2004 Indian Ocean earthquake and tsunami

• 1. Introduction

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  The 2004 Indian Ocean earthquake began at 07:58:53 local time on the 26th of December 2004, deep beneath the sea off the west coast of Aceh, in northern Sumatra, Indonesia. Seismologists call it the Sumatra-Andaman earthquake. The wall of water it set loose has two popular names. British and Commonwealth media called it the Boxing Day tsunami, after the holiday, while others spoke of the Asian tsunami.

  Across the Indian Ocean, the waves killed an estimated 227,898 people in 14 countries. No tsunami in history has killed more, and no other natural disaster of the 21st century has matched that toll. Some coasts were struck within minutes. Others felt the surge only many hours later.

  Three questions run through this story. How can a break in the ocean floor make the entire planet shake? Why did almost nobody on those shores know what was coming? And why did some people and some stretches of coast come through when their neighbours did not?

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• 2. Rank Among Giants

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  Hiroo Kanamori of the California Institute of Technology judges 9.2 to be the most representative size for the quake, yet the number took years to settle. Early reports gave a moment magnitude of 8.8. The United States Geological Survey keeps an official estimate of 9.1, but most recent studies point to 9.2-9.3. A 2016 study arrived at 9.25. A 2021 study revised its own 2007 estimate from 9.1 to 9.2.

  On any of those figures, this is the strongest earthquake of the 21st century and the most powerful ever recorded in Asia. Since modern seismography began in 1900, only two earthquakes are recorded with a greater magnitude. They are the 1960 Chile earthquake at 9.5 and the 1964 Alaska earthquake in Prince William Sound at 9.2. That places the 2004 event second or third in the world.

  Only two other recorded earthquakes reached magnitude 9.0 or more. One struck off Kamchatka, Russia, in November 1952 at 9.0. The other hit Tohoku, Japan, in March 2011 at 9.1. Each of these giants also sent tsunamis across the Pacific. Their death tolls were far lower, mainly because fewer people lived along the nearby coasts.

  Seismologists measure the overall size of an earthquake with a quantity called seismic moment. Of all the moment released worldwide in the 100 years from 1906 through 2005, roughly one-eighth came from this single quake. Together with the Alaskan and Chilean events, it accounts for almost half the total.

  Comparisons with older earthquakes are difficult, because strength was not measured systematically until the 1930s. Researchers sometimes estimate historical quakes from descriptions of damage and from geological records. The 1868 Arica earthquake in Peru and the 1700 Cascadia earthquake in western North America were assessed this way.

  Before 2004, three arguments counted against a giant earthquake near Sumatra. The subducting plate there is older and denser, and it was believed that only young, buoyant crust could produce giant quakes. Plate motion is slow, and it was believed that convergence had to be fast. The region also has back-arc basins, and giant quakes were thought to occur only where such basins are absent. After the event, regions once judged low-risk on these criteria needed a fresh assessment.

  In hindsight, a warning of sorts had come. The 2002 Sumatra earthquake is believed to have been a foreshock, arriving over two years before the main event.

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• 3. A Fault Torn Open

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  The rupture stretched between 1200 and 1300 km, the longest fault break ever observed, and the faulting lasted at least ten minutes, also a record. It began about 160 km off the coast of northern Sumatra, just north of Simeulue island. The origin lay 30 km below sea level, although early reports had put it at 10 km.

  This stretch of seabed is a meeting line between two plates. The Indian plate, part of the larger Indo-Australian plate, travels north-east at an average of 60 mm a year. At the Sunda Trench it dives beneath the Burma plate, which is regarded as part of the great Eurasian plate. The Burma plate carries the Nicobar Islands, the Andaman Islands and northern Sumatra.

  The slip came in two phases over several minutes. The first phase tore open a patch about 400 km long and 100 km wide, 30 km beneath the sea bed. It is the largest rupture ever known from a single earthquake. It raced north-west from Aceh at about 2.8 km/s for roughly 100 seconds.

  Then the fault paused for about another 100 seconds. When it resumed, the break ran north toward the Andaman and Nicobar Islands at a slower 2.1 km/s. It continued for another five minutes, until it reached a boundary where the fault changes from subduction to strike-slip. At such a boundary, the two plates slide past one another in opposite directions.

  Along the fault line the ground moved about 10 m sideways and 4-5 m vertically. The seabed rose by several metres. The Andaman and Nicobar Islands appear to have shifted south-west by around 1.25 m and sunk by 1 m. Early speculation held that some small islands south-west of Sumatra had moved as much as 36 m. More accurate data, released over a month later, found a shift of only about 0.2 m.

  In February 2005, a Royal Navy vessel surveyed the seabed with a high-resolution, multi-beam sonar system. Depths in the zone range from 1000 to 5000 m. The survey showed that ridges 1500 m high, built by earlier geological activity, had collapsed into landslides several kilometres wide. One slide was a single block of rock about 100 m high and 2 km long.

  The momentum of the displaced water had also dragged slabs of rock weighing millions of tonnes as far as 10 km across the seabed. An oceanic trench several kilometres wide lay exposed.

  Secondary splay faults, sometimes called pop-up faults, pushed long, narrow strips of the seabed upward within seconds. That sudden lift increased the height and speed of the waves. It helped destroy the nearby Indonesian town of Lhoknga.

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• 4. A Ringing Planet

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  A seismometer in the U.S. state of Oklahoma recorded the ground moving vertically by 3 mm. Across the globe, the whole planet vibrated by as much as 10 mm. Huge surface waves of the Rayleigh type exceeded 10 mm of vertical motion everywhere on Earth, a result of the quake's enormous energy and its shallow rupture depth.

  Records from the IRIS/USGS Global Seismographic Network let scientists time the waves like runners on a track. The earliest, faintest signal was the compressional wave, which took about 22 minutes to reach the far side of the planet, near Ecuador. The largest surface waves reached that spot after about 100 minutes and reinforced one another there. They then circled the globe and returned to the epicentral region after about 200 minutes.

  Just after that 200-minute mark, the closest stations picked up a major aftershock of magnitude 7.1. Under ordinary circumstances it would have been a headline quake, but beside the mainshock it is dwarfed.

  The planet kept ringing for months. A week after the quake, its reverberations could still be measured, and they supplied valuable data about the Earth's interior. By February 2005 the effect had shrunk to an oscillation of just 20 micrometres. It merged into the Earth's constant natural oscillation more than four months after the quake.

  The shift of mass also altered the Earth's spin. Models published weeks afterwards suggested that the quake shortened the day by 2.68 microseconds, because it reduced the slight flattening of the globe. The planet may also have wobbled on its axis by up to 25 mm in the direction of 145 degrees east longitude, or perhaps by 50 or 60 mm.

  None of this lasted. The Moon's tidal effects lengthen the day by an average of 15 microseconds per year, so the quake's change was quickly lost. The natural Chandler wobble, which in some cases reaches 15 m, eventually offset the small wobble as well.

  Numerous aftershocks were reported in the hours and days after the main event, off the Andaman Islands, the Nicobar Islands and the original epicentre. Aftershocks of up to magnitude 7.2 continued to shake the region daily for three or four months.

  The magnitude 8.6 Nias-Simeulue earthquake of 2005 struck off the Sumatran island of Nias. Despite its proximity, it is not considered an aftershock, and it was most likely triggered by stress changes from the 2004 event. It produced its own aftershocks, some as high as magnitude 6.9.

  Three days before the main quake, a magnitude 8.1 earthquake hit near the uninhabited Auckland Islands, west of New Zealand, and Macquarie Island, south of Australia. Quakes of magnitude eight or more occur only about once a year on average. The U.S. Geological Survey sees no evidence that the two events were causally linked.

  The giant quake is also thought to have stirred volcanoes. It is believed to have triggered activity at Leuser Mountain and Mount Talang in Aceh, while the 2005 Nias-Simeulue earthquake sparked activity in Lake Toba, a massive caldera in Sumatra.

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• 5. Crossing the Ocean

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  Moving at 500 to 1000 km/h across deep water, the tsunami was only a low, broad hump, barely noticeable and harmless. Near the shore it slowed to tens of kilometres per hour. In doing so it piled up into large, destructive waves.

  A tsunami that causes damage far from its source is sometimes called a teletsunami. Vertical motion of the seabed is much more likely to produce one than horizontal motion. Here the seabed rose by several metres and displaced an estimated 30 cubic kilometres of water. The lifted seafloor also reduced the capacity of the Indian Ocean, producing a permanent rise in global sea level of an estimated 0.1 mm.

  Two satellites, TOPEX/Poseidon and Jason-1, happened to pass over the tsunami as it crossed the ocean. Their radars measure the height of the water surface precisely, and they recorded anomalies on the order of 500 mm. The maximum was 600 mm, two hours after the earthquake. These were the first deep-water observations of a tsunami ever made.

  Such readings are valuable because tide gauges at the shore must account for how the coast changes the size and shape of a wave. Measurements from mid-ocean avoid that problem and can be used to work out the parameters of the source earthquake.

  Tad Murty, vice-president of the Tsunami Society, put the total energy of the waves at about 5 megatons of TNT. That is more than twice the explosive energy used in all of World War II, including the two atomic bombs. It is still a couple of orders of magnitude below the energy released by the earthquake itself.

  Scientists investigating the damage in Aceh found that the wave reached 24 m along large stretches of coastline. In some areas it rose to 30 m as it travelled inland. In many places the water pushed as far as 2 km from the shore.

  Because the fault ran nearly north to south, the waves were strongest in an east-west direction. Bangladesh, low-lying and fairly near the epicentre, still had few casualties. The northern part of the rupture moved more slowly, which greatly reduced the energy of the water displacement there.

  Distance and shelter proved poor guides to safety. Waves can bend around a landmass, so the Indian state of Kerala, on the country's western coast, was hit, and the western coast of Sri Lanka suffered substantial impacts. Somalia, much farther away than Bangladesh, was hit harder.

  Arrival times ranged from fifteen minutes to seven hours. Northern Sumatra was struck quickly, while Sri Lanka and the east coast of India were hit roughly 90 minutes to two hours later. Thailand was struck about two hours later even though it was closer, because the waves travelled more slowly through the shallow Andaman Sea.

  At Struisbaai in South Africa, about 8500 km away, a 1.5 m surge came ashore about 16 hours after the earthquake. The delay probably reflects the broad continental shelf there and the westward path along the coast. In Antarctica, tidal gauges at Japan's Showa Base recorded oscillations of up to a metre that lasted a couple of days.

  Part of the energy escaped into the Pacific. Along the western coasts of the Americas it produced small but measurable tsunamis, typically 200 to 400 mm. At Manzanillo, Mexico, a wave height of 89.3 cm was measured, and the waves were detected as far as Vancouver, Canada.

  Many scientists were puzzled that some South American readings were larger than readings in parts of the Indian Ocean. One theory holds that the mid-ocean ridges along the margins of the continental plates focused the waves over long distances.

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• 6. Reading the Sea

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  The last major tsunami in the Indian Ocean came with the 1883 eruption of Krakatoa. Tsunamis are far more frequent in the Pacific Ocean, where earthquakes along the Ring of Fire are common. The extreme western edge of that ring reaches the Indian Ocean, at the very point where this quake struck, yet no warning system existed there.

  Tsunami detection is hard because a wave in deep water has little height, and a network of sensors is needed to catch it. Even with a delay of up to several hours between the earthquake and the waves, nearly all of the victims were taken by surprise.

  Not every large earthquake brings a large tsunami. On the 28th of March 2005, an earthquake reported at magnitude 8.7 struck roughly the same area and caused no major tsunami.

  The earthquake itself is the first warning sign, but tsunamis can strike thousands of kilometres away where the shaking is weak or absent. In the minutes before impact, the sea sometimes pulls back from the coast. This happened on the eastern side of the rupture zone, including Aceh, Phuket island, Khao Lak in Thailand, Penang island in Malaysia, and the Andaman and Nicobar islands.

  The retreat exposed as much as 2.5 km of beach and drew people, especially children, to look and to collect stranded fish. The results were fatal. Not all tsunamis give this signal, and in some cases the sea simply swells with no retreat, leaving people little time to flee.

  On Maikhao Beach in north Phuket City, Thailand, a 10-year-old British tourist named Tilly Smith recognised the danger. She had studied tsunamis in geography at school and spotted the receding ocean and the frothing bubbles. She and her parents warned others, and the beach was evacuated safely.

  John Chroston, a biology teacher from Scotland, saw the same signs at Kamala Bay, north of Phuket. He took a busload of vacationers and locals to higher ground.

  On the Indonesian island of Simeulue, close to the epicentre, folklore did the work of an alarm system. Islanders remembered an earthquake and tsunami in 1907, and after the first shaking they fled to the inland hills before the waves struck. Simeulue was one of the few coastal areas to evacuate ahead of the tsunami.

  Anthropologists had feared for the aboriginal peoples of the Andaman Islands, and especially for the already depopulated Onge tribe, which they worried could be wiped out. Many tribes evacuated and suffered fewer casualties, helped by oral traditions from earlier earthquakes. The Onge folklore speaks of a "huge shaking of ground followed by high wall of water", and almost all the Onge people seem to have survived.

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• 7. Coast by Coast

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  In Banda Aceh, the closest major city, the tsunami arrived about 20 minutes after the earthquake. Eyewitnesses described the sea withdrawing near the port of Ulèë Lheuë after a gentle first rise. Two large, black, steep waves followed and rolled into the capital as a turbulent bore. Witnesses called it a "black giant", a "mountain" and a "wall of water".

  The wave height fell from 12 m at Ulèë Lheuë to 6 m about 8 km to the north-east. Flow depths in the seaside district exceeded 9 m, and the water inundated 3-4 km inland. Within 2-3 km of the shore, nearly every structure was swept away or destroyed, apart from strongly built reinforced concrete houses with brick walls. Footage showed the Aceh River flowing backwards, carrying debris and people up to 40 km inland.

  Lhoknga, a beach town known for surfing, lies about 13 km south-west of Banda Aceh. Locals reported 10 to 12 waves, with the second and third the highest. Black waves about 30 m high approached with explosion-like sounds. The second wave, one witness said, was "taller than the coconut trees and was like a mountain". It stranded cargo ships and barges and reached the fourth level of a cement mining facility near Lampuuk.

  Meulaboh, a remote coastal city, was among the hardest hit. The sea drew back about 500 m, and the destructive second and third waves reached 5 km inland. Helicopter surveys showed entire settlements virtually destroyed, and only a few mosques remained standing. The greatest run-up measured anywhere was 51 m, on a hill between Lhoknga and Leupung. The heavy death toll was attributed mainly to a lack of preparation and limited education about tsunamis.

  Sri Lanka lies about 1700 km from Sumatra, and the tsunami struck around two hours after the earthquake. It hit the east coast first, bent around the southern tip at Dondra Head, and then flooded the south-west after some energy reflected off the Maldives. About 35,000 people died there, the second-highest civilian toll after Indonesia. Roughly 90,000 buildings were destroyed.

  At Galle, amateur video showed a flood carrying debris and sweeping people away. At the resort town of Beruwala, a brown-orange bore reached the first level of a hotel. The largest run-up, 12.5 m, was measured at Yala. Seawalls and breakwaters reduced the waves at some places, and the degraded natural environment contributed to the high death tolls.

  A regular passenger train between Maradana and Matara, carrying over 1,750 passengers, was derailed and overturned by the water. At least 1,000 people died, the largest single rail disaster death toll in history. A high-water mark on a nearby building put the water 2-3 m above the top of the train.

  Thailand lies about 500 km from the epicentre. The tsunami struck its south-western coast about two hours after the quake, at high tide, while the region was crowded with Christmas tourists who had no warning. About 8,000 people died. Khao Lak, in Phang Nga Province, suffered the most, with about 5,000 deaths.

  A video shot from a hill by a local restaurant manager showed the sea retreating from Khao Lak beach as people gathered fish. Moments later a wall of foaming water slammed into the coast. A German family filmed a white line on the horizon that grew until it engulfed a jet skier and lifted two police boats.

  Offshore reefs and shallow seafloor piled the water higher at Khao Lak. The wave reached the third floor of a resort hotel and inundated about 2 km inland. The highest run-up outside Sumatra, 19.6 m, was recorded at Ban Thung Dap on the south-west tip of Ko Phra Thong Island.

  At Patong Beach on Phuket, a small flood first swept away cars. About ten minutes later the sea receded, and then a wall of water towered over the skyline. On the Phi Phi Islands, Israeli tourists filmed a creeping flood that grew until it covered a beach and resort and carried a yacht out to sea. Scuba divers around the Similan and Surin Islands reported a violent, swirling current underwater.

  In India, the tsunami reached Tamil Nadu, Andhra Pradesh and Kerala about two hours after the earthquake. At Marina Beach in Chennai, the water swept across the sand and took morning walkers unaware. In Karaikal, 492 people died. Pondicherry, protected by seawalls, came through relatively unscathed.

  Nagapattinam district in Tamil Nadu was the worst affected, with 6,051 deaths from a 5 m tsunami. Most of the victims were members of the fishing community. At Velankanni, a coastal town with a Catholic basilica and a pilgrimage destination, the waves struck around 9.30 am on a Sunday. Pilgrims were inside attending a Malayalam Mass. The rising water did not enter the shrine, but the receding water swept away hundreds of pilgrims on the beach. About 600 died, and many unidentified bodies were buried in mass graves.

  Kerala was hit because the waves diffracted around Sri Lanka. Its southernmost district, Thiruvananthapuram, had negligible impact, since the waves would have needed a wide-angle turn to reach it. The worst destruction lay on narrow strips of land between the Arabian Sea and the backwaters, where geography blocked inland escape. The Alappad panchayat saw the most deaths, about 131, from waves of 4 m. Seawalls built of loose stones were displaced, and the stones were carried a few metres inland.

  In the Andaman and Nicobar Islands, the waves took only minutes to arrive. On Car Nicobar, at Malacca, the first wave came 5 minutes after the earthquake, and the third and strongest reached 11 m. Waves nearly three stories high devastated the Indian Air Force base nearby, and four oil tankers were thrown almost 800 m from the shore to the air force colony's main gate.

  Katchall Island was the worst affected island in the chain. Of a population of 5,312, 303 people were confirmed dead and 4,354 were missing. In South Andaman, a 20 MW diesel power plant at Bamboo Flat was destroyed. Steep mountainous outcrops shielded Port Blair and Campbell Bay and kept the waves lower there.

  The Maldives lie 2500 km from the epicentre, and the tsunami arrived there about two hours after the quake. Survivors reported three waves, with the second the most powerful. The highest wave measured was 4 m at Vilufushi Island. In the capital, Male, water reached knee level in the streets and spread 250 m inland. Offshore coral reefs, deep channels between atolls, and arrival at low tide all weakened the impact. Some feared the country might be submerged entirely, but that proved untrue.

  Myanmar's coast saw waves of only 0.4-2.9 m, with 71 casualties. Many residents never felt the earthquake. The Myeik Archipelago shielded some coasts, but the flat delta land offered no high ground for escape.

  Somalia was hit about 5000 km from the source, around noon local time, by four waves. Around 289 people died along a 650 km stretch of coast between Garacad and Hafun, part of Puntland province. In Hafun, 19 people died and 160 were presumed missing out of about 5,000 inhabitants, the highest toll for a single African town. The maximum run-up, almost 9 m, was recorded at Bandarbeyla.

  Elsewhere, the sea claimed two lives in Yemen, three in Seychelles, ten in Tanzania and one in Kenya. Peninsular Malaysia was largely shielded by the island of Sumatra. The furthest fatality directly attributed to the tsunami occurred at Rooi-Els, near Cape Town, 8000 km from the epicentre. Tidal surges along the Western Australian coast lasted for hours, set boats adrift and left two people needing rescue.

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• 8. After the Water

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  The Tsunami Evaluation Coalition's final report gives the 227,898 figure used here. The UN Office of the Special Envoy for Tsunami Recovery gives a slightly higher total of 229,866. Indonesia bore most of the loss, with most estimates around 170,000 and some as high as 172,761.

  An initial report from Siti Fadilah Supari, then Indonesia's Minister of Health, suggested as many as 220,000 dead in Indonesia alone, and 280,000 overall. The estimate of dead and missing in Indonesia was later reduced by over 50,000. The coalition cautioned that "all such data are subject to error, as data on missing persons especially are not always as good as one might wish". Higher numbers have also been suggested for Myanmar, based on reports from Thailand.

  Relief agencies reported that about one third of the dead appeared to be children. Many affected regions have a high proportion of young people, and children were the least able to resist the surging water. Oxfam reported that in some regions as many as four times more women than men were killed. Women had been waiting on the beach for the fishers to return and looking after children in houses.

  Up to 9,000 foreign tourists, mostly Europeans enjoying the peak holiday season, were among the dead or missing, especially people from the Nordic countries. Sweden suffered most in relation to its population, with a death toll of 543. France followed with 542, and Germany with 539 identified victims.

  States of emergency were declared in Sri Lanka, Indonesia and the Maldives. The United Nations estimated at the outset that the relief operation would be the costliest in human history. Kofi Annan, then UN Secretary-General, said reconstruction would probably take between five and ten years. Governments and aid groups feared disease might double the toll, which prompted a worldwide humanitarian response. Donations totalled more than US$14 billion.

  The economic damage depended on the scale examined. National economies took a minor hit, but local economies were devastated. Fishing and tourism suffered most. In Sri Lanka, industrial fishery directly employs about 250,000 people, and preliminary estimates said 66% of the fishing fleet and industrial infrastructure in coastal regions was destroyed.

  Over 51,000 vessels were lost across the region. That created a boom in the boatyards of Tamil Nadu, which built fibreglass-reinforced plastic catamarans. The rush lowered quality, and some materials were sacrificed to cut prices for people impoverished by the tsunami.

  Tourism also felt the ripple. Extensive media coverage led many tourists to cancel trips to that part of the world, even to places the water never touched. Inland provinces such as Krabi in Thailand, a starting point for many other destinations, felt the loss. Even untouched beach resorts saw cancellations, and tourists proved reluctant to return for psychological reasons.

  The environmental harm lasted for years. Mangroves, coral reefs, forests, coastal wetlands, dunes and groundwater were all damaged. Saltwater poisoned freshwater supplies and soil, and a layer of salt settled over arable land. In the Maldives, 16 to 17 coral atolls overcome by the waves were left without fresh water. Thousands of rice, mango and banana plantations in Sri Lanka were destroyed almost entirely.

  The Colombo-based International Water Management Institute monitored saltwater effects in Sri Lanka and concluded that wells recovered to pre-tsunami drinking water quality one-and-a-half years after the event. The institute also developed protocols for cleaning contaminated wells, which the World Health Organization endorsed as part of its Emergency Guidelines.

  The Australian government sent ecological experts to help the Maldives, Seychelles and other areas plan reef monitoring and rehabilitation, drawing on experience from the Great Barrier Reef. The United Nations Environment Programme set up an emergency fund and a Task Force. Between 2004 and 2007 it distributed approximately US$9.3 million for environmental recovery and disaster risk reduction, with funding from Finland, Norway, Spain, Sweden and the United Kingdom.

  Evidence suggested that mangroves had given some protection, compared with areas cleared for aquaculture or development. Mangrove restoration therefore became the focus of a number of projects, with varied success. Such ecosystem-based approaches work best when local communities are involved as stakeholders throughout and when planners study the physical conditions of each site to ensure that mangroves can thrive there.

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