Indian Ocean tsunami

At about 7:58 a.m. local time on December 26, 2004, a megathrust earthquake of magnitude about 9.1 ruptured off the west coast of northern Sumatra, Indonesia. The fault tore along roughly 1,200 to 1,300 kilometers of the sea floor over about eight to ten minutes, one of the longest ruptures ever recorded, and lifted the sea bed by several meters. The displaced water formed a tsunami that crossed the Indian Ocean at jet-plane speed, striking Sumatra within minutes and Sri Lanka and India within two hours. Waves reached about 30 meters in places along Aceh's coast. About 230,000 people died in 14 countries, most of them in Indonesia, with large losses also in Sri Lanka, India and Thailand.¹

The cause was the India Plate sliding beneath the Burma microplate along the Sunda Trench, the kind of subduction zone that plate tectonics theory identifies as the source of the largest earthquakes. Plate boundaries lock for centuries until stress is released at once. Although the Pacific Ocean had a tsunami warning system since the 1940s, the Indian Ocean had none, so no official alert reached coastal towns hours away from the source, and many residents had no experience of a tsunami and did not run to higher ground when the sea drew back.

The disaster drew one of the largest international relief efforts in history, with governments and the public pledging billions of dollars. In Aceh, where a long separatist conflict had been underway, the shared catastrophe helped speed a peace agreement in 2005. The United Nations organized an Indian Ocean Tsunami Warning System, which began operating in 2006, with seismic and deep-sea sensors and national warning centers. The tsunami also changed public awareness of tsunami signs worldwide, and its lessons shaped warning efforts elsewhere.¹