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Scientists Propose Quake Alert
System
By ANDREW BRIDGES AP
Science Writer May 01, 2003
Email this story.
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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