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Mid-mantle anisotropy in subduction zones and deep water transport

机译：俯冲带中幔幔各向异性与深水输送

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The Earth's transition zone has until recently been assumed to be seismically isotropic. Increasingly, however, evidence suggests that ordering of material over seismic wavelengths occurs there, but it is unclear what causes this. We use the method of source-side shear wave splitting to examine the anisotropy surrounding earthquakes deeper than 200 km in slabs around the globe. We find significant amounts of splitting (2.4 s), confirming that the transition zone is anisotropic here. However, there is no decrease in the amount of splitting with depth, as would be the case for a metastable tongue of olivine which thins with depth, suggesting this is not the cause. The amount of splitting does not appear to be consistent with processes in the ambient mantle, such as lattice-preferred orientation development in wadsleyite, ringwoodite, or MgSiO3-perovskite. We invert for the orientation of several mechanismssubject to uncertainties in mineralogy and deformationand the best fit is given by updip flattening in a style of anisotropy common to hydrous phases and layered inclusions. We suggest that highly anisotropic hydrous phases or hydrated layering is a likely cause of anisotropy within the slab, implying significant water transport from the surface down to at least 660 km depth.
机译：直到最近，地球的过渡带仍被认为是地震各向同性的。但是，越来越多的证据表明，在地震波长范围内发生了材料排序，但目前尚不清楚是什么原因造成的。我们使用震源侧剪切波分裂的方法，研究了全球平板中深于200 km的地震周围的各向异性。我们发现大量的分裂（2.4 s），证实这里的过渡带是各向异性的。但是，深度分裂的数量并没有减少，就像橄榄石的亚稳定舌头随着深度变薄的情况一样，表明这不是原因。分裂的数量似乎与周围地幔的过程不一致，例如在沃兹利特石，林木或MgSiO3-钙钛矿中晶格优先的取向发展。我们对几种机制的方向进行了反转，这些机制受矿物学和变形的不确定性影响，并且最佳的拟合是通过以水相和层状夹杂物常见的各向异性方式进行上倾展平来实现的。我们认为，高度各向异性的水相或水合分层是板内各向异性的可能原因，这意味着从表面到至少660 km深度的大量水输送。

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