冲带下方找到了更深的散射体，该俯冲带勾勒出深度为 1480 公里的陡峭倾斜伪平面特征，以及在古代（84-144 Ma）印度尼西亚俯冲海沟下方，深度为 1880 公里。我们对俯冲地壳物质的残余物进行成像，很可能是俯冲莫霍面的底面反射。与过去和现在的俯冲有关的深部散射体的存在提供了证据，表明至少对于这些陡峭的俯冲带来说，俯冲地壳确实下降到下地幔中。将相同的技术应用于其他源-接收器路径将增加我们对地幔小尺度结构的了解，并将为地球动力学模型提供进一步的约束。
Small-scale heterogeneities in the mantle can give important insight into the dynamics and composition of the Earth's interior. Here, we analyse seismic energy found as precursors to PP, which is scattered off small-scale heterogeneities related to subduction zones in the upper and mid-mantle. We use data from shallow earthquakes (less than 100 km depth) in the epicentral distance range of 90A degrees-110A degrees and use array methods to study a 100 s window prior to the PP arrival. Our analysis focuses on energy arriving off the great circle path between source and receiver. We select coherent arrivals automatically, based on a semblance weighted beampower spectrum, maximizing the selection of weak amplitude arrivals. Assuming single P-to-P scattering and using the directivity information from array processing, we locate the scattering origin by ray tracing through a 1-D velocity model. Using data from the small-aperture Eielson Array (ILAR) in Alaska, we are able to image structure related to heterogeneities in western Pacific subduction zones. We find evidence for similar to 300 small-scale heterogeneities in the region around the present-day Japan, Izu-Bonin, Mariana and West Philippine subduction zones. Most of the detected heterogeneities are located in the crust and upper mantle, but 6 per cent of scatterers are located deeper than 600 km. Scatterers in the transition zone correlate well with edges of fast features in tomographic images and subducted slab contours derived from slab seismicity. We locate deeper scatterers beneath the Izu-Bonin/Mariana subduction zones, which outline a steeply dipping pseudo-planar feature to 1480 km depth, and beneath the ancient (84-144 Ma) Indonesian subduction trench down to 1880 km depth. We image the remnants of subducted crustal material, likely the underside reflection of the subducted Moho. The presence of deep scatterers related to past and present subduction provides evidence that the subducted crust does descend into the lower mantle at least for these steeply dipping subduction zones. Applying the same technique to other source-receiver paths will increase our knowledge of the small-scale structure of the mantle and will provide further constraints on geodynamic models.