Body Wave Tomography of the Cascadia Subduction Zone and Juan de Fuca Plate System: Identifying Challenges and Solutions for Shore‐Crossing Data
Body Wave Tomography of the Cascadia Subduction Zone and Juan de Fuca Plate System: Identifying Challenges and Solutions for Shore‐Crossing Data
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DOI：
10.1029/2020gc009316
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发表时间：
2020-11
期刊：
影响因子：
3.7
通讯作者：
M. Bodmer;D. Toomey;B. VanderBeek;E. Hooft;J. Byrnes
中科院分区：
文献类型：
--
作者：
M. Bodmer;D. Toomey;B. VanderBeek;E. Hooft;J. Byrnes
卡斯卡迪亚倡议最近的地震结果表明，海洋软流圈的异质性影响俯冲动力学。因此，必须准确描述海洋上地幔的特征，才能充分了解俯冲过程，包括巨型逆冲断层的行为分割。一个关键的挑战是整合陆上和海上数据集，这些数据集涵盖近地表特征的巨大变化，而远震体波层析成像无法解决这些问题。在这里，我们进行了一系列 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。地壳厚度和高程变化在信号中占主导地位，为 P 波贡献约 1 秒的延迟时间差（对于 S 波，延迟时间差大约为两倍）。我们测试了几种反演策略来解释近地表特征，识别潜在的伪影和成像误差的原因。研究发现，无阻尼的空间站静力学会吸收地幔结构并在弧前引入低速伪影。我们首选的反演策略利用上部 50 公里的三维起始模型（包括高程）和强阻尼站静力学，我们发现这可以更好地解析地幔结构，特别是在软流圈深度。这些见解指导对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进行反演。我们推出了新的陆上-海上 S 模型和更新的 P 模型。两个模型的主要特征是相同的，包括局部板片低速异常、板片结构的沿走向变化和近海非均质性，而区域差异可能反映了 Vp/Vs 的变化。
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.