M-wpd: a duration-amplitude procedure for rapid determination of earthquake magnitude and tsunamigenic potential from P waveforms

We present a duration-amplitude procedure for rapid determination of a moment magnitude, M-wpd, for large earthquakes using P-wave recordings at teleseismic distances. Mwpd can be obtained within 20 min or less after the event origin time as the required data are currently available in near real time. The procedure determines apparent source durations, T-0, from high-frequency, P-wave records, and estimates moments through integration of broad-band displacement waveforms over the interval t(P) to t(P) + T-0, where t(P) is the P-arrival time. We apply the duration - amplitude methodology to 79 recent, large earthquakes ( global centroid-moment-tensor magnitude, M-w(CMT) , 6.6-9.3) with diverse source types. The results show that a scaling of the moment estimates for interplate thrust and possibly tsunami earthquakes is necessary to best match M-w(CMT). With this scaling, M-wpd matches M-w(CMT) w typically within +/- 0.2 magnitude units, with a standard deviation of sigma = 0.11, equaling or outperforming other approaches to rapid magnitude determination. Furthermore, M-wpd does not exhibit saturation; that is, for the largest events, Mwpd does not systematically underestimate M-w(CMT). The obtained durations and duration - amplitude moments allow rapid estimation of an energy-to-moment parameter Theta* used for identification of tsunami earthquakes. Our results show that Theta* <= - 5.7 is an appropriate cut-off for this identification, but also show that neither Theta* nor M-w is a good indicator for tsunamigenic events in general. For these events, we find that a reliable indicator is simply that the duration T-0 is greater than about 50 s. The explicit use of the source duration for integration of displacement seismograms, the moment scaling and other characteristics of the duration - amplitude methodology make it an extension of the widely used, M-wp, rapid magnitude procedure. The need for a moment scaling for interplate thrust and possibly tsunami earthquakes may have important implications for the source physics of these events.