LOS
ANGELES -- Scientists 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 notice -- enough time to send school children diving
below their desks or to cut the flow of gas through pipelines,
scientists said. Details appear today in the journal Science.
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 important --
magnitude of an earthquake from as little as four seconds of
measurements of the P wave.
"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, because 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.