The largest slip in great megathrust earthquakes often occurs in the 10–30 km depth range, yet seismic imaging of the material properties in this region has proven difficult. We utilize a dense onshore‐offshore passive seismic dataset from the southernmost Cascadia subduction zone where seismicity in the mantle of the subducted Gorda Plate produces S‐to‐P and P‐to‐S conversions generated within a few km of the plate interface. These conversions typically occur in the 10–20 km depth range at either the top or bottom of a ∼5 km thick layer with a high Vp/Vs that we infer to be primarily the subducted crust. We use their arrival times and amplitudes to infer the location of the top and bottom of the subducted crust as well as the velocity contrasts across these discontinuities. Comparing with both the Slab1.0 and the updated Slab2 interface models, the Slab2 model is generally consistent with the converted phases, while the Slab1.0 model is 1–2 km deeper in the 2–20 km depth range and ∼6–8 km too deep in the 10–20 km depth range between 40.25°N and 40.4°N. Comparing the amplitudes of the converted phases to synthetics for simplified velocity structures, the amplitude of the converted phases requires models containing a ∼5 km thick zone with at least a ∼10%–20% reduction in S wave velocity. Thus, the plate boundary is likely contained within or at the top of this low velocity zone, which potentially indicates a significant porosity and fluid content within the seismogenic zone.

中文翻译：

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局部地震产生的转换相对俯冲戈尔达板块几何的约束

巨大的特大推力地震中最大的滑移通常发生在10–30 km的深度范围内，但是事实证明，对该区域的材料特性进行地震成像很难。我们利用密集的陆上，海上从那里地震在俯冲戈尔达板块的地幔产生最南端的卡斯卡迪亚俯冲带的地震被动集小号至- P和P至-小号在平板界面几公里内产生的转化。这些转换通常发生在10至20 km深度范围内的〜5 km厚层的顶部或底部，具有较高的Vp / Vs，我们认为这主要是俯冲的地壳。我们使用它们的到达时间和振幅来推断俯冲的地壳顶部和底部的位置，以及这些不连续处的速度对比。与Slab1.0和更新后的Slab2接口模型相比，Slab2模型通常与转换后的相位一致，而Slab1.0模型在2-20 km深度范围内约1-2 km，深度约为6-8在40.25°N至40.4°N之间的10–20 km深度范围内，深度过深。为了简化速度结构，将转换后的相位的振幅与合成的振幅进行比较，S波速度。因此，板块边界很可能包含在该低速区域内或顶部，这可能表明在地震发生带内有明显的孔隙度和流体含量。