The magnitude of an earthquake
can be roughly predicted as the shaking begins,
according to new research that promises to
provide several seconds of warning before the
brunt of a catastrophe strikes.
Earthquake prediction has proved
tricky.
Scientists can't pinpoint the timing of a
rupture to within days or even months or years.
But the new method provides an accurate glimpse
of what will happen in just a moment.
"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,"
says Richard Allen, a seismologist at the
University of California, Berkeley.
If an automatic warning system
were set up, there might be time for school
children to dive under desks and others to
hunker down in doorways, Allen figures.
Waves come in waves
The prediction is made possible
by the fact that a typical earthquake sends out
three different types of waves, which Allen has
been studying for the past few years. He and
colleague Erik Olson of the University of
Wisconsin-Madison examined the seismic records
of 71 large temblors.
- Primary waves, or P waves,
come first. They move like a pressure wave,
typically creating a jolt. P waves are the least
destructive.
- Next comes a secondary wave, or S
wave. It shears the ground back and
forth and up and down.
- Then come surface waves, the most
destructive. They jerk the ground
sideways and later roll in like ocean waves.
The frequency of P waves is
related to the intensity of the eventual surface
waves, Allen and his colleagues have determined.
The idea flies in the face of
convention, which holds that a quake's outcome
depends on a cascade of unpredictable events
along a fault that occur after an initial
eruption.
The new results "imply that the
final magnitude of an earthquake depends at
least partially on what happens in its first few
seconds," says Rachel Abercrombie, an expert in
earthquake rupture at Boston University who was
not involved in the study.
While no two earthquakes
are alike, San Francisco would likely get 20
seconds or more of warning for a major temblor,
Allen said.
"It is not a perfect system,"
Allen told LiveScience. But he added
that all of the large-magnitude earthquakes his
team has studied "do show this characteristic of
the first few seconds" that allow a useful
prediction. "The magnitude estimates in the
study are within plus and minus 1 magnitude
unit. For some events the error will be greater
than others."
The science behind the idea is
detailed in the Nov. 10 issue of the journal
Nature.
Getting the word out
Another key to the prediction
system is that not
everyone sits atop the spot where an
earthquake originates, and the waves of an
earthquake travel much more slowly than the
light-speed communications that could send a
warning.
If a rupture occurs at the
extreme northern end of the 800-mile San
Andreas fault, for example, the shaking
could take 80 seconds, traveling about 2 miles
per second, to reach San Francisco. Such a
far-off event might not destroy the City by the
Bay. But no matter where the epicenter, there
would be a window for warning most of the
ultimate victims.
"It is more likely than not that
there would be greater than 20 seconds warning
for the earthquakes most damaging to the city of
San Francisco," Allen's team states on its web
site.
Allen said at least a year of
further testing is needed before a real warning
system could go live.
He envisions using the Internet
to transmit information of a quake. A computer
at the other end would interpret the warning for
the specific location and estimate timing and
intensity of the anticipated shaking. There
might be multiple warning methods.
"Cell phone towers could
determine the impact of the earthquake for the
area they cover and transmit a warning to the
cell phones within range," Allen said.
Allen is working with the U.S.
Geological Survey to determine how accurate the
warnings from his Earthquake Alarm Systems
(ElarmS) would be.