Guided waves propagating in subducted oceanic crust
Guided waves propagating in subducted oceanic crust
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俯冲洋壳中传播的导波
DOI：
10.1029/2003jb002450
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发表时间：
2003
影响因子：
--
通讯作者：
G. Asch
中科院分区：
文献类型：
--
作者：
S. Martin;A. Rietbrock;C. Haberland;G. Asch
[1]我们使用导波上倾沿着纳斯卡板块的表面成像俯冲洋壳在智利-秘鲁俯冲带。在智利北方南纬21°附近观测到的中深度地震的P波起始揭示了波导现象：随着震源深度的增加，低频能量（<2 Hz）变得越来越占主导地位，而高频到达延迟，有时类似于两个不同的相位。为了解释观测结果，我们采用二维有限差分（FD）模拟完整的P-SV波传播沿着俯冲带的上倾剖面。FD计算揭示了一些关于地壳波导的基本问题。模拟了导波随事件焦深的发展。此外，我们发现，所观察到的导波能量必须从波导解耦近100公里的深度，以达到部署站和解耦过程是有关的俯冲角的变化。模拟还产生相对于低速结构的源位置的约束。最后，P起始点的频率含量被用来约束波导的厚度。结果表明，一个平均宽度小于4.5公里、低速率为7%的结构，与周围的地幔相比，在至少160公里的深度内仍然是地震慢的。该层被解释为俯冲洋壳的未改变的下部，这表明在智利-秘鲁俯冲带的完整榴辉岩转变是不可能发生，直到超越火山前。
[1] We use guided waves traveling updip along the surface of the Nazca slab to image subducted oceanic crust at the Chile-Peru subduction zone. Observed P onsets of intermediate depth events near 21°S in northern Chile reveal waveguide behavior: with growing focal depth, low-frequency energy (<2 Hz) becomes more and more dominant, and higher frequencies arrive delayed, sometimes resembling two distinct phases. To explain the observations, we employ two-dimensional finite difference (FD) simulations of complete P-SV wave propagation along an updip profile of the subduction zone. The FD calculations shed some light on several basic issues regarding crustal waveguides. The development of guided waves dependent on event focal depth is simulated. Further, we show that the observed guided wave energy must decouple from the waveguide near 100 km depth to reach the deployed stations and that the decoupling process is related to variations in subduction angle. Simulations also yield constraints on source locations relative to the low-velocity structure. Finally, the frequency content of P onsets is used to constrain the thickness of the waveguide. The results indicate that a structure of <4.5 km average width and 7% low-velocity remains seismically slow compared to the surrounding mantle down to a depth of at least 160 km. The layer is interpreted as the unaltered lower part of the subducted oceanic crust, suggesting that complete eclogite transformation in the Chile-Peru subduction zone is unlikely to take place until beyond the volcanic front.