their calculations the probability of such a strong and
damaging quake hitting somewhere the Golden state is now
more than 99 percent.
more damaging quake of magnitude of 7.5 or greater is at
least 46 percent likely to hit on one of California's
restless web of active fault systems within the same
three decades, but probably in the southern part of the
state, the team of federal and state earthquake
new report by the team of federal and state geologists,
seismologists and geophysicists does not significantly
change the current probability estimates for future
large quakes on the Bay Area's major faults that were
calculated five years ago, but it does provide the first
detailed forecasts for the odds of future quakes on
faults in the Los Angeles area: on the southern San
Andreas, on the San Jacinto and on the Elsinore faults
our two major metropolitan areas where odds are high
that a large quake is coming, people think a lot about
quakes whenever even a smaller one shakes ... but ten
days later most folks forget them, and they shouldn't,"
said David Schwartz, an earthquake geologist at the U.S.
Geological Survey in Menlo Park who served on the
scientific review panel that evaluated the new
probability estimates. The analysis was requested by the
California Earthquake Authority, a public agency created
by the state Legislature in 1996 and funded by companies
throughout the state that offer limited quake insurance
to all comers.
report's details should also prove useful for city
planners, building code designers and home and business
owners "who can use this information to improve public
safety and mitigate damage before the next destructive
earthquake occurs," said geophysicist Ned Field of the
U.S. Geological Survey who headed the Working Group on
California Earthquake Probabilities that developed the
new, comprehensive forecast advances our understanding
of earthquakes and pulls together existing research with
new techniques and data," Field said.
Scientists from the U.S. Geological Survey, the Southern
California Earthquake Center and the California State
Geological Survey participated in the report. Aside from
the scientists in California who evaluated the Working
Group's conclusions, both the California and National
Earthquake Prediction Councils evaluated the study too.
scientists used complex analytical tools that they have
developed over many years and new computer programs to
arrive at their new forecasts of earthquake magnitudes
and the faults they may rupture.
their calculations, the probability that a 6.7 magnitude
quake will hit on any one of the faults in the Bay Area
is now set at 63 percent, only a tiny bit higher than
the 62 percent estimated in 2003. But the probability
for that kind of severely damaging quake on the
Hayward-Rodgers Creek fault was increased in the new
forecast from 27 percent to 31 percent.
analysis was the first the scientists done of
probabilities for quakes on several Southern California
faults. They calculated the odds of a 6.7 magnitude
quake striking within 30 years somewhere in the greater
Los Angeles at two-to-one, a probability of 67 percent,
according to the report.
single fault in all California with the highest
probability of a large quake occurring within the next
30 years is the Southern San Andreas, and the seismic
odds-makers set the number for it at 59 percent.
Looking at Northern California's farthest region -
actually the southern end of the 750-mile-long Cascadia
Subduction Zone which stretches far up the Pacific Coast
into British Columbia - the quake experts set the
probability for a large quake within 30 years at only 10
percent, but concluded it could register a magnitude of
8 or even 9. Quakes that powerful occur once every 500
years on average.
Estimating probabilities for future earthquakes is
highly complicated and calls for analysis of many
factors: the average time between quakes that have
struck on a given fault; the location, the size and the
time when a quake last ruptured a given fault; the type
of quake that hit, the geology of the region, and the
rate at which the Earth's crust is moving.
"The further you are in
time from the last quake on a fault, the higher the
probability is for the next one," Schwartz said.
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