Tectonic plates

The earthquake was unusually large in geographical extent. An estimated 1200 km (750 miles) of faultline slipped about 15 m (50 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 and acoustic data indicate that the first phase involved the formation of a rupture about 400 km (250 miles) long and 100 km (60 miles) wide, located 30 km (19 miles) beneath the sea bed - the longest known rupture ever known to have been caused by an earthquake. The rupture proceeded at a speed of about 2.8 km/s (1.7 miles/s) or 10,000 km/h (6,300 miles/h), beginning off the coast of Aceh and proceeding north-westerly over a period of about 100 seconds. A pause of about another 100 seconds took place before the rupture continued northwards towards the Andaman and Nicobar Islands. However, the northern rupture occurred more slowly than in the south, at about 2.1 km/s (1.3 miles/s), continuing north for another five minutes to a plate boundary where the fault changes from subduction to strike-slip (the two plates push past one another in opposite directions) thus reducing the speed of the water displacement and so reducing the size of the tsunami that hit the northern part of the Indian Ocean [10].

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 Australasian 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 carries 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 drive volatiles out of the subducting plate. These volatiles rise into the mantle above and trigger melt which exits the earth's mantle through volcanoes (see Volcanic arc). The volcanic activity that results as the Indo-Australian plate subducts the Eurasian plate has created the Sunda Arc.

As well as the sideways movement between the plates, the sea bed is estimated to have risen by several metres, displacing an estimated 30 km³ of water and triggering devastating tsunami waves. The waves did not originate from a point source, as mistakenly depicted in some illustrations of their spread, but radiated outwards along the entire 1200 km (750 miles) length of the rupture. This greatly increased the geographical area over which the waves were observed, reaching as far as Mexico, Chile and the Arctic. The raising of the sea bed significantly reduced the capacity of the Indian Ocean, producing a permanent rise in the global sea level by an estimated 0.1 mm.

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