Tremor from Past Caused Sichuan Quake
Connor, Steve, The Independent (London, England)
Discovery may explain unexpected earthquakes and help with prediction, says Steve Connor
SOME OF the most violent earthquakes that have occurred unexpectedly in places with no recent record of tremors may be the aftershocks of earthquakes that took place decades or even centuries ago, scientists have discovered.
The finding could explain many unexpected earthquakes that hit the centre of continental shelves, such as the disastrous quake in Sichuan in the heart of China in May 2008 which killed at least 68,000 people and injured up to 400,000 more. At 7.9 on the Richter scale, it was one of the most deadly quakes in history.
Earthquakes usually occur at the boundary of two or more tectonic plates. But they can also occur many hundreds of miles from a fault line, and it is these earthquakes that scientists believe may be the result of long aftershocks, rather than background seismic activity.
A laboratory study that tested how tectonic faults work has found that the further away an earthquake is from such faultlines, the stronger the likelihood that it could be the long aftershock of a previous earthquake.
Mian Liu, professor of geological sciences at the University of Missouri-Columbia, said that scientists have tried to predict the occurrence of larger earthquakes by looking at the frequency of smaller ones, which is why the Sichuan earthquake was a surprise.
"Until now, we've mostly tried to tell where large earthquakes will happen by looking at where small ones do," Professor Liu said. …