By ANDREW BRIDGES
AP Science Writer
LOS ANGELES (AP)--Scientists working in Southern California have
proposed a way of interpreting feeble tremors that herald a large
earthquake, a step that could help in providing advance warning.
The system theoretically could give anywhere from seconds to tens
of seconds of advance notice--enough time to send school children
diving below their desks or to cut the flow of gas through pipelines
vulnerable to rupture, scientists said. Details appear Friday in the
journal Science.
Similar systems already are used in California and Japan on a
smaller scale. The latest system would not predict or forecast
earthquakes, but rather interpret the staggered way in which a
quake's energy travels to the surface.
The first indication at the surface that a large earthquake has
occurred is typically the jolt caused by the arrival of a
fast-moving but low-energy wave called the primary or P wave.
It is followed by the more energetic but slower-moving S or shear
wave that causes far more violent shaking.
Richard Allen of the University of Wisconsin-Madison and Hiroo
Kanamori of the California Institute of Technology developed a way
to determine the location, origin, time and--most importantly _
magnitude of an earthquake from as little as four seconds of
measurements of the P wave. The system would rely on seismic
instruments already deployed across the greater Los Angeles region.
``If we can detect this P wave and use the information contained
in it to estimate the hazard associated with an earthquake, then
there is the potential to issue a warning before any significant
ground motion reaches the surface,'' Allen said.
The amount of forewarning would depend on the distance of the
sensors from an earthquake's epicenter.
If directly above the epicenter, there would be no time for a
warning, since the S wave would arrive almost immediately after the
P wave. At 37 miles from the epicenter, the system could give a
magnitude estimate 16 seconds before the arrival of the S wave and
the strong ground motion that accompanies it, Allen said.
Allen and Kanamori are now testing their system on the regular
earthquakes in the Los Angeles region to determine if it can provide
accurate magnitude estimates in real time. There are no immediate
plans to develop an actual warning system.
The study may settle the question of whether earthquakes of
different magnitudes begin in different ways, said Lucy Jones,
scientist in charge of the U.S. Geological Survey office in
Pasadena, Calif. The study suggests they do.
``There's debate whether quakes start differently or if a
(magnitude-) 6 is just a 2 that doesn't stop,'' said Jones, who was
not connected with the study.
___
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Science: http://www.sciencemag.org
AP-NY-05-01-03 1753EDT
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