Validity of Resolving the 785 km Discontinuity in the Lower Mantle with P′P′ Precursors?
Validity of Resolving the 785 km Discontinuity in the Lower Mantle with P′P′ Precursors?
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DOI：
10.1785/0220200210
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发表时间：
2020-11
影响因子：
3.3
通讯作者：
Baolong Zhang;Xiangfang Zeng;Jun Xie;V. Cormier
中科院分区：
文献类型：
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作者：
Baolong Zhang;Xiangfang Zeng;Jun Xie;V. Cormier
P ‘ P ’前兆已被用于探测地球下地幔的不连续结构，但一些沿不对称射线路径或散射波传播的地震相位可能被误解为地幔不连续结构的反射。通过标准一维地球模型的正演模拟，我们证明了在P ‘ P ’到达前180 s左右的前体的频率含量、慢度和衰减与震中距离约78°的PKPPdiff相（或PdiffPKP）相一致，而不是来自下地幔界面的反射。此外，一种波束形成技术应用于USArray记录的波形数据表明，PKPPdiff可以从许多地震中普遍观察到。因此，一个没有下地幔不连续的参考一维地球模型可以解释许多观测到的P ‘ P ’前兆信号，如果它们被解释为PKPPdiff，而不是P ‘ 785P ’。然而，这项研究并不排除非洲地下785公里界面的可能性。如果该界面确实存在，为避免PKPPdiff的干扰，最好不要使用距离在78°附近的P ‘ P ’前体进行检测。事实上，它可以用震中距离小于76°的P ‘ P ’前体或其他地震相（如背向散射PKP·PKP波）来探测。
P ′ P ′ precursors have been used to detect discontinuities in the lower mantle of the Earth, but some seismic phases propagating along asymmetric ray paths or scattered waves could be misinterpreted as reflections from mantle discontinuities. By forward modeling in standard 1D Earth models, we demonstrate that the frequency content, slowness, and decay with distance of precursors about 180 s before P′P′ arrival are consistent with those of the PKPPdiff phase (or PdiffPKP) at epicentral distances around 78° rather than a reflection from a lower mantle interface. Furthermore, a beamforming technique applied to waveform data recorded at the USArray demonstrates that PKPPdiff can be commonly observed from numerous earthquakes. Hence, a reference 1D Earth model without lower mantle discontinuities can explain many of the observed P′P′ precursors signals if they are interpreted as PKPPdiff, instead of P′785P′. However, this study does not exclude the possibility of 785 km interface beneath the Africa. If this interface indeed exists, P′P′ precursors at distances around 78° would better not be used for its detection to avoid interference from PKPPdiff. Indeed, it could be detected with P′P′ precursors at epicentral distances less than 76° or with other seismic phases such as backscattered PKP·PKP waves.