Quake's first seconds may affect strength
By Alicia Chang, Associated Press
LOS ANGELES — Scientists have found a way to
estimate an earthquake's ultimate strength by analyzing the initial
seconds of a rupture — a step that could one day provide early
earthquake warning.
Currently, a quake's magnitude — or how much
energy is released — is determined after the shaking stops, usually
minutes after an event.
But researchers from the University of
California, Berkeley, say the measurements of seismic waves soon
after a temblor can signal whether it will be a minor or monster
temblor.
They say the information could possibly be used
in an alert system to give seconds to tens of seconds of advance
notice of an impending quake — enough time for schoolchildren to
take cover, power generators to trip off and valves to shut on
pipelines.
"We're not taking about a massive amount of
time," said Richard Allen, an assistant professor of earth and
planetary science, who led the study. "But one can use our approach
to come up with a magnitude before people have felt the ground
shaking."
Details appear in Thursday's issue of the
journal Nature.
The study suggests different magnitude quakes
begin in different ways, said Lucy Jones, the scientist in charge of
the U.S. Geological Survey in Pasadena, who was not part of the
study.
But Jones was skeptical that the information
can be reliably used to create an early warning system. The United
States is still years away from an alert system because of fears of
false alarms and disagreement among scientists about what physical
forces cause an earthquake to turn into a big one.
In the study, Allen and colleagues analyzed
records of 71 major Pacific Rim quakes in the past decades including
24 events that were greater than a magnitude-6.
Using a mathematical model, they were able to
estimate a quake's size to within one magnitude unit from as little
as four seconds of data of the frequency of the energy in the
primary wave. These low-energy waves typically cause a jolt,
signaling the occurrence of a quake.
Earlier research conducted by Allen showed that
the first few seconds after a quake can be used to estimate quakes
smaller than magnitude-6.
Allen is currently testing the model in
real-time using an intricate network of seismic instruments
scattered in Northern California.
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