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 a tsunami that was among the deadliest disasters in modern history. At a magnitude of 9.0, it was the largest earthquake it was the largest earthquake since the 9.2 magnitude Good Friday Earthquake off Alaska in 1964, and tied for fourth largest since 1900. In February 2005 a new analysis suggested the quake may have been of magnitude 9.3; however, the USGS has not yet confirmed this. [1] (http://www.newscientist.com/article.ns?id=dn6991)

The earthquake originated in the Indian Ocean just north of Simeulue island, off the western coast of northern Sumatra, Indonesia. The resulting tsunami devastated the shores of Indonesia, Sri Lanka, South India, Thailand and other countries with waves up to 15 m (50 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 occuring at Port Elizabeth in South Africa, 8 000km (5 000 miles) away from the epicentre [2] (http://en.wikipedia.org/wiki/Countries_affected_by_the_2004_Indian_Ocean_earthquake#Countries_suffering_some_casualties_and_damage).

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. 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 [3] (http://news.bbc.co.uk/2/hi/asia-pacific/4157947.stm). The plight of the many affected people and countries prompted a widespread humanitarian response.

Quake characteristics

Epicentre of the quake, just north of Simeulue Island

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 US Geological Survey. After further analysis, this was increased to 8.5, 8.9, and 9.0 [4] (http://earthquake.usgs.gov/recenteqsww/Quakes/usslav.htm). In February 2005, some scientists estimated it was actually 9.3, but the USGS has so far not changed its estimate of 9.0. [5] (http://www.newscientist.com/article.ns?id=dn6991)

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 [6] (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 [7] (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 [8] (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 North American Plate); 1827 (Colombia, Gorda Plate and North American Plate); 1812 (Venezuela, Caribbean 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

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 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 [9] (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 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, triggering devastating tsunami waves. The tsunami 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.

Locations of initial quake and aftershocks (Credit: USGS)

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 7.1 off the Nicobar Islands [10] (http://earthquake.usgs.gov/recenteqsww/Quakes/ussmax.htm). Other aftershocks of up to magnitude 6.6 continue to shake the region on a daily basis [11] (http://wwwneic.cr.usgs.gov/neis/bulletin/) [12] (http://earthquake.usgs.gov/recenteqsww/Maps/10/95_5.html).

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 [13] (http://earthquake.usgs.gov/recenteqsww/Quakes/ussjal.htm). This is unusual, since earthquakes of magnitude 8 or more occur only about once per year on average [14] (http://earthquake.usgs.gov/faq/hist.html#8). 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 [15] (http://www.news.com.au/common/story_page/0,4057,11787870%255E28477,00.html). However the US Geological Survey sees no evidence of a causal relationship [16] (http://earthquake.usgs.gov/eqinthenews/2004/usslav/neic_slav_faq.html).

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 [17] (http://earthquake.usgs.gov/recenteqsww/Quakes/uscvad.htm).

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 [18] (http://www.reuters.co.uk/newsArticle.jhtml?type=topNews&storyID=650823).

Power of the earthquake

The total energy released by the earthquake in the Indian Ocean has been estimated as 2.0 exajoules (2.0×10¹⁸ joules) [19] (http://earthquake.usgs.gov/eqinthenews/2004/usslav/neic_slav_faq.html). This is enough to boil 150 litres (40 US gallons) of water for every person on Earth. It is estimated to have resulted in an oscillation of the Earth's surface of about 20 to 30 cm, equivalent to the effect of the tidal forces caused by the Sun and Moon [20] (http://www.reuters.co.uk/newsArticle.jhtml?type=topNews&storyID=650823). The shock waves of the earthquake were felt across the planet - as far away as Oklahoma, vertical movements of 0.12 in (3 mm) were recorded [21] (http://www.muskogeephoenix.com/news/stories/20041228/localnews/1795857.html).

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) [22] (http://www.nasa.gov/home/hqnews/2005/jan/HQ_05011_earthquake.html) 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 [23] (http://slate.msn.com/id/2111443/), [24] (http://www.cnn.com/2004/TECH/science/12/29/quake.wobble.reut/index.html) or perhaps by up to 5 or 6 cm [25] (http://news.xinhuanet.com/english/2004-12/29/content_2389519.htm). 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 ten-meter movement laterally and four to five meters 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 ft). 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. However, measured maximum movement was just over one meter. [26] (http://www.telegraphindia.com/1050209/asp/atleisure/story_4355748.asp) Since movement was both 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 [27] (http://www.timesleader.com/mld/timesleader/10574872.htm).

Tsunami characteristics

Radar imaging of the tsunami two hours after the earthquake

Ripples were felt around the world. See also animation: MPG (http://www.noaanews.noaa.gov/video/tsunami-worldpropagation2004.mpg) / AVI (http://www.noaanews.noaa.gov/video/tsunami-worldpropagation2004.avi) / MOV (http://www.noaanews.noaa.gov/video/tsunami-worldpropagation2004.mov).

The sudden vertical rise of the seabed by several metres during the earthquake displaced massive volumes of water, resulting in a tsunami that struck the coasts of the Indian Ocean. A tsunami which causes damage far away from its source is sometimes called a "teletsunami", and is much more likely to be produced by vertical motion of the seabed than by horizontal motion (Earthquakes and tsunamis, Lorca et al.).

See a full-length animation of how the waves travelled (large file, about 1 MiB) to see exactly how and why some countries were more affected than others

The tsunami, like all others, behaved very differently in deep water than in shallow water. In deep ocean water, tsunami waves form only a small hump, barely noticeable and harmless, which travels at very high speed (500-1,000 km/h); in shallow water near coastlines, a tsunami slows down to only tens of kilometres an hour but in doing so forms large destructive waves [28] (http://www.prh.noaa.gov/itic/library/about_tsu/faqs.html#8).

Radar satellites recorded the heights of tsunami waves in deep water: at two hours after the earthquake, the maximum height was 60 cm (two ft). These are the first such observations ever made. (The observations could not be used to provide a warning, because the satellites were not intended for that purpose and the data took hours to analyze.) [29] (http://www.noaanews.noaa.gov/stories2005/s2365.htm) [30] (http://www.newscientist.com/article.ns?id=dn6854)

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 [31] (http://www.canada.com/montreal/montrealgazette/news/story.html?id=2257b78c-3897-4594-ad86-18c0eb661bea). In many places the waves reached as far as 2 km (1 1/4 mi) inland [32] (http://www.newscientist.com/channel/being-human/mg18524825.100).

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 tsunami's 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 the tsunami 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 tsunami took anywhere from fifteen minutes to seven hours (for Somalia) to reach the various coastlines (see travel time maps: [33] (http://tsun.sscc.ru/tsulab/20041226trt.htm), [34] (http://staff.aist.go.jp/kenji.satake/Sumatra-E.html)). 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 90 minutes to two hours later. Thailand was also struck about two hours later, despite being closer to the epicentre, because the tsunami travelled more slowly in the shallow Andaman Sea off its western coast.

The tsunami was noticed as far as Struisbaai in South Africa, some 8,500km (5,300 miles) away, where a 1.5 metre high ‘tide’ surged onshore about 16 hours after the quake. This spot at the southernmost point of Africa took a relatively long time to reach, probably due to the broad continental shelf off South Africa and the fact that the tsunami would have followed the South African coast from east to west [35] (http://www.iol.co.za/index.php?set_id=1&click_id=13&art_id=vn20041228044643745C770443).

Some of the tsunami's energy escaped into the Pacific Ocean, where it produced small but measurable tsunamis along the western coasts of North and South America (typically 20-40 cm) [36] (http://wcatwc.arh.noaa.gov/IndianOSite/IndianO12-26-04.htm). At Manzanillo, Mexico, a 2.6 m (8.5 ft.) crest-to-trough tsunami was measured. Experts have speculated that this relatively large tsunami at such a great distance was caused by focusing effects of Pacific and local geography [37] (http://wikisource.org/wiki/2004_Indian_Ocean_tsunami_bulletins).

Signs and warnings

Malé, the capital of Maldives, was severely hit.

Despite a lag of up to several hours between the earthquake and the impact of the tsunami, nearly all of the victims were taken completely by surprise; there were no tsunami warning systems in the Indian Ocean to detect tsunamis, and equally importantly, warn the general populace living around the ocean quickly. Tsunami detection is not easy because while a tsunami is in deep water it has a very low height and a network of sensors is needed to detect it. Setting up the communications infrastructure to issue timely warnings is an even bigger problem [38] (http://www.newscientist.com/article.ns?id=dn6839).

Scientists were also hampered by the fact that the initial estimate for the magnitude of the earthquake was 8.1. The determination that the earthquake had actually been much stronger (and the resulting tsunami much larger) was not made until after the tsunami had already struck.

Tsunamis usually occur in the Pacific Ocean due to earthquakes in the "Ring of Fire", and an effective tsunami warning system has long been in place there. Although the extreme western edge of the "Ring of Fire" extends into the Indian Ocean (the point where this earthquake struck), no warning system exists in that ocean due to the rarity of tsunamis there; the last major one was caused by the Krakatoa eruption of 1883.

In the aftermath of the disaster there is a new awareness of the need for a tsunami warning system for the Indian Ocean. The UN has started working on an Indian Ocean Tsunami Warning System and aims to have initial steps in place by end 2005 [39] (http://www.un.org/apps/news/story.asp?NewsID=12932&Cr=tsunami&Cr1=). Some have even proposed creating a unified global tsunami warning system, to include the Atlantic Ocean and Caribbean. See International Early Warning Programme.

See also the 2004 Indian Ocean tsunami timeline, a minute to minute account by the National Oceanic and Atmospheric Administration (NOAA).

Unfamiliarity with warning signs

The first warning sign of a possible tsunami is the earthquake itself; however, tsunamis can strike thousands of miles away, where the earthquake is only felt weakly or not at all. Also, in the minutes preceding a tsunami strike the sea often recedes temporarily from the coast. People in Pacific regions are more familiar with tsunamis and often recognize this phenomenon as a sign to head for higher ground. However, around the Indian Ocean, this rare sight reportedly induced people, especially children, to visit the coast to investigate and collect stranded fish on as much as 2.5 km of exposed beach, with fatal results [40] (http://www.npr.org/templates/story/story.php?storyId=4246573).

One of the few coastal areas to evacuate ahead of the tsunami was on the Indonesian island of Simeulue, very close to the epicentre. Island folklore recounted an earthquake and tsunami in 1907 and the islanders fled to inland hills after the initial shaking — before the tsunami struck [41] (http://www.timesonline.co.uk/article/0,,18690-1422835,00.html). On Maikhao beach in northern Phuket, Thailand, a 10 year old British girl named Tilly Smith had studied tsunamis in geography class at school and recognised the warning sign of the receding ocean. She and her parents warned others on the beach, which was evacuated safely [42] (http://www.telegraph.co.uk/news/main.jhtml?xml=/news/2005/01/01/ugeog.xml&sSheet=/portal/2005/01/01/ixportaltop.html).

Damage and casualties

Devastation to Banda Aceh on the island of Sumatra as a result of the tsunami caused by the 2004 Indian Ocean earthquake (Credit: DigitalGlobe)

The reported death toll from the earthquake, the tsunami and the resultant floods varies widely due to confusion and conflicting reports, but could total to over 235,000 people with tens of thousands reported missing, and over a million left homeless. Early news reports after the earthquake spoke of a toll only in the "hundreds", but the numbers rose steadily over the following week.

Relief agencies report that one-third of the dead appear to be children. This is a result of the high proportion of children in the populations of many of the affected regions and the fact that children were the least able to resist being overcome by the surging waters.

In addition to the large number of local residents, up to 9,000 foreign tourists (mostly Europeans) enjoying the peak holiday travel season were among the dead or missing, especially Scandinavians. The European nation hardest hit may have been Sweden, which reported more than 60 dead and as many as 1,300 missing [43] (http://news.bbc.co.uk/1/hi/world/asia-pacific/4134345.stm).

States of emergency were declared in Sri Lanka, Indonesia, and the Maldives. The United Nations has declared that the current relief operation will be the costliest ever. UN Secretary-General Kofi Annan has stated that reconstruction would probably take between five and ten years. Governments and NGOs fear the final death toll may double as a result of diseases, prompting a massive humanitarian response.

Measured in lives lost, this is one of the ten worst earthquakes in history([44] (http://neic.usgs.gov/neis/eqlists/eqsmosde.html)). It is also the single worst tsunami in history; the previous record was the 1703 tsunami at Awa, Japan, that killed over 100,000 people ([45] (http://www.pbs.org/wnet/savageseas/neptune-side-tsunamis.html)).

For purposes of establishing timelines of local events, the time zones of affected areas are: UTC+3: (Kenya, Madagascar, Somalia, Tanzania); UTC+4: (Mauritius, Réunion, Seychelles); UTC+5: (Maldives); UTC+5:30: (India); UTC+6: (Bangladesh, Sri Lanka); UTC+6:30: (Cocos Islands, Myanmar); UTC+7: (Indonesia (western), Thailand); UTC+8: (Malaysia, Singapore). Since the quake occurred at 00:58:53 UTC, add the above offsets to find the local time of the quake. A list of times can be found at a USGS site (http://neic.usgs.gov/neis/bulletin/neic_slav_tz.html).

Country where deaths occurred	Deaths		Injured	Missing	Displaced
	Confirmed	Estimated1
Indonesia	173,981 (http://story.news.yahoo.com/news?tmpl=story&u=/nm/20050123/ts_nm/quake_dc&cid=564&ncid=2100)	220,000 (http://news.bbc.co.uk/1/hi/world/asia-pacific/4204385.stm)	~100,000 (http://www.channelnewsasia.com/killerwaves)	6,245 (http://www.cnn.com/2005/WORLD/asiapcf/01/19/asia.tsunami/index.html)	400,000- 700,000 (http://www.telegraph.co.uk/news/main.jhtml?xml=/news/2005/01/24/wtsun24.xml&sSheet=/news/2005/01/24/ixworld.html)
Sri Lanka	38,195 (http://news.scotsman.com/latest.cfm?id=4010478)	38,195	15,686 (http://www.cnn.com/2004/WORLD/asiapcf/12/29/asia.quake/index.html)	23,000+ (http://www.cnn.com/2004/WORLD/asiapcf/12/29/asia.quake/index.html)2	~573,000 (http://www.reliefweb.int/w/rwb.nsf/UNID/D39A0A882D6A7E9985256F82006A1158?OpenDocument)
India	10,744 (http://www.ndmindia.nic.in/Tsunami2004/sitrep35.htm)	16,413 (http://www.ndmindia.nic.in/Tsunami2004/sitrep35.htm)	—	5,669 (http://www.ndmindia.nic.in/Tsunami2004/sitrep35.htm)	380,000 (http://www.ndmindia.nic.in/Tsunami2004/sitrep33.htm)
Thailand	5,305 (http://www.channelnewsasia.com/killerwaves)3	11,000 (http://www.abc.net.au/news/newsitems/200501/s1275176.htm)	8,457 (http://www.disaster.go.th/news01/12_47/news_after_shock_34.pdf)	4,499 (http://www.disaster.go.th/news01/12_47/news_after_shock_34.pdf)	—
Somalia	150+ (http://www.reliefweb.int/w/rwb.nsf/UNID/D39A0A882D6A7E9985256F82006A1158?OpenDocument)	298 (http://www.channelnewsasia.com/stories/afp_world/view/125915/1/.html)	—	—	5,000 (http://www.reliefweb.int/w/rwb.nsf/UNID/D39A0A882D6A7E9985256F82006A1158?OpenDocument)
Myanmar (Burma)	59 (http://www.channelnewsasia.com/stories/afp_asiapacific/view/126491/1/.html)	290 (http://news.bbc.co.uk/2/hi/asia-pacific/4145489.stm)–  600 (http://www.asianews.it/view.php?l=en&art=2255)	45 (http://www.irrawaddy.org/aviewer.asp?a=4248&z=24)	7,000 (http://www.alertnet.org/thenews/newsdesk/BKK286589.htm)4	3,200 confirmed (http://www.channelnewsasia.com/stories/afp_asiapacific/view/126491/1/.html)
Malaysia	68 (http://www.irrawaddy.org/default.asp)–
	Results from FactBites:
	2004 Indian Ocean earthquake: Information from Answers.com (6352 words) The 2004 Indian Ocean earthquake, known by the scientific community as the Sumatra-Andaman earthquake, was an undersea earthquake that occurred at 00:58:53 UTC (07:58:53 local time) December 26 2004 with an epicentre off the west coast of Sumatra, Indonesia. The sudden vertical rise of the seabed by several metres during the earthquake displaced massive volumes of water, resulting in a tsunami that struck the coasts of the Indian Ocean. The deadliest earthquakes since 1900 were the Tangshan, China earthquake of 1976, in which at least 255,000 were killed; the earthquake of 1927 in Xining, Qinghai, China (200,000); the Great Kanto earthquake which struck Tokyo in 1923 (143,000); and the Gansu, China, earthquake of 1920 (200,000). Indian Ocean Earthquake and Tsunami (6942 words) The devastating megathrust earthquake of December 26th, 2004 occurred on the interface of the India and Burma plates and was cause by the release of stresses that develop as the India plate subducts beneath the overriding Burma plate. A tsunami (1) was triggered by this earthquake in the Bay of Bengal. The ocean current is driven from the Bay of Bengal to the Arabian Sea during the North-East monsoon and from the Arabian sea to the Bay of Bengal during the South-West monsoon.
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