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.

中文翻译：

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使用P'P'前驱体解决下地幔785公里不连续性的有效性？

P'P'前驱体已被用来探测地球下地幔中的不连续性，但沿不对称射线路径或散射波传播的某些地震相位可能被误解为地幔不连续性的反射。通过在标准1D地球模型中进行正向建模，我们证明了在P'P'到达之前约180 s内，前驱体的频率含量，慢度和衰减与前驱体距离相一致，与周围震中距离处的PKPPdiff相（或PdiffPKP⁠）相一致。 78°，而不是下地幔界面的反射。此外，应用于USArray记录的波形数据的波束成形技术表明，在许多地震中通常都能观察到PKPPdiff。因此，如果没有观测到的地幔不连续性，则参考一维地球模型可以解释许多观测到的P'P'前驱体信号，如果它们被解释为PKPPdiff⁠，而不是P'785P'⁠。但是，这项研究并未排除非洲下方785公里界面的可能性。如果确实存在该界面，则最好不要将P'P'前驱体的距离保持在78°左右，以免受到PKPPdiff⁠的干扰。确实，可以用震中距小于76°的P'P'前驱体或其他地震相（如反向散射PKP·PKP波）检测到它。最好不要将P'P'前驱体的距离设为78°，以免受到PKPPdiff？的干扰。确实，可以用震中距小于76°的P'P'前驱体或其他地震相（如反向散射PKP·PKP波）检测到它。最好不要将P'P'前驱体的距离设为78°，以免受到PKPPdiff？的干扰。确实，可以用震中距小于76°的P'P'前驱体或其他地震相（如反向散射PKP·PKP波）检测到它。