BY ANDREW
BRIDGES
Associated Press
LOS ANGELES -- 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 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.