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The 26 December 2004 earthquake and resulting tsunami in the Indian Ocean were disasters of epic proportions. The earthquake was the second largest ever recorded, and the tsunami killed more than 280,000 people.

The earthquake was also the first major one to be observed with modern instruments, from satellites and digital seismometers to global positioning system (GPS) networks that recorded the movements of the Earth's crust.

A special section in this week's edition of Science provides an overview of what these instruments recorded and scientists' interpretations of what happened.

Two papers by Thorne Lay of the Earth Sciences Department of the University of California at Santa Cruz, United States, Charles Ammon of the Department of Geosciences at Pennsylvania State University, United States, and their colleagues describe the rupture caused by the earthquake.

They show how the quake lasted nearly ten minutes, compared to a few seconds for a moderate earthquake.

It ruptured a 1,300-kilometre stretch of the boundary between the Indo-Australian and Eurasian plates of the Earth's crust, travelling at 2.5 kilometres a second and triggering a displacement of the Earth's surface of up to 15 metres off the Sumatran coast.

The northern end of the rupture zone took longer to slip, which meant that the tsunami's impact was concentrated further south. Had the northern end slipped as fast, many more lives would have been lost.

In a separate paper, published online by Science, R. Banerjee of the Wadia Institute of Himalayan Geology at Dehra Dun, India, and colleagues, estimate that the 26 December quake was about twice as powerful as previously thought, measuring 9.15 on the Richter scale.

Using data from GPS stations around the Pacific and Indian oceans, the team concludes that the earthquake led to a measurable deformation of the Earth's surface as far as 4,500 kilometers away.

The amount of energy released by the earthquake was similar to the total power used by the entire United States over six months. In a four-day period in January 2005, there were more than 150 aftershocks measuring above 5.0 on the Richter scale.

An accompanying commentary by Roger Bilham of the Cooperative Institute for Research in Environmental Sciences and Geological Sciences at the University of Colorado, United States, says that many seismologists are reassessing their conservative estimates of future earthquake risks elsewhere.

Bilham calls the 26 December 2004 earthquake "a wake-up call that conservative seismic forecasts may not serve society well".

Link to full article by Roger Bilham

Link to full paper by Lay and colleagues

Link to full paper by Ammon and colleagues

Link to full paper by Banerjee and colleagues

Read more about tsunamis in SciDev.Net's Tsunami update.
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