Scientists analyzing the aftermath of the December earthquake under the Indian Ocean warned Thursday that another devastating quake is now far more likely to strike the region. The seismic slip off the coast of Sumatra, Indonesia, that triggered the tsunami has piled dangerous levels of stress onto two vulnerable parts of the fault zone, significantly raising the chances of a magnitude 7.5 earthquake. The scientists cannot accurately predict how soon such an earthquake might occur, but they point out that previous examples of so-called coupled earthquakes have struck within a year of each other.
John McCloskey of the University of Ulster in Coleraine, who led the research, said: "Many of us are brought up to understand hazard as whenever you've had your bit of bad luck it doesn't happen again. Lightning never strikes twice. But one great indicator that you're going to have an earthquake is that you've just had one."
He added: "These are very significant and extensive increases in stress. We cannot say for certain it will result in an earthquake, but it's the biggest stress increase over a large area that we've measured since we started doing this research."
A powerful undersea earthquake rocked parts of the region Wednesday but there were no reports of damage or casualties. The quake, which registered 6 on the Richter scale, struck about 19 miles beneath the seabed off Aceh province in Indonesia but did not cause a tsunami.
The new study shows that one of the regions at increased risk of a more powerful event is a 31-mile stretch of the undersea Sunda trench, next to the 745-mile-long zone that ruptured in December. Earthquakes in the Sunda trench triggered fatal tsunamis in 1833 and 1861.
Not all big undersea earthquakes cause tsunamis, but the scientists say their results emphasize the urgent need for a warning system in the Indian Ocean. Countries in the region and U.N. experts agreed on plans for a system last week at a meeting in Paris, but it will not be completed until the end of next year. Until the network of tidal gauges and seabed sensors is in place, Japan and the United States plan to issue alerts on seismic activity in the Indian Ocean.
The second area of concern identified in the new research is a 185-mile region of fault running directly beneath the island of Sumatra, close to the city of Banda Aceh, which was devastated in December and where rebuilding work is underway. The scientists estimate that stress in the Sunda trench region has increased by up to five bars; in the Sumatra fault it has been forced up by as much as nine bars.
McCloskey said there is a worrying recent precedent to consider: In 1998 seismologist Suleyman Nalbant, one of the authors of the new study, used the same technique to show that local seismic activity had increased stress by two bars on a 31-mile stretch of the Anatolian fault in Turkey, which has a very similar structure to the Sumatra fault. Less than 18 months later, the Turkish fault gave way near the city of Izmit and triggered a magnitude 7.4 earthquake that killed 20,000 people. He also warned that earthquakes in subduction zones -- where one continental plate passes underneath another, as happens at the Indian Ocean boundary -- frequently have struck in pairs.
In the Nankai trough to the southeast of Japan, five of the seven large earthquakes over the last 1,500 years have been followed by a similar event within five years. Three occurred within 12 months.
McCloskey said: "It's by studying and reanalyzing what happened in the past that we are able to have some confidence in the relationship between the stresses we're measuring and the occurrence of other earthquakes. But it should be stressed that the mapping is not one to one. It does not mean there will be another earthquake within a year or two, but certainly the risk has increased significantly as a result of what happened in December."
His team used computer simulations of the earthquake prepared by an American team at Caltech in Pasadena, Calif., to re-create movement of the surrounding area. Because some regions of the affected fault slipped further than others, the resulting redistribution of stress through the ground was patchy and uneven -- meaning seismologists could not be sure at first whether the risk of a second quake was raised or lowered.
To find out, McCloskey's group used mathematical models of elastic materials, which essentially view the Earth as a giant rubber ball. For several points along the faults in the two danger zones they worked out whether the movement of the surrounding rocks freed the two surfaces to slide past each other or clamped them together, making an earthquake more or less likely. The results appear in the journal Nature.
Peter Styles, the president of the Geological Society, said: "It has become apparent over the last 10 years that when a major earthquake occurs it changes the stress in adjacent areas. Sometimes this can serve to lock the fault, but sometimes it can make another failure more likely. Every effort should be made to ensure that appropriate monitoring technologies and communication protocols are put in place to monitor the Indian Ocean."
Nick Ambraseys, a seismologist at Imperial College in London, said: "There is nothing in [the new study] that enables, with any degree of certainty, the prediction of the immediacy of the next earthquake -- except that an earthquake such as those of 1833 and 1861 is likely to occur sometime in the future. False alarms and inaccurate timing could create more problems than already exist."
In separate research, marine scientists in the United States have highlighted the risk of a tsunami in the Caribbean, where there is no warning system. The team from the University of North Carolina and the University of Texas say that more than 35 million people could be affected if a powerful earthquake struck along the boundary between the North American and Caribbean tectonic plates. At least 10 significant tsunamis have been recorded in the region since 1492, six of which are known to have caused loss of life.