Calvert Andrew J, Bostock Michael G, Savard Geneviève, Unsworth Martyn J
Department of Earth Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC, V5A 1S6, Canada.
Department of Earth, Ocean and Atmospheric Sciences, 2207 Main Mall, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada.
Nat Commun. 2020 Aug 3;11(1):3874. doi: 10.1038/s41467-020-17609-3.
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.
在俯冲带中，低剪切波速度和泊松比升高的向陆倾斜区域可延伸至至少120公里深度，被解释为俯冲火成洋壳的全部或部分。这种地壳被认为是超压的，因为其中的流体被困在沿上覆板块间边界的不透水密封层之下。我们在此表明，在温哥华岛南部下方的板块边界发生缓慢滑动期间，低频地震立即发生在高泊松比的向陆倾斜区域以及地震反射揭示的6 - 10公里厚剪切带下方。这里的板块边界要么对应于低频地震，要么对应于其上方剪切带下部3 - 5公里处的异常弹性性质。这个高泊松比区域大致与一个导电层重合，可由以近静水孔隙压力被困的板片衍生流体来解释。
