Alarm system may warn of quakes Sensors would recognize By Andrew Bridges, Associated Press Scientists have proposed
a means of interpreting the initial, feeble tremors that herald the
arrival of a large earthquake to give residents of Southern
California advance warning of more violent shaking to come.
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 today in the journal Science.
The alarm system would not predict or forecast earthquakes.
Instead, it would exploit the staggered way in which energy travels
from the underground source of quakes 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 shear, or S 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 frequency of the energy in the P wave. The
system would rely on a network of 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 of a major quake, 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.
A similar system is already in use in Japan, where individual
sensors are used to provide early warnings.
Following the 1989 Loma Prieta earthquake that rocked the San
Francisco Bay area, seismologists tracked P waves to warn rescue
workers on a section of collapsed freeway about oncoming
aftershocks. And a Nevada company has sold siren devices triggered
by P waves to several fire stations in California.
Other systems in place in Taiwan and Mexico rely on measurements
of the peak ground motion associated with the S wave to relay
warnings to locations far from an earthquake's epicenter but still
vulnerable to shaking.
Allen and Kanamori used data from past earthquakes to simulate
how their system would work. They are now testing it on the regular
earthquakes that shake the Los Angeles region to determine if it can
provide accurate magnitude estimates in real time.
There are no immediate plans to go on and develop an actual
warning system.
If developed, it would work best if plugged into a region's
infrastructure, allowing it to automatically prompt shutdowns of
everything from trains to factories during major quakes, Allen said.
Such a system would allow a utility to quickly cut power to its
grid, minimizing but not eliminating damage, said Philip Mo, a
structural engineer with Southern California Edison, a Rosemead,
Calif., utility.
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: http://www.sciencemag.org/
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