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."
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The work was supported by the USGS, University of Wisconsin,
Madison, and UC Berkeley.
Editor's Note: The original news release can be found here.