Hydroacoustic observations of the 2004 megathrust earthquake and other events

Abstract

Hydroacoustic signals (T-waves) generated by the 2004 Great Sumatra earthquake were recorded by a network of 5 small hydroacoustic arrays located in the Indian Ocean at distances of 2800 to 7000 km from the epicenter. The array configurations allow for accurate determination of the receiver-to-source azimuth given coherent arrivals. Analysis of a series of 10-s windows within the T-wave coda shows that the receiver-to-source azimuth varies smoothly as a function of time, suggesting a nonstationary T-wave source. Apparent motion in the T-wave source has also been observed for many small-scale earthquakes and, for those events, reflects T-wave excitation over a broad expanse of the seafloor. For small-scale earthquakes, it is shown that the variations in the source azimuth are predictable using a seafloor scattering model. The azimuthal variations for the great Sumatra event are shown to be inconsistent with a small-scale source and are thus indicative of the rupture velocity. The data indicate that the rupture proceeded in two distinct phases; initially it progressed northwest at approximately 2.4 km/s along the Sunda trench. At 600 km from the epicenter, the rupture slowed to approximately 1.5 km/s.