The 2004 Indian Ocean tsunami was triggered by an undersea earthquake off Sumatra. The earthquake occurred off the west coast of northern Sumatra, Indonesia, near Aceh, along the Sunda megathrust where tectonic plates meet.
On 26 December 2004, the Indian Plate was forced beneath the Burma Plate in a massive subduction-zone rupture. The seafloor shifted vertically over a huge area, displacing an enormous volume of seawater and generating a series of tsunami waves. The earthquake is generally estimated at magnitude 9.1–9.3.
Northern Sumatra and Aceh were hit first and suffered catastrophic destruction. The waves then spread across the Indian Ocean, reaching Sri Lanka and India in about 90 minutes to two hours, Thailand in roughly two hours, and East Africa many hours later.
A common mix-up is naming Sri Lanka or Thailand because those countries suffered severe damage and high casualties. They were impact zones, however, not the earthquake's source region. Bali and Java are Indonesian islands, but the rupture responsible for this event was located off northern Sumatra.