AN ENERGY-DURATION PROCEDURE FOR RAPID DETERMINATION OF EARTHQUAKE MAGNITUDE AND TSUNAMIGENIC POTENTIAL

Abstract

We introduce a rapid and robust, energy-duration procedure, based on the Haskell, extended-source model, to obtain an earthquake moment and a moment magnitude, MED. Using seismograms at teleseismic distances (30°–90°), this procedure combines radiated seismic energy measures on the P to S interval of broadband signals and source duration measures on high-frequency, P-wave signals. The MED energy-duration magnitude is scaled to correspond to the Global Centroid-Moment Tensor (CMT) moment-magnitude, MCMTw, and can be calculated within about 20 min or less after origin time (OT). The measured energy and duration values also provide the energy-to-moment ratio, Θ, used for identification of tsunami earthquakes. The MED magnitudes for a set of recent, large earthquakes match closely MCMTw, even for the largest, great earthquakes; these results imply that the MED measure is accurate and does not saturate. After the 2004 December 26 Sumatra-Andaman mega-thrust earthquake, magnitude estimates available within 1 hr of OT ranged from M= 8.0 to 8.5, the CMT magnitude, available about 3 hr after OT, was MCMTw= 9.0, and, several months after the event, Mw= 9.1–9.3 was obtained from analysis of the earth normal modes. The energy-duration magnitude for this event is MED= 9.2, a measure that is potentially available within 20 min after OT. After the 2006 July 17, Java earthquake, the magnitude was evaluated at M= 7.2 at 17 min after OT, the CMT magnitude, available about 1 hr after OT, was MCMTw= 7.7; the energy-duration results for this event give MED= 7.8, with a very long source duration of about 160 s, and a very low Θ value, indicating a possible tsunami earthquake.