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Abstract. The December 26, 2004 Sumatra-Andaman Island earthquake, which ruptured the Sunda Trench subduction zone, is one of the three largest earthquakes to occur since global monitoring began in the 1890s. Its seismic moment was M0 = 1.0010 23 1.151023 N-m, corresponding to a moment-magnitude of Mw=9.3. The rupture propagated from south to north, with the southerly part of fault rupturing at a speed of 2.8 km/s. Rupture propagation appears to have slowed in the northern section, possibly to ~2.1 km/s, although published estimates have considerable scatter. The average slip is ~5 meters along a shallowly-dipping (8), N31W striking thrust fault. The majority of slip and moment release appears to have been concentrated in the southern part of the rupture zone, where slip locally exceeded 30 meters. Stress loading from this earthquake caused the section of the plate boundary immediately to the south to rupture in a second, somewhat smaller earthquake. This second earthquake occurred on March 28, 2005 and had a moment magnitude of Mw=8.5