The 2004 Indian Ocean earthquake had a magnitude of 9.1. It struck off the west coast of northern Sumatra on December 26, 2004, when a massive section of the seafloor ruptured along the boundary between the India and Burma tectonic plates.
The undersea rupture displaced the ocean floor and generated a devastating tsunami that crossed the Indian Ocean. Waves struck coastlines in Indonesia, Sri Lanka, India, Thailand, the Maldives, and parts of East Africa. More than 220,000 people died or were reported missing, making it one of the deadliest natural disasters in modern history.
Some references give the earthquake a magnitude of 9.1 to 9.3 because estimates vary with the measurement method and later reanalysis. However, 9.1 is the standard value used by the U.S. Geological Survey and is a fair canonical answer. A common mix-up is confusing the earthquake's magnitude with the tsunami's wave height or the overall death toll.