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Aug.
24, 2007
LOS ANGELES
ENJOYING 1,000-YEAR SEISMIC LULL
The Los
Angeles
basin appears to be in a seismic “lull”
characterized by relatively smaller and
infrequent earthquakes, according to a study in the September issue of Geology.
By contrast, the Mojave Desert
is in a seismically
active period. Seismic activity alternates between the two regions, the
study
suggests.
The lull in the Los Angeles
basin began 1,000
years ago, said the authors, led by James Dolan, USC
College
associate professor of earth sciences.
“The past 1,000 years has been relatively quiet,”
Dolan said, referring to what
he calls the “urban fault network” under the Los Angeles
metropolitan area.
The claim will come as news to anyone who has lived through a big quake
in Southern California.
But Dolan said that even the Northridge earthquake of 1994, the
costliest
natural disaster in U.S.
history at the time, was “a drop in the bucket”
compared to the massive jolts
that would strike the basin during a period of high seismic activity.
The study comes with some caveats. Among them:
• The urban fault network does not include the more distant
San Andreas fault.
Though the San Andreas is storing energy at a slower than average rate,
a major
quake along the fault is always possible. About 10 San Andreas
“big ones” have
occurred during the current lull on the urban fault network.
• The authors developed their theory from the discovery of
several “clusters”
of intense seismic activity in the geological record. It is not yet
known if
the clusters are statistically significant.
The authors studied the geological record going back 12,000 years.
During that
period, they found several clusters of seismic
“bursts,” with the most recent
lasting 4,000 years and ending about 1,000 years ago.
The seismic clusters were separated by periods of relative calm lasting
about
1,500 to 2,000 years.
Remarkably, the lulls in the Los
Angeles region
corresponded with seismic clusters in the Mojave Desert, as described
in 2000
by Thomas Rockwell of San Diego State University
and his colleagues.
“When we're having earthquakes in L.A.,
generally we don't have as many earthquakes in the Mojave,”
and vice versa,
Dolan said.
The study in Geology proposes a mechanism by which
periods of high
seismic activity alternate between the urban fault network and the Mojave Desert.
The two main cogs in the mechanism are the section of the San Andreas
fault
north of Los Angeles
and the desert fault system
known as the eastern California
shear zone.
Rapid motion along one fault causes slower motion along the other, the
authors
suggest. During relatively rare periods when the San Andreas fault is
moving
slowly, the strain in the urban fault network drops accordingly,
leading to a
seismic lull in Los Angeles
and to more seismic activity in the desert.
“The San Andreas is always dominant. It's always the big
brother,” Dolan said.
“But at times the eastern California
shear zone takes up its share of the load.”
During the current lull in Los Angeles,
major
earthquakes in the eastern California
shear zone have included the magnitude 7.1 Hector Mine of 1999, the 7.3
Landers
of 1992 and the 7.6 Owens Valley of 1872.
Each packed four to 20 times the energy of the Northridge quake.
While all three quakes occurred in sparsely populated areas, Palm Springs and other desert
communities lie close to the eastern California
shear zone
and could be vulnerable.
“These are very large earthquakes,” Dolan said.
If the authors' theory is confirmed, detecting the start and end of a
lull will
become extremely important. Predicting the end of the current lull is
impossible at present, Dolan said.
“We do know that the Mojave part of the eastern California
shear zone is still
storing energy much more rapidly than usual (by a factor of about two),
so I
would tend to doubt that the recent 1994 (magnitude) 6.7 Northridge and
1971
(magnitude) 6.7 San Fernando earthquakes indicate that we are coming
out of'
the current lull,” he said.
Dolan studies fault systems in Southern California and in Turkey,
whose simpler fault
geography helps Dolan to understand the “extremely
complicated place” that he
calls home.
In a 2003 study published in Science, he estimated
the size and
frequency of past earthquakes on the Puente Hills fault, one of the Los
Angeles-area faults currently in a lull.
The study found that all four major earthquakes on the Puente Hills
fault in
the past 11,000 years exceeded magnitude 7.0.
“We're stuck with living here, so we have to understand what
we can about this
system,” Dolan said.
Dolan's co-authors were Charles Sammis, professor of earth sciences at USC College,
and David Bowman, associate professor of geological sciences at California State
University,
Fullerton.
Funding for the research came from the National Science Foundation and
the U.S.
Geological Survey through the Southern California Earthquake Center as
well as
from the California Department of Transportation and the City and County
of Los Angeles.
##
Contact:
Contact:
Carl
Marziali
University
of Southern California
213-740-4751
marziali@usc.edu
This
text derived from:
http://www.usc.edu/uscnews/stories/14160.html
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