A Correction to Body-wave Magnitude mb Based on Moment Magnitude Mw

Abstract

Although body-wave magnitude mb (as well as surface-wave magnitude Ms ) is rapidly being supplanted by moment magnitude MW as the magnitude of record for moderate to large earthquakes, the first, automatically determined teleseismic estimate of earthquake size is still mb . While efforts are underway to implement various automated estimates of MW , these have not, to date, proven entirely successful, nor are they rapid enough to satisfy the rapid reporting requirements of the National Earthquake Information Center (NEIC). For example, estimates of MWP , computed by integrating the broadband P -wave displacement pulse, appear to be biased for some magnitude ranges and are probably still subject to saturation, although at a higher magnitude than Mb . Likewise, automated moment-tensor algorithms do not yet yield a rapid enough estimate of scalar moment, from which MW can be derived. Thus, initial teleseismically derived earthquake response is still almost entirely based upon mb . It is well known, however, that mb is biased with respect to MW for nearly all magnitude ranges, not just the larger magnitudes where mb is expected to saturate. If the bias could be quantified, a correction term could be used to yield a more representative estimate of earthquake size. This would be useful during the initial, automated stages of the earthquake alerting process, aiding both seismic analysts and emergency response personnel in assessing potential earthquake damage and casualties. The examination of the relationships between various magnitude scales has a long history. This has been necessitated, in part, because although almost all magnitude scales have been calibrated to be consistent with the “Richter scale” (originally the local magnitude ML introduced by Richter [1935]) at some frequency, epicentral distance, focal depth, earthquake size, etc. …