Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009geoji.176..200l&link_type=abstract
Geophysical Journal International, Volume 176, Issue 46, pp. 200-214.
Astronomy and Astrophysics
Astronomy
7
Tsunamis, Earthquake Dynamics, Earthquake Source Observations, Seismic Monitoring And Test-Ban Treaty Verification, Body Waves, Wave Propagation
Scientific paper
We present a duration-amplitude procedure for rapid determination of a moment magnitude, Mwpd, 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, T0, from high-frequency, P-wave records, and estimates moments through integration of broad-band displacement waveforms over the interval tP to tP + T0, where tP is the P-arrival time. We apply the duration-amplitude methodology to 79 recent, large earthquakes (global centroid-moment-tensor magnitude, MCMTw, 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 MCMTw. With this scaling, Mwpd matches MCMTw typically within +/-0.2 magnitude units, with a standard deviation of σ = 0.11, equaling or outperforming other approaches to rapid magnitude determination. Furthermore, Mwpd does not exhibit saturation; that is, for the largest events, Mwpd does not systematically underestimate MCMTw. The obtained durations and duration-amplitude moments allow rapid estimation of an energy-to-moment parameter Θ* used for identification of tsunami earthquakes. Our results show that Θ* <= -5.7 is an appropriate cut-off for this identification, but also show that neither Θ* nor Mw is a good indicator for tsunamigenic events in general. For these events, we find that a reliable indicator is simply that the duration T0 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, Mwp, 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.
Lomax Anthony
Michelini Alberto
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