May 26, 2008	Monday	Jamadi-ul-Awwal 20, 1429

PARIS, May 25: A major quake such as the one that left at least 60,000 dead in southwestern China this month can trigger other earthquakes half way around the world, according to a study released on Sunday.

This unexpected finding could one day help make better predictions about the frequency and intensity of aftershocks, the lead researcher said.

A team of geologists in the United States found that 12 out of 15 major quakes — registering a magnitude of 7 or higher — since 1990 generated surface waves that set off smaller seismic events in fault systems on distant continents.

The China quake, which measured 8 on the Richter scale, was not included in the study, which was published in the British journal Nature Geoscience.

“It was known that these surface waves could travel,” explained co-author Tom Parsons of the US Geological Survey.

“But most scientists thought these so-called dynamically-triggered earthquakes were a special case. In fact, they happen all the time, everywhere, and that was something of a surprise,” he said in an interview.

The terrible December 2004 mega-quake off the coast of Sumatra in Indonesia, for example, provoked seismic events as far away as Alaska, California and Ecuador.

There is a better than 95 per cent likelihood that the earthquake rate in distant areas will be much higher in the immediate aftermath of a big quake than before or after, the study found.

And while the seismic movements triggered by faraway quakes were generally smaller — in the three-to-five magnitude range — there is no reason they could not be as big or bigger than the first.

“They could be any size,” said Parsons, who in previous research identified eight cases in the last quarter century in which a 7-or-bigger earthquake led to another that was even larger.

“The big question is aftershocks, and what happens after you have a big earthquake,” said Parons, adding that there were two competing theories as to how such follow-on quakes were unleashed.

Static triggering occurs within a few fault lengths of the main rupture, often in a cascading effect. But impact generally peters out beyond a 100-to-200km radius.

The spike in quake activity further afield can only be explained by dynamic triggering, explained Parsons.—AFP