The top of the subducting plate is a thick, complex zone where the heterogeneous structure likely controls earthquake rupture processes. Imaging this heterogeneous channel typically involves active-source methods with limited depth penetration, or low-resolution teleseismic methods. To access short wavelengths at greater depth, we use high-frequency P-to-S (PS, 1–15 Hz) mode-converted arrivals from nearby earthquakes >50 km deep to image the plate interface at vertical scales <1 km. We use 37 broadband stations in southcentral Alaska between 2007 and 2008 at 10–15 km spacing, spanning the great 1964 earthquake rupture zone and adjacent deeper slow slip and tremor regions. The central 21 stations record high-amplitude PS arrivals converting from the megathrust region, at depths corresponding to the top of a prominent low-velocity zone (LVZ) in receiver function images. The PS/P amplitude ratio (A_{PS_P}) varies along strike and with depth of the conversion point but is independent of earthquake location and varies slowly between adjacent stations. A_{PS_P} changes with slab depth, indicating changes in lithology or fluid content of the plate interface, consistent with transitions in slip behavior from locked to slow slip. High A_{PS_P} cannot be explained by a velocity step or a single low-velocity, high Vp/Vs layer, but requires several alternating high and low-velocity layers. These observations indicate that the LVZ is a highly heterogeneous channel at multiple scales, resembling a subduction channel or sheared zone of metasediment and altered crust as observed in many exhumed subduction zones.

中文翻译：

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使用局部板内地震的 P 模式转换到 S 模式对阿拉斯加-阿留申巨型逆冲断层进行高分辨率成像

俯冲板块的顶部是一个厚而复杂的区域，其中的异质结构可能控制着地震破裂过程。对这种异质通道进行成像通常涉及深度有限的主动源方法或低分辨率远震方法。为了获取更大深度的短波长，我们使用来自附近>50公里深度的地震的高频P-to-S（PS，1-15 Hz）模式转换波来对垂直尺度<1公里的板块界面进行成像。2007 年至 2008 年间，我们在阿拉斯加中南部使用了 37 个宽带站，间隔为 10-15 公里，跨越 1964 年大地震破裂带和邻近的更深的慢滑移和震动区域。中央 21 个台站记录了从巨型逆冲区域转换而来的高振幅 PS 到达，其深度对应于接收器功能图像中突出的低速区 (LVZ) 的顶部。PS/P 振幅比 (A _{PS_P} ) 沿走向和转换点深度变化，但与地震位置无关，并且在相邻台之间变化缓慢。PS_P随_板块深度变化，表明板块界面岩性或流体含量的变化，与滑移行为从锁定滑移到慢速滑移的转变一致。高 A _{PS_P}不能用速度阶跃或单个低速、高 Vp/Vs 层来解释，而是需要多个交替的高速层和低速层。这些观测表明，LVZ 是一个在多个尺度上高度异质的通道，类似于在许多挖出的俯冲带中观察到的变质沉积物和蚀变地壳的俯冲通道或剪切带。