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QUAKE LENGTH WREAKS HAVOC

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When an earthquake strikes, the first thing people want to know is the magnitude: How big was it?

But the magnitude, expressed by numbers on the Richter scale, is only part of a complex story, one that can't be told with a single number, says Caltech geologist Hiroo Kanamuri. An earthquake's duration may be just as important.In a presentation to the American Geophysical Union, he made the point dramatically, by comparing the October Loma Prieta earthquake that rocked San Francisco - magnitude 6.9 - with the utter devastation of the Armenian quake a year earlier - also magnitude 6.9.

Magnitude is measured by the strength of the initial waves monitored by seismographs. But in the case of the San Francisco quake, the shaking lasted only about 20 seconds. In Armenia, it lasted more than a minute.

Although inferior building materials and construction methods contributed to the horrific death toll in Armenia, they were not the whole story, Kanamuri says. Engineers say that had the San Francisco quake lasted even 10 seconds longer, the consequences might have been much worse. Bridges, highways and buildings that were weakened might have collapsed completely.

Duration is not the only aspect of earthquakes that fascinates Kanamuri. He is installing what may be the world's most sophisticated network of seismic monitors in Southern California. The first of the digitized stations is in place and has already produced "a lot of interesting results," Kanamuri says. He expects it to provide far more detailed information about future earthquakes than has ever been gathered.

"We have many different faults here," he said, and this network will enable scientists "to understand the difference in behavior of these different faults."

By providing such detailed information, the network may reveal precursors that would help in earthquake prediction. And, as a byproduct of this work, the network will also allow detailed studies of the Earth's interior structure, by measuring how earthquake waves bend and bounce around the planet.

The network will provide unusually detailed information about earthquakes anywhere in the world, including variations in quake duration, which has been little studied. "That's a very important parameter," Kanamuri says. "It's really not widely understood, even among seismologists."