Rayleigh Wave Attenuation and Amplification Measured at Ocean‐Bottom Seismometer Arrays Using Helmholtz Tomography
Rayleigh Wave Attenuation and Amplification Measured at Ocean‐Bottom Seismometer Arrays Using Helmholtz Tomography
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DOI：
10.1029/2022jb025174
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发表时间：
2022-10
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影响因子：
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通讯作者：
J. Russell;C. Dalton
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作者：
J. Russell;C. Dalton
剪切衰减提供了对上地幔物理和化学状态的深入了解。然而，尽管最近海底地震仪（OBS）阵列不断增加，但在海洋中观测到衰减的情况并不常见。海洋环境衰减研究必须克服与海底强海洋噪声和 OBS 恢复期间数据丢失相关的独特挑战，此外还必须解决弹性聚焦、局部站点放大和表面波振幅的滞弹性衰减的竞争影响。我们将亥姆霍兹断层扫描应用于 OBS 数据，以同时解析阵列平均瑞利波衰减和 20-150 秒周期的站点放大图。该方法使用波场曲率明确地解释了由于横向速度异质性而导致的弹性聚焦和散焦。我们使用 NoMelt 实验和胡安德富卡 (JdF) 板块的真实波场模拟来验证该方法，它们分别代表端元公海和海岸线邻近环境。两个 OBS 阵列均成功恢复了聚焦校正，包括覆盖高度衰减软流圈层的周期 1,500（Qμ ${Q}_{\mu }$ ∼ 50 至 70）。在 JdF，我们发现衰减的宽峰 (Qμ ${Q}_{\mu }$ ∼ 50 到 60) 集中在 100–130 km 的深度。我们还报告了 JdF 海脊的强烈局部站点放大（31 秒周期>10%），这可用于完善地壳和浅地幔结构的模型。
Shear attenuation provides insights into the physical and chemical state of the upper mantle. Yet, observations of attenuation are infrequent in the oceans, despite recent proliferation of arrays of ocean‐bottom seismometers (OBSs). Studies of attenuation in marine environments must overcome unique challenges associated with strong oceanographic noise at the seafloor and data loss during OBS recovery in addition to untangling the competing influences of elastic focusing, local site amplification, and anelastic attenuation on surface‐wave amplitudes. We apply Helmholtz tomography to OBS data to simultaneously resolve array‐averaged Rayleigh wave attenuation and maps of site amplification at periods of 20–150 s. The approach explicitly accounts for elastic focusing and defocusing due to lateral velocity heterogeneity using wavefield curvature. We validate the approach using realistic wavefield simulations at the NoMelt Experiment and Juan de Fuca (JdF) plate, which represent endmember open‐ocean and coastline‐adjacent environments, respectively. Focusing corrections are successfully recovered at both OBS arrays, including at periods 1,500) overlying a highly attenuating asthenospheric layer ( Qμ ${Q}_{\mu }$ ∼ 50 to 70). At JdF, we find a broad peak in attenuation ( Qμ ${Q}_{\mu }$ ∼ 50 to 60) centered at a depth of 100–130 km. We also report strong local site amplification at the JdF Ridge (>10% at 31 s period), which can be used to refine models of crust and shallow mantle structure.