The worst may not be over for the people of China's Sichuan province. New seismic studies by an international team show that the major earthquake that occurred there earlier this year could have increased stress on two adjoining faults, hiking the risk of a moderately powerful temblor on each. The findings could give scientists a much better handle on the dynamics of fault zones in some of the world's most populated areas. They may also provide researchers with "truly testable prospective forecasting," says Tom Parsons, a geophysicist at the U.S. Geological Survey in Menlo Park, California, who was not involved with the research.
On 12 May, the Wenchuan quake struck along the Longmenshan fault zone near Chengdu, a city of 11 million in the south-central part of the country. The event registered at magnitude 7.9 and killed almost 70,000 people outright; another 18,000 are missing and presumed dead. Earthquakes are common in Asia, but this one was unusual, because it occurred well within the continent, along a fault that had not registered a major event in thousands of years. The quake was also next to two much-more-active seismic zones. That allowed a team comprised of scientists from Japan, Europe, and the United States to test an idea gaining popularity among geophysicists: that instead of reducing stress by releasing energy, a major earthquake in one fault zone could actually increase stress in neighboring zones.
The researchers collected all public seismic data about the Wenchuan temblor and combined it with records from the past decade. They then used a computer model to calculate increased stresses and earthquake risks along the Xianshuihe fault to the southwest and the Kunlun-Min Jiang fault to the northwest. Geophysicist and co-author Ross Stein, also of the U.S. Geological Survey in Menlo Park, likens the process to taking a square slab of stiff rubber and making three unconnected cuts into it in the shape of a Z. When you slide the sections on each side of the center cut in opposite directions, you can then watch how the disturbance stresses the other cuts and the rest of the slab.
Stein says the team members figured that if they could calculate precisely how much deformity the side-slipping Wenchuan quake caused in the surrounding crust, they could infer increased stresses and predict future activity in the region. Based on the model's predictions, the team reports 9 September in Geophysical Research Letters that there should be a doubling of quake activity of magnitude 3 and above along the other two faults over the next decade--with most of the activity along the Xianshuihe fault. "It's very active and we're worried about it," Stein says.
These kinds of calculations could improve understanding of the dynamics of multiple fault systems, says Stein, and might even help quake forecasting in places such as southern California, where a significant quake along, say, the San Andreas fault could set off activity in the Hayward, Northridge, or other nearby faults.