Recent seismic results from the Cascadia Initiative indicate that heterogeneity in the oceanic asthenosphere affects subduction dynamics. Accurate characterization of the oceanic upper mantle is thus necessary to fully understand subduction processes, including the behavioral segmentation of the megathrust. A key challenge is integrating onshore and offshore datasets, which span large variations in near‐surface features that teleseismic body wave tomography is ill‐posed to resolve. Here, we perform a series of P and S forward modeling predictions to better understand the relative contribution of elevation, crustal thickness, offshore sedimentation, and near‐surface velocity structure to teleseismic delay times. Crustal thickness and elevation variations dominate the signal, contributing ∼1 s of delay time difference for P‐waves (roughly double for S). We test several inversion strategies to account for near‐surface features, identifying potential artifacts and causes of imaging errors. Undamped station statics are found to absorb mantle structures and introduce low‐velocity artifacts beneath the forearc. Our preferred inversion strategy utilizes a three‐dimensional starting model (including elevation) of the upper 50 km and heavily damped station statics, which we find leads to better resolution of mantle structure, particularly at asthenospheric depths. These insights guide inversions of observed delay times from the Cascadia subduction zone and Juan de Fuca plate system. We present a new onshore‐offshore S model and an updated P model. Major features are common to both models, including localized subslab low‐velocity anomalies, along‐strike variations in slab structure, and offshore heterogeneity, while regional differences may reflect changes in Vp/Vs.

中文翻译：

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卡斯卡迪亚俯冲带和胡安·德富卡板块系统的体波层析成像：识别跨岸数据的挑战和解决方案

卡斯卡迪亚倡议的最新地震结果表明，海洋软流圈的非均质性影响俯冲动力。因此，要充分了解俯冲过程，包括大推力的行为分段，就必须对海洋上地幔进行准确描述。一个关键的挑战是整合陆上和海上数据集，这些数据集跨越近地表特征的巨大变化，而远震体波层析成像难以解决。在这里，我们进行了一系列的P和S正演模拟预测，以更好地了解海拔，地壳厚度，近海沉积和近地表速度结构对远震延迟时间的相对贡献。地壳厚度和高度变化主导信号，对P波贡献约1 s的延迟时间差（对于S波，约为两倍）。我们测试了几种反演策略来说明近地表特征，确定潜在的伪影和成像误差的原因。发现无阻尼的站静点会吸收地幔结构，并在前臂下方引入低速伪影。我们首选的反演策略是利用高空50 km的三维起始模型（包括高程）和高度阻尼的站点静力学，我们发现这会导致更好的地幔结构分辨率，尤其是在软流圈深度。这些见解指导了从卡斯卡迪亚俯冲带和胡安德富卡板块系统观测到的延迟时间的反演。我们提出了一个新的陆上-海上S模型和一个更新的P模型。两种型号的主要特点是共通的，