The 26 December 2004 M9 Sumatra-Andaman Islands earthquake caused a tsunami that devastated Indian Ocean coasts within three hours after the earthquake. Improved tsunami warning and emergency response for future great earthquakes require knowing an earthquake's size within minutes after the event. Although the hypocenter of a distant earthquake is routinely determined from the first seismic P waves within about 15 min, several hours may pass before a reliable size determination for very large earthquakes is available (e.g., for the 2004 Sumatra-Andaman earthquake see Menke and Levin <2005>). Seismologists specify the size of an earthquake using seismic scalar moment (M0), a physical quantity proportional to the product of fault area and mean slip on the fault. Currently, analyses of long-period seismograms (period > 100 s) provide reliable estimates of M0 for very large events Dziewonski et al., 1981>, but these seismograms are not available until an hour or more after the event.