Sensitivities of diffracted PKKPab waves to the velocity structures in the lowermost mantle
Sensitivities of diffracted PKKPab waves to the velocity structures in the lowermost mantle
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衍射 PKKPab 波对最下地幔速度结构的敏感性
DOI：
10.1093/gji/ggad033
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发表时间：
2023-01
影响因子：
2.8
通讯作者：
Yi Wang
中科院分区：
文献类型：
--
作者：
Yulin Chen;Sidao Ni;Baolong Zhang;Li Zhao;Xiaodong Song;Yi Wang
在最近的研究中，PKKPab （${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$）沿核幔边界（CMB）的衍射已经远远超出了它的截止距离，这有助于改进空间采样以约束最下层的地幔速度结构。然而，PKKPab波的衍射可能发生在CMB的三个相互作用点之一（岩心入口、底面反射和出口），从而导致采样区域的不确定性。此外，最底地幔非几何${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$的灵敏度核很难用经典射线理论获得，并且可以预期比直接到达的典型香蕉甜甜圈形状更复杂。在本研究中，我们通过分析基于全波理论的数值算法计算的${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$波的有限频率延迟-时间灵敏度核来解决这两个问题。我们发现${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$波的衍射效应在其核心入口和出口区域附近最为显著。在1 s的优势期，这两个地区地幔侧第一菲涅耳带的宽度估计约为60 km。为了进一步研究${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$对最下层地幔不同结构的敏感性，我们进行了一系列1D和2D高频（~ 1 Hz）模拟实验。我们的研究结果表明，${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$波的传播时间和振幅对大尺度p波异常（Vp扰动为±2%，厚度大于100 km）和小尺度超低速度带（Vp减小5%或10%，厚度为数十km）很敏感。然而，${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}}\ $波的慢度在扰动模型中几乎保持不变。我们用相似震中距离的站点的差分延迟-时间灵敏度核来解释这一意想不到的结果。我们的结果显示了${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$波在研究地幔基底结构方面的优点和局限性。
Diffractions of PKKPab (${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$) along the core mantle boundary (CMB) have been observed well beyond its cutoff distance in recent studies, making it useful for improving the spatial sampling to constrain the lowermost mantle velocity structures. However, the diffractions of PKKPab waves may occur at one of the three CMB interaction points (core entry, underside reflection and exit), giving rise to uncertainties of the sampling region. Moreover, the sensitivity kernels of the non-geometrical ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$ in the lowermost mantle are difficult to obtain with classical ray theory, and can be expected to be more complicated than the typical banana-doughnut shape for direct arrivals. In this study, we address these two issues by analyzing the finite-frequency delay-time sensitivity kernels of the ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$ waves computed by numerical algorithms based on full-wave theory. We find that the diffraction effects for the ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$ waves are most significant near their core entry and exit regions. For a dominant period of 1 s, the estimated widths of the first Fresnel zones on the mantle side of these two areas are about 60 km. To further investigate the sensitivities of ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$ to different structures in the lowermost mantle, we conduct a series of 1D and 2D high-frequency (∼1 Hz) modelling experiments. Our results show that the travel times and amplitudes of the ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$ waves are sensitive to large-scale P-wave anomalies (with Vp perturbations of ± 2 per cent and thicknesses of more than 100 km) and small-scale ultra-low velocity zones (ULVZs) (with Vp reduction of 5 per cent or 10 per cent and thickness of tens of kilometers). However, the slownesses of the ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}\ $waves remain nearly unchanged in the perturbed models. We explain this unexpected result by the differential delay-time sensitivity kernels for stations at similar epicentral distances. Our results demonstrate both the advantages and limitations of the ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$ waves in studying the structures at the base of the mantle.