2004 Indian Ocean Earthquake

Quake characteristics

Epicentre of the quake, just north of Simeulue IslandThe earthquake was initially reported as 6.8 on the Richter scale. The Pacific Tsunami Warning Center (PTWC) also estimated it at 8.5 shortly after the earthquake. On the moment magnitude scale, which is more accurate for quakes of this size, the earthquake's magnitude was first reported as 8.1 by the U.S. Geological Survey. After further analysis, this was increased to 8.5, 8.9, and 9.0 (USGS, 2004, [4] (http://earthquake.usgs.gov/recenteqsww/Quakes/usslav.htm)). In February 2005, some scientists revised the estimate of magnitude to 9.3. Although the Pacific Tsunami Warning Center has accepted this, the USGS has so far not changed its estimate of 9.0 (McKee, 9 Feb 2005, [5] (http://www.newscientist.com/article.ns?id=dn6991)). The most definitive estimate so far has put the magnitude at 9.15 [6] (http://www.sciencemag.org/cgi/content/summary/308/5725/1125).

The hypocentre of the main earthquake was at 3.316°N, 95.854°E (3° 19' N, 95° 51.24' E), some 160 km (100 miles) west of Sumatra, at a depth of 30 km (18.6 miles) below mean sea level (initially reported as 10 km). This is at the extreme western end of the Ring of Fire, an earthquake belt that accounts for 81 percent of the world's largest earthquakes (USGS FAQ, [7] (http://earthquake.usgs.gov/faq/hist.html#1)). The earthquake itself (apart from the tsunami) was felt as far away as Bangladesh, India, Malaysia, Myanmar, Thailand, Singapore and the Maldives.

Since 1900 the only earthquakes recorded with a greater magnitude were the 1960 Great Chilean Earthquake (magnitude 9.5), the 1964 Good Friday Earthquake in Prince William Sound (9.2), and the March 9, 1957 earthquake [8] (http://neic.usgs.gov/neis/eq_depot/usa/1957_03_09.html) in the Andreanof Islands (9.1). The only other recorded earthquake of magnitude 9.0 was in 1952 off the southeast coast of Kamchatka [9] (http://neic.usgs.gov/neis/eq_depot/world/1952_11_04.html) (see Top 10 earthquakes (http://neic.usgs.gov/neis/eqlists/10maps_world.html)). Each of these megathrust earthquakes also spawned tsunamis (in the Pacific Ocean), but the death toll from these was significantly lower; a few thousand for the worst one, probably because of the lower population density along the coasts near affected areas and the much greater distances to more populated coasts.

Other large megathrust earthquakes occurred in 1868 (Peru, Nazca Plate and South American Plate); 1827 (Colombia, Nazca Plate and South American Plate); 1812 (Venezuela, Caribbean Plate and South American Plate) and 1700 (Cascadia Earthquake, western US and Canada, Juan de Fuca Plate and North American Plate). These are all believed to have been of greater than magnitude 9, but no accurate measurements were available in those days.

Tectonic plates

Animation of the tsunami caused by the earthquake (see also the full-length version) From NOAA/PMEL (http://www.pmel.noaa.gov/tsunami).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 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 km/s (1.2 miles/s) or 7200 km/h (4470 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 [10] (http://www.guardian.co.uk/life/feature/story/0,13026,1383675,00.html). However, the northern rupture occurred more slowly than in the south, reducing the speed of the water displacement and so reducing the size of the tsunami that hit the northern part of the Indian Ocean.

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 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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