2004 TSUNAMI DISASTER
INDIAN OCEAN EARTHQUAKE
The Indian Ocean earthquake was an undersea earthquake that occurred at 00:58:53 UTC (07:58:53 local time) on December 24, 2004. The earthquake generated a tsunami that was among the deadliest disasters in modern history.
At a magnitude of 9.0, it was the largest earthquake since the 9.2 magnitude Good Friday Earthquake off Alaska in 1964. In February 2005, new analysis suggested the magnitude was underestimated, and one study estimates it at 9.3; however, the USGS has not yet confirmed this. The earthquake originated in the Indian Ocean just north of Simeule island, off the western coast of northern Sumatra, Indonesia. The resulting tsunami devastated the shores of Indonesia, Sri Inka, South India, Thailand and other countries with waves up to 30 m (100 feet) high. It caused serious damage and deaths as far as the east coast of Africa, with the furthest recorded death due to the tsunami occurring at Port Elizabeth in South Africa, 8 000km (5 000 miles) away from the epicenter.
Anywhere from 228,000 to 310,000 people are thought to have died as a result of the tsunami, and the count is not yet complete. In Indonesia in particular, 500 bodies a day were still being found in February 2005 and the count was expected to continue past June. The true final toll may never be known due to bodies having been swept out to sea, but current estimates use conservative methodologies. Relief agencies warn of the possibility of more deaths to come as a result of epidemics caused by poor sanitation, but the threat of starvation seems now to have been largely averted . The plight of the many affected people and countries prompted a widespread humanitarian response.
Quake characteristics 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. 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. Aftershocks and other earthquakes Numerous aftershocks were reported off the Andaman Islands, the Nicobar Islands and the region of the original epicentre in the hours and days that followed. The largest aftershock was 8.7 epicentred off the Sumatran island of Nias. Other aftershocks of up to magnitude 6.6 continue to shake the region on a daily basis. The 2004 Indian Ocean earthquake came just three days after a magnitude 8.1 earthquake in an uninhabited region west of New Zealand's sub-Antarctic Auckland Islands, and north of Australia's Macquarie Island. This is unusual, since earthquakes of magnitude 8 or more occur only about once per year on average. Some seismologists have speculated about a connection between these two earthquakes, saying that the former one might have been a catalyst to the Indian Ocean earthquake, as the two quakes happened on opposite sides of the Indo-Australian Plate. Power of the earthquake The total energy released by the earthquake in the Indian Ocean has been estimated as 4.3 exajoules (4.3×1018 joules). This is equivalent to 100 gigatons of TNT, or about as much energy as is used in the United States in 6 months. It is estimated to have resulted in an oscillation of the Earth's surface of about 20-30 cm (8 to 12 inches), equivalent to the effect of the tidal forces caused by the Sun and Moon. The shock waves of the earthquake were felt across the planet; as far away as Oklahoma, vertical movements of 3 mm (0.12 inches) were recorded. The entire Earth's surface is estimated to have moved vertically by up to 1 cm. The shift of mass and the massive release of energy very slightly altered the Earth's rotation. The exact amount is yet undetermined, but theoretical models suggest the earthquake shortened the length of a day by 2.68 microseconds (2.68 µs) (or about one billionth of the length of a day) due to a decrease in the oblateness of the Earth. It also caused the Earth to minutely "wobble" on its axis by up to 2.5 cm (1 inch) in the direction of 145° east longitude or perhaps by up to 5 or 6 cm (2.0 to 2.4 inches). However, due to tidal effects of the Moon, the length of a day increases at an average of 15 µs per year, so any rotational change due to the earthquake will be lost quickly. Similarly, the natural Chandler wobble of the Earth can be up to 15 m (50 ft). More spectacularly, there was 10 m (33 feet) movement laterally and 4 to 5 m (13 to 16 feet) vertically along the fault line. Early speculation was that some of the smaller islands southwest of Sumatra may have moved southwest by up to 20 m (66 feet). There were also calculations that the northern tip of Sumatra, which is on the Burma Plate (the southern regions are on the Sunda Plate), may have moved up to 36 m (118 ft) southwest. Since movement was vertical as well as lateral, some coastal areas may now be below sea level. Measurements using GPS and satellite imagery are being used to determine the extent and nature of actual geophysical change. The Andaman and Nicobar Islands appear to have shifted southwest by around 4 m, according to GPS data. In February 2005, the Royal Navy vessel HMS Scott surveyed the sea bed around the earthquake zone, which varies in depth between 1,000 m (3,300 feet) and 5,000 m (16,500 feet) west of Sumatra. The survey, conducted using a high-resolution multi-beam sonar system, revealed that the earthquake had had a huge impact on the topography of the sea bed. It had created large thrust ridges, about 1,500 m high, which have collapsed in places to produce large landslides several kilometres across. One landslide consisted of a single block of material some 100 m (300 feet) high and 2 km (1.25 miles) long. The force of the displaced water was such that individual blocks of rock, massing millions of tons apiece, were dragged as much as 10 km (7 miles) across the sea bed. An newly-formed oceanic trench several kilometres wide was also found in the earthquake zone.
