Events as powerful as the Sumatra earthquake are usually caused by ruptures in subduction zones, in which the edge of one tectonic plate slips underneath the edge of the adjoining plate. An earthquake of this type on the sea bed can cause massive tsunamis because of vertical displacement of water above the point of slippage. A recent example was the magnitude 9 earthquake off Japan last year.
The Sumatra earthquake’s magnitude surprised scientists because it was the most powerful strike-slip earthquake ever recorded. The quake was also unusual in that the rupture was in the middle of an oceanic plate rather than at the boundary between two plates. The slippage distance was surprisingly large; the 1906 San Francisco earthquake produced a slippage of 4.5 meters, while in Sumatra the distance was 21.3 meters.