Study says quake's first waves denote
magnitude Warning system cost prohibitive By Becky Oskin, Staff Writer
PASADENA -- Though an earthquake's first tremors often pass
without notice, scientists say they now can estimate the size of
coming quakes based on this early bump and rattle an important step
toward building an earthquake early warning system in Southern
California.
With a warning system in place, alarms could give people a few
seconds to find shelter, shut down machinery and pipelines, slow
trains and abort aircraft landings.
However, even the researchers responsible for the study say it
would take an enormous investment in training and technology to make
the system a reality.
"There's a big distance between technological development and
actual implementation,' said Caltech seismologist Hiroo Kanamori.
Kanamori and Richard Allen, a geophysicist at the University of
Wisconsin in Madison and leader of the study, modeled the warning
system on Southern California's dense network of 155 seismic
monitors. Their results appear today in the journal Science.
The method does not predict quakes. It relies on the time lag
between different types of earthquake waves.
The energy released when one block of rock slips past another
during an earthquake travels in more than one way. One type of
energy travels as a P-wave, which moves the ground in a
push-and-pull motion. At the epicenter, the P-wave moves outward
simultaneously with the more damaging S-wave, which shakes up and
down and side to side.
"The P-wave is usually very small, but it carries all the
information regarding what happened at the source,' Kanamori said.
Because an S-wave travels more slowly than P-wave, the S- wave
arrives later at areas some distance from the epicenter.
Many people feel this gap as a quiet period between the slap of a
P-wave and the violent shaking caused by S-waves.
Devising a system that can calculate an earthquake's magnitude
from a P-wave in just a few seconds was the fundamental step
achieved by Allen and Kanamori.
"This is something that was not known,' said Lucy Jones, a
seismologist and scientist-in- charge at the United States
Geological Survey's Pasadena office.
"This gives us a piece of information about the size of a quake
in the first few seconds. That's important, but it doesn't give us a
practical system,' said Jones, a member of the state's Seismic
Safety Commission.
Early-warning systems in Japan and Mexico rely on the great
distance between major cities and the epicenters of large
earthquakes.
Mexico City is about 185 miles from the source of its most
damaging quakes, near the Pacific coast. A network of seismometers
there gives the city 70 seconds to prepare.
Because Southern California is built above its most dangerous
faults, the warning times offered by Allen and Kanamori's system
would be much shorter.
Someone 18 miles from the epicenter of the 1987 Whittier Narrows
quake would experience an eight-second delay between passage of the
P-wave and the S-wave.
Much of that delay would be used in detecting the P-wave, sending
the data to a central processors, calculating the location, origin,
time and magnitude, then disseminating the warning.
With a significant investment in telemetry and electronics, it is
possible to reduce all of that to a few seconds. Allen and Kanamori
are now testing their method in real-time to gauge the possibility.
"People who need warning the most will have less time, but at
least the system can give people a chance to react,' Allen said.
"In an earthquake, every second counts.'
But Kanamori points out the difficulty of training millions of
people to react to a warning that may come once in a lifetime.
"Maybe people can react quickly enough to take cover, but I
can't. I was sitting here (in my office) during Whittier Narrows.
All the ceiling tiles came down and I just watched.'
The most practical option would be linking the warning system to
building monitors and utility company controls and infrastructure.
Engineers in Japan are designing skyscrapers that can stiffen or
soften in reaction to earthquake warnings, Kanamori said.
-- Becky Oskin can be reached at (626) 578-6300, Ext.
4451, or by e-mail at becky.oskin@sgvn.com. |