US: August 17, 2007


LONDON - Earthquakes on long, straight faults can rupture faster than previously thought and trigger powerful shock waves that make quick-moving quakes even more destructive, according to a new study published on Thursday.


The findings could help researchers pinpoint the most dangerous faults in the world and better predict areas potentially at risk of greater damage from earthquakes, said Shamita Das, who led the study.

"Given the potential for increased destruction, we must take such information into account when planning earthquake-resistant construction worldwide," Das, a seismologist at Oxford University, wrote in the journal Science.

Earthquakes generally move, or rupture, at a pace of about 2.5 kilometres to 3 kilometres per second, a speed many researchers for a long time believed quakes could not exceed, Das said.

But this study bolsters theories that they can, in fact, move much faster than that, she added.

Using more powerful computers, higher quality seismograms and new imaging techniques, the researchers showed that a 2001 earthquake in Central Tibet along the Kunlun fault line moved at about 5 kilometres per second. That quake was a magnitude 7.8.

The study also found that long, straight fault lines create the opportunity for these fast-moving quakes in an effect Das compared to driving in a car on a straight road where it is possible to build up a lot of speed.

"If the road bends you have to slow down, or if you have a bump you have to slow down," she said in a telephone interview. "Earthquakes behave in a similar way."

After showing that quakes could move much faster, Das turned her attention to California's San Andreas fault and found that many parts of it run in the kind of long, straight lines that make it prone to these quick quakes.

The fault -- which triggered the 1906 San Francisco quake and deadly fires that followed -- runs from the Southern California desert to northern California and marks the boundary of the Pacific and North American tectonic plates.

"We know there are going to be more earthquakes on the San Andreas Fault," she said. "If the next one behaves this way, there would be a greater potential for damage than previously believed."

These highly destructive quakes which can generate powerful shock waves -- similar to the sonic boom from a supersonic aircraft -- are also most likely to hit in so-called strike-slip quakes in which two sides of the fault slide past one another horizontally, she said.

She noted that thrust-fault earthquakes where plates go underneath each other -- like the one that struck Peru on Wednesday and killed hundreds of people -- would probably not produce such shock waves.

But many other earthquakes probably have and the next step is to identify faults around the world that could in the future fuel these more powerful quakes, Das said.


Story by Michael Kahn


REUTERS NEWS SERVICE