We present a regional short‐period Sn magnitude scale mb(Sn) for small earthquakes along the northern Mid‐Atlantic Ridge. Surface‐wave magnitudes, teleseismic body‐wave magnitudes, and seismic moments cannot be reliably determined for small earthquakes along this and other midocean ridges. Local magnitudes that rely on Lg waves are likewise not generally useful due to the substantial oceanic paths for earthquakes along midocean ridges. In contrast, Pn and Sn arrivals for earthquakes along the northern Mid‐Atlantic Ridge are generally well recorded by the existing seismographic networks, and, in fact, Sn arrivals are larger than Pn arrivals for about one‐third of the ridge events. For this reason, we have developed a new regional Sn magnitude scale that is tied to Mw⁠, so that seismic moments can be readily approximated. In our least‐squares fit of peak amplitudes from 120 earthquakes having a published moment magnitude, we solved for the attenuation curve for paths in the oceanic mantle lid, for event magnitude adjustments (EMAs) to account for differences between long‐period moment magnitude Mw and short‐period Sn magnitude, and for station corrections. We find regional EMAs that are well correlated with the style of faulting: they are positive for normal‐faulting earthquakes along spreading ridges and negative for strike‐slip earthquakes along transform faults. These source‐specific EMAs are approximately +0.11 magnitude units for normal‐fault earthquakes and −0.26 magnitude units for strike‐slip earthquakes on transform faults, and are consistent with previously reported apparent stresses from these regions. The amplitude distance curve determined for Sn for the northern Atlantic Ocean is similar to that determined for Pn in the northern Atlantic out to a distance of about 500 km, but at larger distances is more similar to the western U.S. Pn curve, likely reflective of the warmer temperatures at greater upper‐mantle depths.

中文翻译：

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北部中大西洋海岭地震的区域Sn震级mb（Sn）和矩震级估计

我们为中大西洋北部北部小地震提供了一个区域性的短期Sn震级尺度mb（Sn）。对于沿着该洋脊和其他中洋洋脊的小地震，不能可靠地确定表面波震级，远震体波震级和地震矩。由于沿洋中脊的地震的大量海洋路径，依靠Lg波的局部震级通常也没有用。相比之下，现有地震波网通常记录了中大西洋北部北部地震的Pn和Sn到达，实际上，在大约三分之一的山脊事件中，Sn到达比Pn到达要大。因此，我们开发了与Mw？相关的新的区域Sn震级标度，以便可以轻松估算地震矩。在我们的120个地震的峰值幅度的最小二乘拟合中，我们得出了大地幔盖上路径的衰减曲线，并对事件量级调整（EMA）进行了解释，以解释长期矩量级Mw之间的差异。和短周期的Sn幅值，并用于测站校正。我们发现区域EMA与断层的样式密切相关：它们对沿扩散脊的正断层地震为正，而对沿转换断层的走滑地震为负。对于特定断裂地震，这些源特定的EMA约为+0.11震级单位，对于转换断层，对于走滑地震，这些震源约为-0.26震级单位，与先前报道的来自这些地区的视应力一致。