Subsequent calculations by McCloskey and colleagues show this displacement and slip resulted in a large buildup of stress in a 31-mile (50-kilometer) stretch of the Sunda trench just south of the rupture zone. They saw an even stronger loading of stress along a 186-mile (300-kilometer) section of the Sumatra fault near Banda Aceh.
"We are very confident in our calculations, and it is clear that there is a relationship between these increased stresses and the increased risk of further activity on either fault," McCloskey said. "However, how this relationship works out in detail is not known."
For example, he added, there is not a one-to-one relationship between earthquake occurrence and stress increases. More research is needed to understand why.
Kerry Sieh is a geologist at the California Institute of Technology who has studied the Sumatra tectonic system for the past ten years. He said the findings of McCloskey and colleagues will come as little surprise but are nevertheless a nice quantitative analysis of the stress buildup in the region.
"Whether or not that means an immediate threat of another big earthquake to the south is a much more complex question to answer," he said. "When a fault is being loaded up by a nearby event, a number of factors go into making the evaluation of whether it will rupture soon."
Ross Stein is a seismologist with the U.S. Geological Survey in Menlo Park, California. He said the stress calculations by McCloskey and colleagues are reasonable but cautioned that they are not a calculation of the earthquake risk.
The last time a major earthquake struck the southern section of the Sunda trench was in 1866. "In other words, it has stored up quite a bit of stress on its own. Add to it the sudden hit from the earthquake and it would surprise no one if it would rupture again," he said.
According to Sieh, the last large and devastating earthquake anywhere along the Sunda trench was in 1907. Though seismologists are uncertain as to the exact 1907 epicenter, it appears to be the section immediately south of the 2004 rupture.
Sieh added that he believes the section of the trench at most risk for failure lies between about 186 and 620 miles (300 to 1,000 kilometers) south of the 2004 epicenter. That section fails about every 200 years. It last ruptured in a couplet of giant earthquakes in 1797 and 1833.
"A repeat of that episode would produce devastation along the West Sumatran coast similar to that along the coast of Aceh last year," he said.
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