Vertical-incidence seismic reflection profiles generated from global phases for 16 earthquakes recorded by stations of the Transportable Array (TA) show distinctive patterns of P-wave reflectivity in the uppermost mantle beneath the central and eastern United States. The overall distribution of reflections identified objectively using the sign test statistic applied to bootstrapped stacks is consistent with a westward increase in depth of the lithosphere-asthenosphere boundary (LAB) from roughly 110 to 250 km that is marked within the lower lithosphere by piecewise continuous segments of elevated horizontal P-wave reflectivity. For some profiles, the onsets of zones of increased reflectivity closely match depths corresponding to the maximum negative S-wave velocity gradients found by surface-wave tomography, suggesting that P-wave reflectivity can be used to help characterize properties of the lower lithosphere. We suggest that the vertical change in horizontal reflectivity straddles the lithosphere-asthenosphere transition, encompassing a broad zone of layering caused by increased strain in the lower lithosphere as well as drag-induced flow in the asthenosphere. Some of the lines also show waveforms that fall within the depth range of arrivals identified as midlithospheric discontinuities (MLDs) in overlapping Sp receiver-function profiles. The reflection waveforms observed in the TA lines are mostly multicyclic with a mix of polarities indicating a layered transition, consistent with previous observations and model studies that show the breakup of single Sp waveforms into a series of less prominent, shorter-period P-wave reflections as the dominant frequency of incident energy is increased.

中文翻译：

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使用水平 P 波反射率的对比度绘制大陆岩石圈的底部：美国中部和东部的结果


可移动阵列 （TA） 站点记录的 16 次地震的全球相位生成的垂直入射地震反射剖面显示了美国中部和东部下方最上层地幔中独特的 P 波反射率模式。使用应用于自举堆栈的符号检验统计量客观识别的反射的总体分布与岩石圈-软流圈边界 （LAB） 深度从大约 110 公里向西增加一致，这在下岩石圈内由升高的水平 P 波反射率的分段连续段标记。对于某些剖面，反射率增加的区域的开始与表面波断层扫描发现的最大负 S 波速度梯度相对应的深度非常匹配，这表明 P 波反射率可用于帮助表征下岩石圈的特性。我们认为水平反射率的垂直变化跨越了岩石圈-软流圈过渡，包括由下岩石圈应变增加以及软流圈阻力诱导流引起的广泛分层区。一些线路还显示了落在重叠的 Sp 接收器功能剖面中被确定为中岩石圈不连续性 （MLD） 的到达深度范围内的波形。在 TA 线中观察到的反射波形大多是多循环的，极性混合表明分层过渡，这与之前的观察和模型研究一致，这些研究显示，随着入射能量的主频率增加，单个 Sp 波形会分解成一系列不太突出、周期较短的 P 波反射。