London, May 10 : UK and Chinese geophysicists have developed a new approach to seismic monitoring that should allow forecasting of the time, size and sometimes the location of earthquakes.
According to a report in Nature News, the new method exploits a phenomenon called shear-wave splitting, in which seismic 'shear' waves travelling through rocks are split into two components, which vibrate parallel and perpendicular relative to microscopic cracks.
These two shear wave components travel through the ground at different speeds, and so reach detectors at different times.
The alignment of these microscopic cracks reflects the amount of stress in the Earth's crust. The more stress, the more the cracks are aligned, and the bigger the gap between the two waves' arrival.
In the past, researchers have gauged the build-up of strain in earthquake zones indirectly, by looking at satellite maps of surface ground movement.
But this gives only a rough idea of the stress changes deeper down, where earthquakes originate. Stresses have also been monitored directly at fault lines, but such localized data doesn't show when and where slippage will occur.
"Local effects are chaotic and unpredictable," said Stuart Crampin of the University of Edinburgh. "This unpredictability can be eliminated, however, by monitoring stress changes over a much wider area," he added.
In October 1999, seismic stations in southwest Iceland reported an increase in shear-wave splitting time delays at a rate similar to that seen before a magnitude 5.1 earthquake four months earlier.
Crampin and his colleagues forecast that another quake of this magnitude would happen "soon", or a magnitude-6 within three months.
There was a magnitude-5 quake three days later, which offered a hint that shear-wave splitting might be providing important information.
Crampin and his colleagues now say that detailed monitoring can make more accurate forecasts of time, size and place.
They list 15 other quakes from California to China that have been preceded by characteristic changes in shear-wave splitting time delays, giving an indication of the timing and size of the coming event.
According to them, a quake's location can be deduced from other factors, including how and where the rate of change in the shear-wave time delays abruptly slows hours to weeks before the event.
This may be caused by stress release, as small cracks merge into larger ones before the rupture.
Crampin has determined that a system could be set up for monitoring all damaging quakes in a region within 400 km of a single three-borehole stress-monitoring site, for around 4 -10 million dollars.
Crampin and colleagues have proposed the establishment of a global network of 1,500 such sites.
"It's very ambitious, certainly, but a huge amount of evidence demonstrates that it would work," he said.