Unprecedented hydroacoustic observations of the megathrust earthquake of 26 Dec, 2004 were afforded by a network of 5 small hydroacoustic arrays located in the Indian Ocean, at distances of 2800 to 7000 km from the epicenter. Each array recorded acoustic waves, called T waves, generated by this event. Analysis of a series of short time windows within the T wave coda shows that the receiver to source azimuth varies smoothly as a function of time, indicating that the apparent T wave source is not stationary. The apparent T wave source moves northward along the Sunda trench at an average velocity of 2 km/s, closely tracking event rupture. The hydroacoustic data suggest that the rupture proceeded in two distinct phases; initially it progressed northwest along the Sunda trench with a velocity of approximately 2.4 km/s. At 600 km from the epicenter the rupture slowed to approximately 1.5 km/s, as it continued to propagate to the northwest.