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New Study Shows Odds High for Big California Quakes
|California has more than a 99%
chance of having a magnitude 6.7 or larger earthquake within the next
30 years, according scientists using a new model to determine the
probability of big quakes.
The likelihood of a major quake of magnitude 7.5 or greater in the next
30 years is 46%—and such a quake is most likely to occur in the
southern half of the state.
The new study determined the probabilities that different parts of
California will experience earthquake ruptures of various magnitudes.
The new statewide probabilities are the result of a model that
comprehensively combines information from seismology, earthquake
geology, and geodesy (measuring precise locations on the Earth’s
surface). For the first time, probabilities for California having
a large earthquake in the next 30 years can be forecast statewide.
“This new, comprehensive forecast advances our understanding of
earthquakes and pulls together existing research with new techniques
and data,” explained USGS geophysicist and lead scientist Ned Field.
“Planners, decision makers and California residents can use this
information to improve public safety and mitigate damage before the
next destructive earthquake occurs.”
The new information is being provided to decision makers who establish
local building codes, earthquake insurance rates, and emergency
planning and will assist in more accurate planning for inevitable
future large earthquakes.
The official earthquake forecasts, known as the “Uniform California
Earthquake Rupture Forecast (UCERF),” were developed by a
multidisciplinary group of scientists and engineers, known as the
Working Group on California Earthquake Probabilities. Building on
previous studies, the Working Group updated and developed the
first-ever statewide, comprehensive model of California.
The organizations sponsoring the Working Group include the U.S. Geological Survey, the California Geological Survey,
and the Southern California Earthquake
Center . An independent scientific review panel, as well as the
California and National Earthquake Prediction Evaluation Councils, have
evaluated the new UCERF study.
The consensus of the scientific community on forecasting California
earthquakes allows for meaningful comparisons of earthquake
probabilities in Los Angeles and the San Francisco Bay Area, as well as
comparisons among several large faults.
The probability of a magnitude 6.7 or larger earthquake over the next
30 years striking the greater Los Angeles area is 67%, and in the San
Francisco Bay Area it is 63%, similar to previous Bay Area estimates.
For the entire California region, the fault with the highest
probability of generating at least one magnitude 6.7 quake or larger is
the southern San Andreas (59% in the next 30 years).
For northern California, the most likely source of such earthquakes is
the Hayward-Rodgers Creek Fault (31% in the next 30 years). Such quakes
can be deadly, as shown by the 1989 magnitude 6.9 Loma Prieta and the
1994 magnitude 6.7 Northridge earthquakes.
Earthquake probabilities for many parts of the state are similar to
those in previous studies, but the new probabilities calculated for the
Elsinore and San Jacinto Faults in southern California are about half
those previously determined. For the far northwestern part of the
State, a major source of earthquakes is the offshore 750-mile-long
Cascadia Subduction Zone, the southern part of which extends about 150
miles into California. For the next 30 years there is a 10% probability
of a magnitude 8 to 9 quake somewhere along that zone. Such quakes
occur about once every 500 years on average.
The new model does not estimate the likelihood of shaking (seismic
hazard) that would be caused by quakes. Even areas in the state
with a low probability of fault rupture could experience shaking and
damage from distant, powerful quakes. The U.S. Geological Survey (USGS)
is incorporating the UCERF into its official estimate of California’s
seismic hazard, which in turn will be used to update building codes.
Other subsequent studies will add information on the vulnerability of
manmade structures to estimate expected losses, which is called
“seismic risk.” In these ways, the UCERF will help to increase public
safety and community resilience to earthquake hazards.
The results of the UCERF study serve as a reminder that all
Californians live in earthquake country and should be prepared.
Although earthquakes cannot be prevented, the damage they do can be
greatly reduced through prudent planning and preparedness. The ongoing
work of the Southern California Earthquake Center, USGS, California
Geological Survey, and other scientists in evaluating earthquake
probabilities is part of the National Earthquake Hazard Reduction
Program’s efforts to safeguard lives and property from the future
quakes that are certain to strike in California and elsewhere in the
The full UCERF report,
a summary fact sheet,
and supplemental information ,
are available online.