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 today 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.
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 testing their system to
determine if it can provide accurate magnitude estimates
in real
time.
Home
