Important Historical Events
2004 Asia Tsunami Information
The 2004 Indian Ocean earthquake was an undersea earthquake that occurred at 00:58:53 UTC (07:58:53 local time) on December 26, 2004. The earthquake generated tsunamis that were 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, and tied for fourth largest since 1900.
The earthquake originated in the Indian Ocean just north of Simeulue island, off the western coast of northern Sumatra, Indonesia. The resulting tsunamis devastated the shores of Indonesia, Sri Lanka, India, Thailand, and other countries with waves of up to 15 m (50 feet) high, even reaching the east coast of Africa, 4500 km (2,800 miles) west of the epicenter.
At least 150,000 people are known to have died as a result of the tsunamis and the count is still taking place. The true final toll may never be known due to bodies swept out to sea, but it is likely to be higher than the current estimate. Relief agencies warn of the possibility of more deaths to come as a result of epidemics because of poor sanitation, but the threat of starvation seems now to have been averted. The plight of the many affected people and countries prompted a widespread humanitarian response.
The 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 finally to 9.0.
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 in the Andreanof Islands (9.1). The only other recorded earthquake of magnitude 9.0 was in 1952 off the southeast coast of Kamchatka. 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.
Locations of the initial earthquake and aftershocks.
The hypocenter was at 3.316°N, 95.854°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. The earthquake itself (apart from the tsunamis) was felt as far away as Bangladesh, India, Malaysia, Myanmar, Thailand, Singapore and the Maldives.
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 indicates 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 proceding 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.
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.
As well as the sideways movement between the plates, the sea bed is estimated to have risen by several metres, triggering devastating tsunami waves. The tsunamis did not originate from a point source, as mistakenly depicted in many illustrations of their spread, but radiated outwards along the entire 1200 km length of the rupture. This greatly increased the geographical area over which the waves were observed, reaching as far as Mexico and Chile.
The 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 tectonic plate. However the USGS sees no evidence of a causal relationship.
Coincidentally, the earthquake struck almost exactly one year (to the hour) after a magnitude 6.6 earthquake killed an estimated 30,000 people in the city of Bam in Iran.
As well as continuing aftershocks, the energy released by the original earthquake continued to make its presence felt well after the event. A week after the earthquake, its reverberations could still be measured, providing valuable scientific data about the Earth's interior
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 may have shortened the length of a day by as much as three microseconds (3 µs) and caused the Earth to minutely "wobble" on its axis by up to 2.5 cm (1 inch) or perhaps by up to 5 or 6 cm. However, due to tidal effects of the Moon, the length of a day increases by 15 µs every year, so any rotational speedup due to the earthquake will be quickly lost. Similarly, the natural Chandler wobble of the Earth can be up to 15 m (50 ft).
More spectacularly, some of the smaller islands southwest of Sumatra may have moved southwest up to 20 m (66 ft). The northern tip of Sumatra, which is on the Burma Plate (the southern regions are on the Sunda Plate), may also have moved up to 36 m (118 ft) southwest. Movement was likely both vertical as well as lateral. Measurements using GPS and satellite imagery are being used to determine the extent and nature of actual geophysical change.
According to Tad Murty, vice-president of the Tsunami Society, the total energy of the tsunami waves was about five megatons of TNT (20 petajoules). This is more than twice the total explosive energy used during all of World War II (including the two atomic bombs), but still a couple of orders of magnitude less than the energy released in the earthquake itself.
Two radar satellites that happened to be overhead at the right moment recorded two wavefronts 500–850 km apart with a height of 50 cm. These are the first such observations ever made.
Because the 1,200 km of faultline affected by the quake was in a nearly north-south orientation, the greatest strength of the tsunami waves was in an east-west direction. Bangladesh, which lies at the northern end of the Bay of Bengal, had very few casualties despite being a low-lying country.
Coasts that have a land mass between them and the tsunamis' location of origin are usually safe; however, tsunami waves can sometimes diffract around such land masses. Thus, the Indian state of Kerala was hit by tsunamis despite being on the western coast of India, and the western coast of Sri Lanka also suffered substantial impacts. Also, distance alone is no guarantee of safety: Somalia was hit harder than Bangladesh despite being much farther away.
Due to the distances involved, the tsunamis took anywhere from fifteen minutes to seven hours (for Somalia) to reach the various coastlines. The northern regions of the Indonesian island of Sumatra were hit very quickly, while Sri Lanka and the east coast of India were hit roughly two hours later. Thailand was also struck about two hours later, despite being closer to the epicenter, because the tsunami travelled more slowly in the shallow Andaman Sea off its western coast.