Downdip velocity changes in subducted oceanic crust beneath Northern Japan-insights from guided waves
Downdip velocity changes in subducted oceanic crust beneath Northern Japan-insights from guided waves
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日本北部俯冲洋壳的下倾速度变化——来自导波的见解
DOI：
10.1093/gji/ggu206
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发表时间：
2014
影响因子：
2.8
通讯作者：
Garth T
中科院分区：
文献类型：
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作者：
Garth T
在日本北部俯冲带前部观测到的分散纵波到达表明低速俯冲洋壳波导持续到至少 220 公里的深度。 150-220公里深度处事件的首次到达表明，波导的速度对比度随着深度的增加而减小。高频能量（>2赫兹）被低速地壳波导保留并延迟，而低频能量（<0.5赫兹）以周围地幔物质的更快速度传播。然后，由于板片的弯曲，导波能量与低速地壳波导解耦，并在低频到达后 1-2 秒在地表可见。直接将 150-220km 深度 WBZ 地震到达的色散纵波与 2-D 和 3-D 全波形有限差分模拟产生的合成波形进行比较。通过比较观测波形和合成波形的频谱图和速度谱，我们能够完全约束色散波形，从而直接比较观测波形和合成波形。使用这种完整的波形建模方法，我们能够严格约束导致观察到的导波色散的速度结构。使用二维弹性波形模拟进行的分辨率测试表明，色散可以由 6-8 公里厚的低速洋壳来解释，其速度对比随深度而变化。推断出的这种可变低速洋壳的速度可以用硬钠石轴承组合来解释，并表明低速矿物可能存在比以前认为的更深的深度。 2D 模拟以 3D 全波形模拟为基准，并表明通过 2D 近似推断的结构在 3D 中产生类似的色散。二维粘弹性模拟表明，在地幔楔中增加衰减可以改善离散波形的拟合。然而，可以排除低速层中的衰减增加。
DispersedP-wave arrivals observed in the subduction zone forearc of Northern Japan suggest that low velocity subducted oceanic crustal waveguide persists to depths of at least 220 km. First arrivals from events at 150–220 km depth show that the velocity contrast of the waveguide reduces with depth. High frequency energy (>2 Hz) is retained and delayed by the low velocity crustal waveguide while the lower frequency energy (<0.5 Hz) travels at faster velocities of the surrounding mantle material. The guided wave energy then decouples from the low velocity crustal waveguide due to the bend of the slab and is seen at the surface 1–2 s after the low frequency arrival. DispersiveP-wave arrivals from WBZ earthquakes at 150–220 km depth are directly compared to synthetic waveforms produced by 2-D and 3-D full waveform finite difference simulations. By comparing both the spectrogram and the velocity spectra of the observed and synthetic waveforms we are able to fully constrain the dispersive waveform, and so directly compare the observed and synthetic waveforms. Using this full waveform modelling approach we are able to tightly constrain the velocity structures that cause the observed guided wave dispersion. Resolution tests using 2-D elastic waveform simulations show that the dispersion can be accounted for by a 6–8 km thick low velocity oceanic crust, with a velocity contrast that varies with depth. The velocities inferred for this variable low velocity oceanic crust can be explained by lawsonite bearing assemblages, and suggest that low velocity minerals may persist to greater depth than previously thought. 2-D simulations are benchmarked to 3-D full waveform simulations and show that the structures inferred by the 2-D approximation produce similar dispersion in 3-D. 2-D viscoelastic simulations show that including elevated attenuation in the mantle wedge can improve the fit of the dispersed waveform. Elevated attenuation in the low velocity layers can however be ruled out.