SHdiff‐SVdiff splitting in an isotropic Earth
SHdiff‐SVdiff splitting in an isotropic Earth
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DOI：
10.1029/2009jb006795
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发表时间：
2010-07
影响因子：
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通讯作者：
D. Komatitsch;L. Vinnik;S. Chevrot
中科院分区：
文献类型：
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作者：
D. Komatitsch;L. Vinnik;S. Chevrot
[1]垂直极化的SVdiff震相有时比水平极化的SHdiff地震震相来得晚。这两个震相都在地幔底部的D“层中传播，它们到达时间的差异通常用D”的地震各向异性来解释。数值模拟结果表明，即使在各向同性参考地球模型中，以及在D“方向S速度相对于IASP91的速度较高的模型中，SVdiff相对于SHdiff也存在显著的(∼1 S)延迟。延迟伴随着SVdiff随距离的强烈幅度衰减。这种关系类似于滞弹性效应。在D“中低S速度的模式中，SVdiff的波形可能与SHdiff非常不同，这种差异可能被误认为SHdiff相对于SVdiff的延迟。在横向不均匀的各向同性D“中，SVdiff也可以被延迟，因为与SHdiff不同，SVdiff具有沿快速路径的强幅度衰减。因此，它的旅行时间被慢传播路径所主导，并倾向于正向偏向。对于两个半球的S速度差为3%的D“模式，SHdiff到达∼的时间比第一个清晰可见的SVdiff早到15 S。这是在一条几百公里宽的走廊上观察到的。如果这些影响在实际地震数据中被错误地解释为D“各向异性，则各向异性的估计是有偏差的，并可能导致在D的高、低S速度区地震各向异性不同的想法”。
[1] The vertically-polarized SVdiff seismic phase sometimes arrives later than the horizontally-polarized SHdiff. Both phases propagate in the D″ layer at the base of the mantle and the difference in their arrival times is usually interpreted in terms of seismic anisotropy in D″. Using numerical simulations we demonstrate that a significant (∼1 s) delay of SVdiff relative to SHdiff is present even in the IASP91 isotropic reference Earth model and in a model with a high S velocity in D″ relative to IASP91. The delay is accompanied by strong amplitude decay of SVdiff with distance. This relationship resembles the effect of anelasticity. In a model with a low S velocity in D″, the waveform of SVdiff can be very different from SHdiff, and this difference can be mistaken for a delay of SHdiff relative to SVdiff. In a laterally-heterogeneous isotropic D″ SVdiff can also be delayed because, unlike SHdiff, SVdiff has strong amplitude decay along fast paths. Its traveltime is thus dominated by slow propagation paths and tends to be positively biased. For D″ models with an S velocity difference of 3% between two hemispheres, there are regions where SHdiff arrives up to ∼15 s earlier than the first clearly visible SVdiff. This is observed in a corridor several hundred kilometers wide. If these effects are misinterpreted in terms of D″ anisotropy in actual seismic data, estimates of anisotropy are biased and may contribute to the idea of seismic anisotropy being different in high- and low-S-velocity regions of D″.