Cascadia low frequency earthquakes at the base of an overpressured subduction shear zone
Cascadia low frequency earthquakes at the base of an overpressured subduction shear zone
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超压俯冲剪切带底部的卡斯卡迪亚低频地震
DOI：
10.1038/s41467-020-17609-3
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发表时间：
2020
影响因子：
16.6
通讯作者：
M. Unsworth
中科院分区：
文献类型：
--
作者：
A. Calvert;M. Bostock;G. Savard;M. Unsworth
在俯冲带中，低横波速度和泊松比升高的向陆倾斜区域可延伸至至少120 km深度，被解释为全部或部分的俯冲火成岩洋壳。这个地壳被认为是超压的，因为它内部的流体被困在沿上覆的板块间边界的不透水封条下面。研究表明，在南温哥华岛板块边界慢滑期间，低频地震发生在高泊松比向陆倾斜区和地震反射显示的6-10 km厚剪切带的正下方。这里的板块边界要么对应于低频地震，要么对应于紧接在低频地震上方的剪切带下3-5公里处的异常弹性特性。这个高泊松比区与导电层近似重合，可以用在接近静岩孔隙压力下被捕获的板状流体来解释。由于流体被困在沿上覆板块间边界的不透水封条下，俯冲的海洋地壳区域通常被认为是超压的。在这里，作者表明，卡斯卡迪亚俯冲带的慢滑地震发生在6-10公里厚剪切带的正下方，在这个剪切带中，板块衍生的流体可能被困在接近静岩石的孔隙压力下。
In subduction zones, landward dipping regions of low shear wave velocity and elevated Poisson’s ratio, which can extend to at least 120 km depth, are interpreted to be all or part of the subducting igneous oceanic crust. This crust is considered to be overpressured, because fluids within it are trapped beneath an impermeable seal along the overlying inter-plate boundary. Here we show that during slow slip on the plate boundary beneath southern Vancouver Island, low frequency earthquakes occur immediately below both the landward dipping region of high Poisson’s ratio and a 6–10 km thick shear zone revealed by seismic reflections. The plate boundary here either corresponds to the low frequency earthquakes or to the anomalous elastic properties in the lower 3–5 km of the shear zone immediately above them. This zone of high Poisson’s ratio, which approximately coincides with an electrically conductive layer, can be explained by slab-derived fluids trapped at near-lithostatic pore pressures. Regions of the subducting oceanic crust are often considered to be overpressured, owing to fluid trapped beneath an impermeable seal along the overlying inter-plate boundary. Here, the authors show that slow slip earthquakes at the Cascadia subduction zone occur immediately below a 6-10 km-thick shear zone, in which slab-derived fluids are likely trapped at near-lithostatic pore pressures.