New system for earthquake early
warning
A University
of California, Berkeley, seismologist has discovered a way to provide
seconds to tens of seconds of advance warning about impending ground
shaking from an earthquake. |
Image: This view along the fault
plane of the 1999 Chi-Chi earthquake in Taiwan shows that the 7.6-magnitude
rupture started at the star and radiated outward, with each circle representing
3 seconds of time. The green, yellow and red colors indicate regions of high
slip. Using ElarmS, it was possible to estimate the final magnitude of the
earthquake a mere two seconds after it started, when only the area within the
red circle had ruptured. (Douglas Dreger/UC Berkeley [2001])
While a
few seconds may not sound like much, it is enough time for school children to
dive under their desks, gas and electric companies to shut down or isolate their
systems, phone companies to reroute traffic, airports to halt takeoffs and
landings, and emergency providers to pinpoint probable trouble areas. Such
actions can save lives and money.
An early warning system like this is
possible thanks to the work of Richard Allen, UC Berkeley assistant professor of
earth and planetary science, who in the last five years has demonstrated that
within a few seconds of an earthquake rupture, he can predict the total
magnitude of the quake and its destructive potential. In San Francisco, for
example, Allen estimates that it's likely the city could receive 20 seconds'
warning of an impending temblor.
"We can determine the magnitude within
a couple of seconds of initiation of rupture and predict the ground motion from
seconds to tens of seconds before it's felt," Allen said. He and his colleagues
are now testing a system, ElarmS, that would make these predictions, and the
researchers are working with the U.S. Geological Survey (USGS) to determine how
accurate these warnings would be.
Allen and coauthor Erik L. Olson, a
former graduate student at the University of Wisconsin, Madison, published their
data on early earthquake ground motion predictions in the Nov. 10 issue of
Nature.
Seismologists, especially those in the United States, have
become increasingly pessimistic about being able to predict earthquakes.
Experiments at the intensively monitored Parkfield, Calif., site have dampened
enthusiasm that earthquake ruptures could be predicted hours or days before they
happen. To reduce loss of life and property, earthquake-prone regions generally
rely on a combination of advance preparation and post-earthquake assessment and
notification between five and 10 minutes after a quake.
Allen's early
warnings come after a quake rupture has already begun but before the shaking is
felt tens of miles from the epicenter.
San Francisco, for example, sits
about midway along the northern half of the 800-mile San Andreas fault. If a
rupture occurs at the extreme northern end, it could take 80 seconds, traveling
nearly 2 miles per second, to reach the city. An early warning system could
provide a critical buffer for residents, businesses and emergency responders,
even if the time isn't sufficient to evacuate a building.
The early
warning information also would feed directly into the new active-response
building designs that change the mechanical properties of a structure to let it
ride out shaking and minimize damage both inside and out. Active response
buildings are already operational in Japan, Allen said.
"That is our
long-term goal, to have the building feel the earthquake, not the occupants,"
Allen said.
Two years ago, while at the University of Wisconsin, Allen
reported differences in the frequency of seismic signals emanating from small
and medium earthquakes during the first four seconds of the rupture, with the
larger quakes showing lower frequency signals than the smaller quakes. The
signal is part of the primary wave, or P wave, that is the first, though least
destructive, wave to arrive after a rupture. Most people experience the P wave,
which is a pressure wave that travels through rock like sound through air, as a
jolt.
This P wave is followed by a secondary wave, or S wave,
that shears the ground back and forth and up and down. Shortly after, more
destructive surface waves arrive that jerk the ground sideways and later roll in
like ocean waves.
In the current study, Allen shows that the
relationship between P wave frequency and the total magnitude of the quake holds
for major quakes, up to magnitude 8 and higher, as well as for medium and small
quakes. Based on the correlation, he can predict the total magnitude of the
quake to within 1 magnitude, and for a specific area, like the San Andreas
Fault, to within half a magnitude. Magnitude is a measure of the total area that
ruptures underground and the average amount of slip along the rupture. A half a
magnitude amounts to a factor of 3 difference in ground motion.
"Most
seismologists are surprised, and frequently skeptical, that you can predict the
magnitude of an earthquake before it has ended, but this is telling us that
there is something very different from what we thought about the physics of the
processes involved in a rupture," Allen said.
Allen's findings conflict
with the current model of earthquake rupture. The "cascade" model assumes that
earthquake faults are made up of lots of different-sized patches, each under
some degree of stress. When one of the patches is stressed enough to slip, the
slip propagates to adjacent patches, which rupture in turn like falling
dominoes. The rupture stops only when the stress propagating along the fault
zone reaches a patch that is too solidly locked to slip.
Inherent in
this model is the idea that the initiating rupture is the same for big and small
quakes. Allen's findings suggest this is wrong. Instead, the rupture is
different for large and small quakes from the beginning, and the initial rupture
contains information that can be used to predict the final size.
He
proposes that if the initial rupture generates a large "slip pulse" that travels
continuously in all directions across the fault plane, the pulse can supply the
necessary energy to propagate through patches that would not otherwise have
ruptured. Only when the energy in the pulse drops to a level insufficient to
overcome the grip of rock on rock does the rupture stop.
"If the rupture
pulse initiates with a large slip, it is more likely to evolve into a large
earthquake," he and Olson wrote in their report.
Allen's demonstration
that this observation holds in earthquakes around the world, from California to
Taiwan and Japan, provides a solid basis for constructing an early warning
system. Once the magnitude of the quake has been estimated, computers can
predict areas of serious ground shaking based on an understanding of a
particular fault. Within five seconds, warnings could be sent to cities in the
areas calculated to expect damaging ground motion.
Because humans
couldn't respond fast enough, Allen said, these warnings would have to rely on
computers programmed to respond to quakes of a certain magnitude.
"This
allows people to get information about an event before the ground starts shaking
and the system goes down," he said.
The ElarmS system also could warn
rescue and clean-up personnel of aftershocks, which can cause collapse of
unstable debris.
As the rupture proceeds, Allen said, analysis of
seismic waves can refine magnitude and ground motion estimates, finally merging
into the standard shake map typically produced within minutes of the end of an
earthquake.
"We're at the stage where we need to test the accuracy of
the system, which we're now doing," Allen said. "Next, we will determine whether
the telemetry is fast enough to get data to us within seconds of a rupture."
Source: UC Berkeley Related stories:
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» The Science
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