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Authors: Lomax, A.* 
Michelini, A.* 
Title: Mwpd: A Duration-Amplitude Procedure for Rapid Determination of Earthquake Magnitude and Tsunamigenic Potential from P Waveforms
Issue Date: 2007
Keywords: earthquakes
Richter magnitude
seismic moment
earthquake­source mechanism
Subject Classification05. General::05.01. Computational geophysics::05.01.05. Algorithms and implementation 
Abstract: We present a duration-amplitude procedure for rapid determination of an earthquake moment magnitude, Mwpd, from P-wave recordings at teleseismic distances. The Mwpd magnitude can be obtained within 20 minutes or less after the event origin time since the required data is available in near-real time. The procedure determines apparent source durations, T0, from high-frequency, P-wave records, and estimates moments through integration of broadband displacement waveforms over the interval tP to tP+T0, where tP is the P arrival time. We apply the duration-amplitude methodology to a number of recent, large earthquakes (Global Centroid-Moment Tensor magnitude, MwCMT, 6.6 to 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 MwCMT. With this scaling, Mwpd matches MwCMT typically within ±0.2 magnitude units, with a standard deviation of σ=0.10, outperforming other approaches to rapid magnitude determination. In addition, Mwpd does not exhibit saturation for the largest events, or, equivalently, ΔM=Mwpd-MwCMT does not become more negative with increasing MwCMT. 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, Mwp, rapid­magnitude procedure. The obtained durations and duration-amplitude moments allow rapid estimation of the energy-to- moment ratio Θ used for identification of tsunami earthquakes. The need for a moment scaling for interplate thrust and possibly tsunami earthquakes may have important implications for the source physics of these events.
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