Sept. 12, 2008 -- After a magnitude 7.9 earthquake struck central China last May, killing 69,000 people, injuring hundreds of thousands and leaving millions homeless, geologists believe there may be more devastation to come.
Stress on the nearby Kunlun, Xianshuihe and Min Jiang faults has increased, they say, doubling the risk that one of them will unleash a magnitude 6.0 or 7.0 tremor in the next decade.
Led by Shinji Toda of the Geological Survey of Japan, a team of researchers used data from the massive quake to calculate how tectonic stresses in Earth's crust were redistributed. They found that the Longmen Shan fault, which slipped between 6 and 9 meters (19.7 to 29.5 feet) during the powerful quake was relieved of stress. But faults to the northwest and southwest loaded up on tension, making them more prone to rupture.
Over the next decade, the team estimates there's a 50-50 chance of a magnitude 6.0 quake or greater and a 10 percent chance of a quake of 7.0 or higher on one of the three faults.
"You tend to think a big earthquake relieves stress in the crust, but there are areas that were pushed closer to failure," team member Ross Stein of the United States Geological Survey said.
The idea that a powerful earthquake could lead to chain reactions first took hold in early 2005. The Indian Ocean coastline from Indonesia to Somalia was recovering from the horrifying December 2004 tsunami, which had been sparked by a magnitude 9.2 quake off the coast of Sumatra.
On March 16, 2005 seismologists warned that the quake had caused stress build up on the Sunda and Sumatra faults, and that they were primed for another massive temblor. Twelve days later their calculations were grimly verified: a magnitude 8.7 earthquake struck on the Sunda fault off Sumatra's coast, killing 1,300 people.
"For the 2005 Sumatra quake, researchers made a prediction that seemed to be confirmed" Eric Kirby of Pennsylvania State University said. "[The work in China] will be another one of those tests."
But even in highly active seismic regions, earthquakes in excess of magnitude 7.0 are rare, and it might be years before additional stresses from a big quake trigger another one in its wake.
So the team working in China made an additional forecast. They estimate that if their calculations are correct, the increased stress on the Kunlun, Xianshuihe, and Min Jiang faults will cause an increased rate of small quakes of around magnitude 3.2 or greater. The normal background rate of small quakes each decade should double or triple. If it does, a big one could soon follow.
"This doesn't mean that what we do is prediction in any sense of the word," Stein said. "But a calculation like this can have a bearing on assessing earthquake hazard in a region."
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