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Scientists Propose Quake Alert System
By ANDREW BRIDGES
AP Science Writer
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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