Seismic waves released by some earthquakes can release so much energy that the Earth actually rings like a bell.

The scale of such events is matched by the devastation they cause. Only a few weeks ago 4,000 people died in the earthquake that hit Afghanistan. Such disasters have made earthquake prediction an urgent topic of research, but so far the science has been relatively ineffectual. Research published recently in Physical Review Letters could provide a breakthrough.It might seem perverse, but researchers suggest that finding what makes earthquakes stop could prove more useful than discovering how they start.

Earthquakes occur because the plates that make up the Earth's crust are slowly moving relative to each other along the fault lines that divide the plates. This movement builds up stresses that eventually "go critical."

Seismographs detect a million or more earthquakes around the Earth every year, but only 50,000 or so of these are large enough to be noticed by the population in a region.

However, according to the model that is currently used to simulate and help predict earthquakes, there is nothing to stop every single tremor growing and growing until it shoots off the Richter scale.

"The simple theory that everyone believes has it that the crack should go on forever," says John Rundle, professor of geological science at the University of Colorado. "As it grows larger, its presence concentrates more stress on the crack's tips, which drives it further on. So the question is, if you have a small earthquake, why doesn't it grow into a magnitude 9 earthquake?"

Magnitude 9 corresponds to the most powerful earthquakes on record (the Afghanistan quake was magnitude 6.9). The 1964 magnitude 9 earthquake that hit Alaska resulted in a crack in the Earth's crust 620 miles long.

The existing models assume that these stresses are uniform in the rock around the area of a crack, but Rundle and his colleagues have now called this into question. They made a different assumption: that there must be large variations in the stresses, and these variations hold the earthquake back.

Recognizing the signature of an impending disaster may enable them to predict events up to six months in advance, Rundle says.

Dist. by Scripps Howard News Service