Seismic rays travelling just below the Moho provide insights into the thermal and compositional properties of the upper-mantle, and can be detected as Pn phases from regional earthquakes. Such phases are routinely identified in the continents, but in the oceans, detection of Pn phases is limited by a lack of long-term instrument deployments. We present estimates of upper-mantle velocity in the equatorial Atlantic Ocean from Pn arrivals beneath, and flanking, the Mid-Atlantic Ridge, and across several transform faults. We analyzed waveforms from 50 earthquakes with magnitude MW >3.5, recorded over 12 months in 2012-2013 by five autonomous hydrophones and a broadband seismograph located on the St. Peter and St. Paul archipelago. The resulting catalog of 152 ray paths allows us to resolve spatial variations in uppermantle velocities, which are consistent with estimates from nearby wide-angle seismic experiments. We find relatively high velocities near to the St. Paul transform system (~8.4 km s-1), compared to lower ridge-parallel velocities (~7.7 km s-1). Hence, this method is able to resolve ridge-transform scale velocity variations. Ray paths in lithosphere younger than 10 Myr have mean velocities of 7.9 ± 0.5 km s-1, which is slightly lower than those sampling lithosphere older than 20 Myr (8.1 km ± 0.3 s-1). There is no apparent systematic relationship between velocity and ray azimuth, which could be due to thickened lithosphere or complex mantle upwelling, although uncertainties in our velocity estimates may obscure such patterns. We also do not find any correlation between Pn velocity and shear wave speeds from the global SL2013sv model at depths < 150 km. Our results demonstrate that data from long-term deployments of autonomous hydrophones can be used to obtain rare and insightful estimates of uppermost mantle velocities over hundreds of kilometers, in otherwise inaccessible parts of the deep oceans.

中文翻译：

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大西洋中脊下的最高地幔速度和赤道大西洋的转换断层

在莫霍面下方传播的地震射线提供了对上地幔热和成分特性的洞察，并且可以从区域地震中检测为 Pn 相。这些相在大陆经常被识别，但在海洋中，由于缺乏长期仪器部署，Pn 相的检测受到限制。我们根据 Pn 到达下方和侧翼的大西洋中脊，以及穿过几个转换断层，对赤道大西洋上地幔速度的估计值。我们分析了 50 次 MW > 3.5 地震的波形，这些地震在 2012-2013 年的 12 个月内由位于圣彼得和圣保罗群岛的五个自主水听器和一个宽带地震仪记录。由此产生的 152 条射线路径目录使我们能够解决上地幔速度的空间变化，这与附近广角地震实验的估计一致。与较低的脊平行速度（~7.7 km s-1）相比，我们发现在 St. Paul 变换系统附近的速度相对较高（~8.4 km s-1）。因此，该方法能够解决脊变换尺度速度变化。年龄小于 10 Myr 的岩石圈中射线路径的平均速度为 7.9 ± 0.5 km s-1，略低于采样年龄大于 20 Myr 的岩石圈（8.1 km ± 0.3 s-1）。速度和射线方位之间没有明显的系统关系，这可能是由于岩石圈增厚或复杂的地幔上涌，尽管我们速度估计的不确定性可能会掩盖这种模式。我们也没有从全球 SL2013sv 模型中发现深度 < 150 km 的 Pn 速度和剪切波速度之间存在任何相关性。