The string of seismic events began when an enormous earthquake occurred off the West coast of Sumatra today (26 December 2004) at 11:59am ESST (0.58 UT). It measured magnitude 8.9.

The epicentre was approximately 320 km west of Medan, Sumatra. This earthquake is the result of plate collision, caused by the Australian plate moving northwards about 7cm per year. "This is the largest earthquake in the world for forty years, and the fifth largest in the world since 1900," said Dr Cvetan Sinadinovski, Duty Seismologist with Geoscience Australia.

This animation (MPG) illustrates how the large tsunami caused by the Great 1833 Sumatra earthquake spreads throughout the Indian Ocean basin. This earthquake was very similar in size and location to the one which occurred on Sunday, 26 Dec, 2004. While the tsunami struck the coast of Sumatra within minutes, it takes several hours to travel the breadth of the Indian Ocean.

This animation illustrates how the large tsunami caused by the Great 1833 Sumatra earthquake spreads throughout the Indian Ocean basin. This earthquake was very similar in size and location to the one which occurred on Sunday, 26 Dec, 2004. While the tsunami struck the coast of Sumatra within minutes, it takes several hours to travel the breadth of the Indian Ocean.

See also http://www.ga.gov.au/image_cache/GA5059.pdf