The earthquake was unusually large in geographical extent. An estimated 1200 km (750 miles) of faultline slipped about 15 m (45 ft) along the subduction zone where the India Plate dives under the Burma Plate. The slip did not happen instantaneously but took place in two phases over a period of several minutes. Seismographic data indicate that the first phase involved the formation of a rupture about 400 km long and 100 km wide, located 30km beneath the sea bed. The rupture proceeded at a speed of about 2 km/s, beginning off the coast of Aceh and proceeding north-westerly over about 100 seconds. A pause of about another 100 seconds took place before the rupture continued northwards towards the Andaman and Nicobar Islands [7] (
http://www.guardian.co.uk/life/feature/story/0,13026,1383675,00.html).
The India Plate is part of the great Indo-Australian Plate, which underlies the Indian Ocean and Bay of Bengal, and is drifting northeast at an average of 6 cm/year (2 inches/year). The India Plate meets the Burma Plate (which is considered a portion of the great Eurasian Plate) at the Sunda Trench. At this point the India Plate subducts the Burma Plate which includes the Nicobar Islands, the Andaman Islands and northern Sumatra. The India Plate slips deeper and deeper beneath the Burma Plate until the increasing temperature and pressure turns the subducting edge of the India Plate into magma which eventually pushes the magma above it out through the volcanoes (see Volcanic arc). This process is interrupted by the locking of the plates for several centuries until the build up of stress causes their release resulting in a massive earthquake and tsunami. The volcanic activity that results as the Indo-Australian plate subducts the Eurasian plate has created the Sunda Arc.
