This week's earthquake struck off the Indonesian island of Nias

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A powerful earthquake struck off the coast of Sumatra, Indonesia, on Monday (28 March), killing about 1,000 people according to the latest estimates.

Unlike the earthquake that hit the same region on 26 December 2004, the quake did not trigger a tsunami. The difference in the depth and strength of the two earthquakes could explain why.

Measuring 8.7 on the Richter scale, Monday's earthquake was probably 12-15 times less powerful than its predecessor, a crucial difference as the likelihood that an earthquake will cause a tsunami by moving the seabed increases with the amount of energy it releases.

This week's quake is also thought to have occurred about 30 kilometres deeper than last year's.

Earlier this month John McCloskey and colleagues at the University of Ulster, United Kingdom, warned that stress caused by December's earthquake had greatly increased the risk of another severe earthquake measuring 8.0 to 8.5 on the Richter scale (see Tsunami quake 'increased risk of further disasters').

Link to full BBC Online news stories

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