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NSF PR 95-81 - December 7, 1995
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When The San Andreas Fault Moves: Predicting The Effects
of a 7.75 Earthquake in The L.A. Basin
National Science Foundation scientists report that
a magnitude 7.75 earthquake along the San Andreas
fault in southern California - one they give a 27
percent chance of occurring in the next 30 years -
would shake the densely populated Los Angeles basin
far more severely than they had expected. The evidence
comes from a detailed computer simulation, which they
describe as the most ambitious ever attempted for
a major earthquake.
"I never expected that the ground motion would be
so great over such a large area from an earthquake
of this size," says Ralph Archuleta of the University
of California at Santa Barbara, one of the pioneering
study's authors.
The simulation showed that long-period ground motion
the motion created by successive seismic waves arriving
at intervals greater than 2.5 seconds in the Los Angeles
area would be four to 10 times greater than that of
the 1992 magnitude 7.3 Landers earthquake, California's
most powerful in 40 years.
The study, published in the December 8th issue of
Science, was undertaken by seismologists Kim Olsen
of UCSB and Joseph Matarese of MIT, along with Archuleta,
to explore the risks of living in large, sediment-filled
bowls like the Los Angeles basin, even at some distance
from a major fault. It was funded by the National
Science Foundation Southern California Earthquake
Center (SCEC).
As was shown by the devastating effects of the 1985
Michoacan earthquake on Mexico City and the 1989 Loma
Prieta earthquake on San Francisco's Marina District,
the compliant sediments filling such basins can greatly
amplify ground motion, prolonging shaking and increasing
its intensity.
Still, explains Archuleta, no one had ever undertaken
a detailed study of the likely shaking in the 8,500-square
mile greater Los Angeles area with its population
of more than 10 million people, in part because the
three dimensional nature of the problem presented
an enormous computing challenge. The scientists overcame
this obstacle by developing special computer techniques
that enabled them to simulate an earthquake originating
on the section of the San Andreas fault closest to
the L.A. basin.
They chose the San Andreas fault - rather than other
faults in the L.A. region because it has been the
source of the area's largest quakes in the past, and
is likely to be in the future. The San Andreas is
California's great 700 mile seismic divide, marking
the boundary where two of the earth's major crustal
slabs ("tectonic plates") slide past each other.
Their postulated event began with a rupture six miles
deep under Quail Lake, near Tejon Pass. It continued
southeastward "like a zipper opening," Archuleta says
racing through the Mojave Desert and San Bernardino
Mountains until it reached Mill Creek, east of the
city of San Bernardino, 104 miles away. Total ground
displacement on opposite sides of the fault was just
under 16 feet. The rupture itself lasted 68 seconds,
though the ground vibrated for nearly another minute.
The simulation indicated that the area between Interstates
5 and 405 from downtown Los Angeles to Anaheim and
Santa Ana would experience strong swaying motion for
about 60 seconds. Even so, the authors point out,
such longperiod motion is generally not associated
with damage to structures less than 10 stories high.
Nor would it probably pose a serious threat to most
family dwellings in the L.A. basin, they add. However,
the simulation suggested that cities closest to the
fault, such as Ontario, Pomona, San Bernardino, and
Redlands, might feel even more intense ground motion.
In spite of the "what-if" nature of their exercise,
the scientists emphasized that it was firmly grounded
in reality: on December 12, 1812, the same stretch
of the San Andreas fault was hit by an earthquake
of similar size. Moreover, because large earthquakes
along the fault's Mojave and San Bernardino segments
recur at similar intervals about once every 150 years
- it was "reasonable," they felt, to consider a scenario
that had both segments rupturing together.
Archuleta and Olsen, who have done similar computer
studies of Northridge-type earthquakes on faults in
Los Angeles, said that the simulation showed that
a major earthquake on the San Andreas fault would
trigger still larger long-period ground motions. "This
may require a shift in thinking, because many people
have recently tended to believe that large earthquakes
(between magnitude 6 and 7) within the L.A. basin
present a greater threat than major earthquakes on
the San Andreas," says Archuleta.
"And there are still other scenarios that have to
be examined for us to get a better sense of the range
of ground motion that might be expected from a real
earthquake on the San Andreas. This work is part of
a larger program by SCEC scientists to produce realistic
ground motion estimates for future earthquakes on
the San Andreas and other faults within southern California,"
he adds.
NOTE TO REPORTERS, EDITORS, AND NEWS DIRECTORS: A color
video showing the effects of the simulated earthquake
on the Los Angeles basin is available upon request.
On Monday, December 11, Archuleta will talk about
the work at the American Geophysical Union meeting
in San Francisco, at 11:45 a.m., in room 124 of the
Moscone Center. The three scientists will also be
available that afternoon during a poster session on
their study.
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