Abstract Seismic recordings from the USArray of the October 1, 2013 Sea of Okhotsk earthquake and the May 24, 2010 Western Brazil earthquake include high-amplitude SxP signals due to S-wave to P-wave scattering about 150 to 200 km below the 660-km discontinuity below these events. The scattering structures have been mapped previously and interpreted as fragments of folded crust that subducted into the uppermost lower mantle. The USArray recordings are unique because SxP is visible at all stations of the USArray over a distance of 2000 km. The arrival times of SxP with respect to the P arrival times vary by 2–3 s across the USArray. The 2–3 s arrival time variations of SxP suggest the finite dimensions of the scattering structures. We estimate the dimensions by beamforming waveforms for subsets of USArray stations and the uncertainty ranges by bootstrapping. In our analysis we assume that the high-frequency wave traveltimes can be modeled using ray theory and that wave-speed heterogeneity between the scattering point and the array does not affect SxP-P differential traveltimes. The results indicate that the scattering structures beneath the Sea of Okhotsk and western Brazil have dimensions of 20–160 km but the uncertainties are large. The variation of the SxP amplitude by a factor of two to three is likely due to the complex geometry of the folded crust. We attribute the down-and-up SxP waveform to the interference of S-wave to P-wave conversions at the top and bottom of a 10–20 km thick portion of the folded crust. Our results suggest that the subducted crust can retain thicknesses of 10–20 km even after being folded by subduction into the viscous lower mantle.

中文翻译：

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使用 USArray 记录的 S 波到 P 波转换的下地幔中散射体的尺寸

摘要 USArray 对 2013 年 10 月 1 日鄂霍次克海地震和 2010 年 5 月 24 日巴西西部地震的地震记录包括高振幅 SxP 信号，这是由于 S 波到 P 波散射在 660 至 200 公里以下约 150 至 200 公里处。 km 不连续性低于这些事件。散射结构以前已经被绘制出来，并被解释为俯冲到最上层下地幔中的折叠地壳碎片。USArray 记录是独一无二的，因为 SxP 在 2000 公里距离内的 USArray 的所有站点上都是可见的。SxP 的到达时间相对于 P 的到达时间在整个 USArray 中相差 2-3 秒。SxP 的 2-3 秒到达时间变化表明散射结构的有限维度。我们通过 USArray 站子集的波束成形波形估计维度，并通过自举法估计不确定性范围。在我们的分析中，我们假设可以使用射线理论对高频波走时进行建模，并且散射点和阵列之间的波速异质性不会影响 SxP-P 差分走时。结果表明鄂霍次克海和巴西西部海底的散射结构尺寸为20~160 km，但不确定性较大。由于折叠地壳的复杂几何形状，SxP 振幅的变化可能是 2 到 3 倍。我们将上下 SxP 波形归因于折叠地壳 10-20 公里厚部分顶部和底部的 S 波到 P 波转换的干扰。