2004 Indian Ocean earthquake and tsunami

On the morning of 26 December 2004, one of the largest earthquakes ever measured ruptured the seabed off northern Sumatra and sent tsunami waves racing across the Indian Ocean. Over the following hours the waves killed roughly 228,000 people in fourteen countries, most of them in Indonesia, Sri Lanka, India and Thailand. It is the deadliest tsunami on record, and it prompted one of the largest international relief efforts ever mounted.

The shock struck at 07:58 local time, with its epicentre between the island of Simeulue and the mainland of Sumatra. It was caused by a slip along the boundary where the Indian plate dives beneath the Burma plate. Estimates of its moment magnitude range from 9.1 to 9.3, which places it among the three strongest earthquakes since 1900 and makes it the most powerful ever recorded in Asia.

The fault broke over a length of about 1,200 to 1,300 kilometres, the longest rupture yet observed, and the shaking lasted at least ten minutes. The sea floor was pushed up by several metres, displacing an enormous volume of water along the whole length of the break. The earthquake made the entire planet vibrate measurably.

In deep water the tsunami was a low swell travelling at 500 to 1,000 kilometres an hour; near shore it slowed and reared up. It reached the coast of Aceh, at the northern tip of Sumatra, within about twenty minutes. Waves of 15 to 30 metres hit the province's west coast, and the water ran up to 51 metres on a hillside near Lhoknga. In Banda Aceh the flood reached more than three kilometres inland and swept away most houses within two to three kilometres of the shore.

Sri Lanka and the east coast of India were struck around one and a half to two hours after the earthquake, and Thailand about two hours after, because the waves slowed over the shallow Andaman Sea. Waves curved around Sri Lanka to hit its western shore and reached the Indian state of Kerala. The tsunami went on to strike Somalia and the low-lying Maldives, caused a death near Cape Town some 8,000 kilometres away, and was detected as far as Mexico, Canada and Antarctica.

The Indian Ocean had no tsunami warning system, so almost everyone was taken by surprise despite the hours between the earthquake and the waves' arrival on distant shores. In several places the sea first drew back, and people walked out onto the exposed seabed. A few communities were saved by memory: the islanders of Simeulue, recalling an earthquake and tsunami of 1907, fled to the hills, and traditions among the Onge people of the Andaman Islands helped them survive. On a beach in Phuket, a ten-year-old British girl who had studied tsunamis at school raised the alarm in time for the beach to be cleared.

Counts of the dead differ: the Tsunami Evaluation Coalition gave 227,898, while a UN office gave 229,866. Aceh suffered by far the worst, with Indonesia's toll estimated at well over 160,000; Sri Lanka lost around 35,000 people. About a third of the victims are believed to have been children. Up to 9,000 foreign tourists, mainly Europeans on Christmas holidays, were among the dead or missing, with Sweden the hardest-hit European country.

Governments, charities and individuals around the world pledged more than 14 billion US dollars. Fears of epidemics led to hurried mass burials, and the World Food Programme fed more than 1.3 million people. In Aceh, the disaster helped end a long conflict: the Free Aceh Movement declared a ceasefire two days after the tsunami, and a peace agreement with the Indonesian government followed on 15 August 2005. An Indian Ocean Tsunami Warning System was set up in 2005.

Our 2000 and 2010 maps show the countries around the Indian Ocean before and after the disaster, including Indonesia, India and Sri Lanka.