The Sumatra, Indonesia, earthquake on 26 December 2004 was one of the most devastating earthquakes in history. With a magnitude of Mw = 9.3 (revised based on normal-mode amplitudes by Stein and Okal, http://www.earth.northwestern.edu/people/seth/research/sumatra.html), it is the second largest earthquake recorded since 1900. It occurred about 100 km off the west coast of northern Sumatra, where the relatively dense Indo-Australian plate moves beneath the lighter Burma plate, resulting in stress accumulation.
The average relative velocity of the two plates is about 6 cm/yr. On 26 December 2004, however, the two plates moved by a distance of several meters, releasing the stress accumulated over hundreds of years. The result was a devastating tsunami that hit coastlines across the Indian Ocean, killing about 300,000 people in Indonesia, Sri Lanka, India, Thailand, Burma, Malaysia, Somalia, and other countries (Guardian, 29 January 2005, http://www.guardian.co.uk/tsunami/story/0,15671,1380895,00.html).