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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On the Net:
Science: http://www.sciencemag.org
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