The Batu section of the Sunda trench, south of the Mentawai islands, last ruptured in 1935 and has slowly slipped ever since. As a result, McCloskey said, the total stresses there are probably too low to cause a giant rupture.
Of greater concern to McCloskey and colleagues is the section of the trench south of Siberut, which is at the northern end of the Mentawai islands.
This section last ruptured in 1797, which means it has more than 200 years of accumulated stress waiting to be released. The seismic history of this section indicates that major quakes strike there about every 230 years.
According to the seismologists' calculations, the stress buildup south of Siberut is sufficient to produce an earthquake of magnitude 8.5, with a high potential for a tsunami, McCloskey said.
In addition, the new calculations show that the March earthquake expanded Sumatra fault' stressed section by about 125 miles (200 kilometers).
The fault runs the length of Sumatra. Previous calculations from the stress induced by the December earthquake suggested the region immediately south of the hard-hit city of Banda Aceh was at great risk for further temblors.
"That [threat] hasn't gone away," McCloskey said. "There's no sense [that the March event] has relieved any stress there. Rather, it has increased the area of stress along the Sumatra fault."
Roger Bilham, an earthquake hazard specialist at the University of Colorado at Boulder, said McCloskey and his colleagues are doing the right thing by making their findings public. But he cautioned that the findings are not a prediction of when the next earthquake will occur.
"There's no question the strain changes have increased in the region on the faults they mention," Bilham said. But, he added, seismologists lack sufficient knowledge of how the Earth moves beneath its crust to predict when the next earthquake will occur.
The Mentawai section of the Sunda trench might rupture next week, or it might rupture in 50 years. "Basically, we'd be fools to ignore this" stress buildup, Bilham said.
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