Analysis of the earth’s longest period normal modes shows that the December 2004 Sumatra–Andaman earthquake was much larger (Mw 9.3) than initially inferred from surface-wave data and involved slip on a much longer fault than initially inferred from body-wave data. The seismic moment and relative excitation of the normal modes indicate that the entire aftershock zone ruptured, consistent with the large tsunami amplitudes in Thailand, Sri Lanka, and India. An apparent increase in seismic moment with period results from interference between parts of the fault. The earthquake resulted from subduction of the Indian plate beneath the Burma microplate, a sliver plate between the Indian and Sunda plates. Hence, the rate and direction of convergence depends on the motion of the Burma plate, which is not well known. Convergence would be highly oblique if the rate of motion between Burma and Sunda is that inferred from spreading in the Andaman Sea, and less if a slower rate is inferred from the Sagaing fault. The December earthquake was much larger than expected from a previously proposed relation, based on the idea of seismic coupling, in which such earthquakes occur only when young lithosphere subducts rapidly. Moreover, a global reanalysis finds little support for this correlation. Hence, we suspect that much of the apparent differences between subduction zones, such as some trench segments but not others being prone to Mw >8.5 events and hence oceanwide tsunamis, may reflect the short earthquake history sampled. This possibility is supported by the variability in rupture mode at individual trench segments.

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