Determining Moho Depth beneath Sedimentary Basins Using Regional Pn Multiples
Determining Moho Depth beneath Sedimentary Basins Using Regional Pn Multiples
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DOI：
10.1785/0120180325
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发表时间：
2019-06
影响因子：
3
通讯作者：
Chunquan Yu;Z. Zhan;E. Hauksson;E. Cochran;D. Helmberger
中科院分区：
文献类型：
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作者：
Chunquan Yu;Z. Zhan;E. Hauksson;E. Cochran;D. Helmberger
对涉及天然地震的厚沉积盆地下的莫霍面的研究具有挑战性，因为低速物质经常引起掩盖莫霍面信号的强烈混响。在这里，我们开发了一种方法来确定深度的莫霍面利用沉积物的存在。该方法利用了区域地震PnPn的第一Pn地壳多次波及其相对于Pn的走时差。PnPn的振幅通常较弱，因此，在没有沉积物覆盖的地区很难识别。然而，在低速沉积物的存在下，由于近地表P-到-P反射系数的增加，PnPn显着放大。PnPn的到达时间、振幅和波形，如果用参考Pn归一化，则对震源参数，如震源机制和震源深度不敏感。我们证明了这种方法的潜力，使用1D和2D波形模拟。合成波形表明PmpPn和PnPmp（一个Pn分支在震源或接收器附近合并到PmP）对PnPn振幅有很大贡献，这取决于其自由表面P-to-P反射点处的近地表结构。我们进一步验证的方法与两个领域的例子在帝国谷，一个是美国-墨西哥边境附近，另一个是在美国中部的俄克拉荷马州。这两个例子表明，该方法可以用来研究莫霍面附近的源或接收器。
The study of the Moho beneath thick sedimentary basins involving natural earthquakes is challenging, as low‐velocity materials often cause strong reverberations that mask Moho signals. Here, we develop a method to determine the depth of the Moho by taking advantage of the presence of the sediments. The method utilizes the first Pn crustal multiple from regional earthquakes PnPn and its differential travel time with respect to Pn. PnPn is usually weak in amplitude; thus, it is difficult to identify in regions without a sedimentary cover. However, PnPn is significantly amplified in the presence of low‐velocity sediments because of an increase in the near‐surface P‐to‐P reflection coefficient. The arrival time, amplitude, and wave shape of PnPn, if normalized by the reference Pn, are insensitive to earthquake source parameters, such as focal mechanism and focal depth. We demonstrate the potential of this method using both 1D and 2D waveform simulations. Synthetic waveforms suggest that PmpPn and PnPmp (one Pn leg merges to PmP near the source or the receiver) largely contribute to the PnPn amplitudes, which depend on the near‐surface structure at their free‐surface P‐to‐P reflection points. We further validate the method with two field examples in the Imperial Valley; one is near the United States–Mexico border, and the other is in Oklahoma in the central United States. Both examples suggest that the method can be used to study the Moho either near the source or the receiver.