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The 2004 Indian Ocean earthquake was an undersea megathrust earthquake that occurred at 00:58:53 UTC on Sunday, 26 December 2004, with an epicentre off the west coast of Sumatra, Indonesia. The quake itself is known by the scientific community as the Sumatra–Andaman earthquake. The resulting tsunami is given various names, including the 2004 Indian Ocean tsunami, South Asian tsunami, Indonesian tsunami, and the Boxing Day tsunami.

The earthquake was caused by subduction and triggered a series of devastating tsunamis along the coasts of most landmasses bordering the Indian Ocean, killing over 230,000 people in fourteen countries, and inundating coastal communities with waves up to 30 meters (98 ft) high. It was one of the deadliest natural disasters in recorded history. Indonesia was the hardest-hit country, followed by Sri Lanka, India, and Thailand.

With a magnitude of Mw 9.1–9.3, it is the third largest earthquake ever recorded on a seismograph. The earthquake had the longest duration of faulting ever observed, between 8.3 and 10 minutes. It caused the entire planet to vibrate as much as 1 centimetre (0.4 inches)and triggered other earthquakes as far away as Alaska. Its epicentre was between Simeulue and mainland Indonesia. The plight of the affected people and countries prompted a worldwide humanitarian response. In all, the worldwide community donated more than $14 billion (2004 US$) in humanitarian aid.