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The earthquake that caused last year's tsunami significantly increased the chance that another large earthquake — and possibly another tsunami — could occur, says research published today (17 March) in Nature.

The greatest immediate threat to the area off the coast of Sumatra, Indonesia, is an earthquake measuring 7.0–7.5 on the Richter scale, says the study.

The researchers, led by John McCloskey at the University of Ulster, United Kingdom, found that last year's earthquake put a large amount of stress on two fault zones — regions where there are numerous fractures in the Earth's crust — called the Sunda trench and the Sumatra fault.

The fault zones are already prone to earthquakes, but the increased stress caused when December's earthquake struck means the risk of another major earthquake has risen significantly.

Suleyman Nalbant, one of McCloskey's co-authors, told SciDev.Net that although the risk of another earthquake has increased, the team could not predict when it might actually occur.

Nalbant said that if an earthquake occurs in a 'subduction zone', a region where the one of the plates of the Earth's crust slides beneath the next one, it could generate another tsunami.

The researchers say that this threat highlights the urgency of setting up a tsunami warning system in the Indian Ocean, adding that, as in December 2004, earthquakes in the Sunda trench in 1833 and 1861 both caused fatal tsunamis.

"It is also very important that people are educated about the danger of tsunamis," says Nalbant. Many deaths could have been avoided last year if people had recognised signs of danger and run to higher land, he explains.

Research shows that major subduction-zone earthquakes like the one that shook Sumatra often come in pairs, separated by a period ranging from a few years to as little as a few months.

In the Nankai trough subduction zone, off the coast of Japan, five of the seven large earthquakes in the past 1,500 years were followed by similar events within five years. For three of these, the second earthquake followed within one year.

The researchers plan to undertake further studies to better assess the risk of an earthquake and predict where it might occur.

Link to full paper by McCloskey et al in Nature

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

Reference: Nature 434, 291 (2005)