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By Robert Lee Hotz & Kenneth Reich
Times Staff Writers
Northridge trembler in January, the most costly American earthquake
since 1906, measurably rearranged more than 1,900 square miles of
metropolitan Los Angeles, leaving portions two feet higher than
before, according to a wider-ranging report released today.
shock -- now reassessed at magnitude 6.7 -- also forced scientists
to revise their estimates of the seismic hazard facing the densely
inhabited suburbs of metropolitan Los Angeles.
In the most
comprehensive assessment of the Northridge earthquake so far, the
scientists of the U.S. Geological Survey and the Southern California
Earthquake Center say that people in the Los Angeles area should
expect at least one earthquake every year of magnitude 5.0 or more
for the foreseeable future. There is a 45% likelihood of another
Northridge aftershock above magnitude 5.0 by the end of July.
said that with almost 100 faults in the Los Angeles area known to
be capable of a magnitude 6.0 or greater earthquake, emergency planners
must recognize that urban earthquakes can no longer be considered
rare events. Over the long run, contractors should expect that homes
and buildings might be subjected to severe ground shaking -- at
the extremes of the stresses considered in present building codes
-- several times in the structure's lifetime.
The full extent
of the urban corridor from San Bernardino through Los Angeles and
northwest to Santa Barbara is at risk.
should expect at least one earthquake of magnitude 5.0 or greater
every year, and they should strengthen buildings to withstand the
repeated shocks, according to the most comprehensive report so far
on the Jan. 17 quake.
There is a
45 percent likelihood of another aftershock above a magnitude of
5.0 from the Northridge Quake by the end of July, the study found.
The 6.7 magnitude earthquake left 61 dead and caused at least $20
billion in damage.
A team of scientists
from the California Institute of Technology, University of Southern
California, NASA's Jet Propulsion Laboratory, Columbia University
and University of California, San Diego, used data from sensors
that have been installed across the region. They paid particular
attention to buildings and codes.
a debate over what happens when you are right on top of an earthquake,"
said Lucille M. Jones, a U.S. Geological Survey seismologist who
headed the team."That argument is over." The study, reported
Friday in the journal Science, found "that the northern Los Angeles
region faces one of the greatest seismic hazards in Southern California."
so many faults that any safety efforts have to be regional in scope,
said Thomas L. Henry, executive director of the Southern California
Earthquake Center at USC. Nearly 100 faults in the Los Angeles area
are capable of a magnitude 6.0 or greater earthquake, so they shouldn't
be considered rare events, the scientists said.
"This is an
incredibly complex web of faults underlying all of Los Angeles -
more complex than we thought," Jones said. The Northridge Quake
left more than 3,000 buildings unsafe, but the scientists blamed
much of the damage on faulty construction and inadequate building
such as securing computers, lights, pipes and bookcases could save
millions, the study suggested. "The emphasis on protecting against
loss of life says nothing about the cost of repair to restore the
buildings to usability," said Douglas Dreger, a UC Berkeley seismologist
who agreed with the findings.
present codes, the buildings may not collapse but may not ever be
usable again. It may be advisable to strengthen the codes to prevent
much more of the damage that can occur in strong earthquakes."