China quake raised stresses; aftershocks to come
WASHINGTON, July 6 (Reuters) - The giant earthquake that devastated China's Sichuan region in May, killing more than 70,000 people, worsened geological stresses on important faults in the area, U.S. geologists said on Sunday.
These stresses are likely to lead to large aftershocks, some greater than 150 miles (240 km) away, they reported in the journal Nature.
The team at the U.S. Geological Survey said their method could also predict the likelihood of quakes elsewhere.
"The 12 May 2008 magnitude 7.9 earthquake that struck the eastern Sichuan region caused grievous losses, yet its legacy includes possible large aftershocks in the near future because it increased failure stress on important faults within and around the Sichuan basin," Tom Parsons of the USGS in Menlo Park, California and colleagues wrote.
"Given that delays of years to decades between mainshocks and large aftershocks are commonly observed around the world, identifying potential future rupture zones will be useful in focusing mitigation efforts," they added.
The Sichuan quake struck the eastern edge of the Tibetan plateau, causing losses exceeding 1 trillion yuan ($146 billion), and destroying the homes of 10 million people.
Seismologists have already recorded at least 150 aftershocks above a magnitude of 4, with the highest a 6, and say a major aftershock of 6.9 is still possible.
"The mainshock of the 12 May earthquake ruptured with as much as 9 metres (30 feet) of slip along the boundary between the Longmen Shan and Sichuan basin, and demonstrated the complex strike-slip and thrust motion that characterizes the region," Parsons and colleagues wrote.
They used stress-transfer analysis, a method used to predict big aftershocks after an enormous quake hit Sumatra in Indonesia in December of 2004.
Such big quakes distort surrounding rocks, said geophysicist John McCloskey of the University of Ulster.
"Rapid methods for estimating the details of the slip distribution in the mainshock -- where and by how much the fault moved during the earthquake -- mean that the size of these stress changes can be calculated quickly," he said in a statement.
"Parsons and his team have illustrated how this can be done rapidly following an earthquake and they have shown clearly that several large faults which have experienced increases in stress big enough to trigger other large earthquakes.
Some of their calculated stresses are many times bigger than those known to be responsible for triggering great earthquakes in the past." (Reporting by Maggie Fox, editing by Alan Elsner)
