A Regional Sn Magnitude Scale mb(Sn) and Estimates of Moment Magnitude for Earthquakes along the Northern Mid-Atlantic Ridge

We present a regional short-period Sn magnitude scale m(b)(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 M-w, 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 M-w 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.