THURSDAY, May 1 (HealthScoutNews)
-- Earthquakes are nature's scary surprises. By the time you
realize you're experiencing a temblor, the shaking is often
half over, leaving you to wonder if another one will follow
quickly.
Long-time residents of earthquake country swear that their
animals sometimes alert them by acting strangely, while others
warn that it's "earthquake weather" when the mercury
rises.
Soon there may be a more scientific warning for residents
of southern California.
In the May 2 issue of Science, researchers describe
an early-warning system for the region that could give as much
as 40 seconds' notice before a strong ground motion was due.
Once in place, the system could help stem disaster by giving
people enough time to take shelter, evacuate buildings, stop
trains, and divert aircraft.
The amount of warning time will depend on how far a person
is from the epicenter, says Richard M. Allen, a professor of
geology and geophysics at the University of Wisconsin-Madison
and the lead author of the paper. "If you are at the
epicenter, we're talking about zero, one, maybe two seconds.
If you are 60 kilometers [37 miles] away, we are talking about
20 seconds of warning."
The proposed new early warning system, called ElarmS, uses
a network of 155 seismic stations already in place in southern
California, called TriNet, now part of the California
Integrated Seismic Network.
"TriNet has two rapid reporting systems," Allen says. "CUBE
is a paging system and it gives the magnitude and location of
the earthquake. The other is the Shake Map -- it shows the
distribution of the ground motion across southern California.
They're available within minutes."
TriNet stations record ground motion, including P waves,
the first seismic arrival from an earthquake, and the usually
larger amplitude S waves, which are responsible for most of
the damage to buildings during a quake.
"We are using the P waves to locate and determine the
magnitude of the earthquake and, based on that, issue a
warning," Allen says.
To test the concept of ElarmS, Allen and his co-author,
Hiroo Kanamori of the California Institute of Technology in
Pasadena, gathered waveform data from 53 recent California
earthquakes and estimated the magnitude from the P waves and
other data.
Early-warning systems are already in place in other
locations, Allen says. Mexico City, for instance, has an early
warning system based on measurements of ground motion along
the coast, where the quakes tend to occur, and can transmit
alerts to the city before the ground motion arrives.
Southern Californians live above many active faults, some
of which are directly below densely populated metropolitan
areas. So measuring the P waves, rather than waiting for the S
waves or waiting until the ground motion reaches a particular
threshold, as other systems do, is a better idea, Allen
says.
The next step, Allen says, is to test the new system in the
"real time system that Trinet already runs" to see how
accurate the magnitude estimates based on P waves are.
Other experts praise the paper's scientific value but
question whether the early warning system will be logistically
feasible.
"The paper is excellent work," says Bill Spencer, a
professor of civil engineering at the University of Illinois
at Urbana-Champaign. The system might work best, he adds, in
conjunction with buildings equipped with shock absorber-like
systems that cause the building to adapt to the magnitude of
the quake to sustain the least damage. "These are already
inside some large buildings," he says.
"Scientifically it's a very good paper," agrees David Wald,
a seismologist with the United States Geological Survey in
Golden, Colo., who helped develop the Shake Map. "But I have
reservations about implementation of an early-warning system."
Putting the system into practice, he says, would involve
complicated logistics.
"You could stop elevators at a certain floor [if the
warning was issued]. You could install electronic freeway
signs, saying 'Don't go over this overpass.' But a lot of
things rely on human response." And that might not be so
predictable, he says.
"The amount of [warning] time this system would provide is
very little for the people closest," he says. "People who
would need the most warning would get the least."
Yet, Allen counters that any warning is better than none.
After the Northridge quake of 1994, he says, "buildings were
red-tagged up to 60 kilometers from the epicenter." In the
1989 Loma Prieta quake, highways and buildings 50 miles from
the epicenter collapsed.
More information
For details on how earthquakes occur, visit the Institute
for Crustal Studies. Learn more facts about tremors from
the U.S.
Geological Survey.