Influence of shear wave velocity heterogeneity on SH-wave reverberation imaging of the mantle transition zone
Influence of shear wave velocity heterogeneity on SH-wave reverberation imaging of the mantle transition zone
复制标题
剪切波速度不均匀性对地幔过渡带SH波混响成像的影响
DOI：
10.1093/gji/ggac321
复制
发表时间：
2022
影响因子：
2.8
通讯作者：
Chaves, Carlos A. M.
中科院分区：
文献类型：
--
作者：
Liu, Meichen;Ritsema, Jeroen;Chaves, Carlos A. M.
长周期(T&gT；10，S)地震台下地表和反射边界之间的横波反射对于研究地幔过渡带的相变是有用的，但剪切速度不均匀和有限频率效应使波形叠加的解释变得复杂。我们跟进了Sheeller&Buehler(以下简称SSD)最近对顶部横波反射SSD的研究，作为绘制美国阵列下410公里和660公里不连续面深度的探针。和SB19一样，我们观察到记录的Ss410s-S和Ss660s-S在美国西部的车站比美国中东部的车站走时差更长。如果使用基于一维地震模型(在我们的例子中为PREM)的共反射点映射方法将Ss410-S和Ss660-S旅行时差转换为深度，则410公里和660公里的不连续面在美国西部下面比美国中东部深约40-50公里。然而，如果根据全球层析成像考虑地幔三维横波速度变化，则在CRP图像中MTZ由东向西加深的现象消失了。此外，从谱元方法的合成中，我们发现射线理论高估了混响的旅行时间延迟。当410公里和660公里的不连续面的波长小于MTZ中波混响的菲涅尔区时，它们的起伏被低估了。因此，上地幔分层的模拟必须基于3-D参考结构和混响旅行时的准确计算。
Long-period (T> 10 s) shear wave reflections between the surface and reflecting boundaries below seismic stations are useful for studying phase transitions in the mantle transition zone (MTZ) but shear-velocity heterogeneity and finite-frequency effects complicate the interpretation of waveform stacks. We follow up on a recent study by Shearer & Buehler (hereafter ) of the top-side shear wave reflection Ssds as a probe for mapping the depths of the 410-km and 660-km discontinuities beneath the USArray. Like SB19, we observe that the recorded Ss410s-S and Ss660s-S traveltime differences are longer at stations in the western United States than in the central-eastern United States. The 410-km and 660-km discontinuities are about 40–50 km deeper beneath the western United States than the central-eastern United States if Ss410s-S and Ss660s-S traveltime differences are transformed to depth using a common-reflection point (CRP) mapping approach based on a 1-D seismic model (PREM in our case). However, the east-to-west deepening of the MTZ disappears in the CRP image if we account for 3-D shear wave velocity variations in the mantle according to global tomography. In addition, from spectral-element method synthetics, we find that ray theory overpredicts the traveltime delays of the reverberations. Undulations of the 410-km and 660-km discontinuities are underestimated when their wavelengths are smaller than the Fresnel zones of the wave reverberations in the MTZ. Therefore, modelling of layering in the upper mantle must be based on 3-D reference structures and accurate calculations of reverberation traveltimes.