Seismic evidence for broken oceanic crust in the 2004 Sumatra earthquake epicentral region

The 2004 Sumatra earthquake was one of the largest events to occur in a subduction zone in the past 50 years. Seismic reflection data for this subduction zone reveal thrust faults cutting across the entire oceanic crust. This observation, coupled with the hypocentres of aftershocks, suggests that the megathrust—the interface between the Indo-Australian plate and the Sunda plate—currently lies in the oceanic mantle. The great Sumatra earthquake of 26 December 2004 was the third largest event to occur in a subduction zone in the past 50 years. The rupture initiated at 30–40 km depth northwest of Simeulue Island 1 and propagated for ∼1,300 km to the northern Andaman Islands 2 . The earthquake was caused by sudden slip along the plate interface between the subducting Indo-Australian plate and the overriding Sunda plate 3 , 4 . Although detailed knowledge of the structure of the subduction interface is important to define potential sources of large megathrust earthquakes, available data 5 , 6 , 7 , 8 have not provided such information so far. Here we present a high-quality seismic section of the focal region, from the abyssal plain down to 40 km depth below the fore-arc. The seismic data reveal that the subducting crust and oceanic Moho—the crust–mantle boundary—are broken and displaced by landward-dipping thrust ramps, suggesting that the megathrust now lies in the oceanic mantle. We image active thrust faults at the front of the accretionary wedge, consistent with thrust aftershocks on steeply dipping planes. Our observations imply that very strong coupling leading to brittle failure of mantle rocks accounts for the initiation of such an exceptionally large earthquake.