Major California Earthquake All But Certain During Next 30 Years

Scientists released the first official statewide forecast of earthquake probabilities for California Monday, reporting a 99 percent probability of a strong and damaging earthquake of magnitude of 6.7 or greater occurring along one of the state’s major seismic faults within the next 30 years.

The report, submitted by federal and state seismologists, geologists and geophysicists, said there was a 46 percent chance of an even more damaging magnitude 7.5 or greater earthquake occurring during the next three decades, most likely along one of the active fault systems in the southern part of the state.

The new statewide forecast is the result of a model that combines information from seismology, earthquake geology, and geodesy ““ the measuring of precise locations on the Earth’s surface.  The model makes possible, for the first time, the ability to forecast statewide probabilities of California having a large earthquake in the next 30 years.

The projections are not a significant change from current probability estimates conducted five years ago, but they do provide the first detailed predictions of a deadly quake in the Los Angeles area San Andreas, San Jacinto and Elsinore faults. 

"This new, comprehensive forecast advances our understanding of earthquakes and pulls together existing research with new techniques and data," explained U.S. Geological Survey (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," said Field, in a USGS news release about the report.

Statewide, the report found the fault with the highest probability of at least one magnitude 6.7 quake or larger is the southern San Andreas, with a 59 percent probability in the next 30 years.

For northern California, the highest probability source of a large earthquake is the Hayward-Rodgers Creek Fault, which shows a 31 percent of a major quake within the next 30 years.

Such quakes can be strong and deadly, as demonstrated by the 1989 magnitude 6.9 Loma Prieta and the 1994 magnitude 6.7 Northridge earthquakes.

The report is being sent to businesses and state and local agencies that establish building codes, earthquake insurance rates, participate in emergency planning and those who would assist in more accurate planning for inevitable future large earthquakes.

The state’s official earthquake forecasts, called “˜UCERF’ for Uniform California Earthquake Rupture Forecast, were developed by a multidisciplinary group of scientists and engineers known as the Working Group on California Earthquake Probabilities.  The forecast built on previous studies, which the Working Group updated and used to developed the first-ever statewide, comprehensive model for California.

The Working Group was sponsored by multiple organizations, including the U.S. Geological Survey, the California Geological Survey and the Southern California Earthquake Center.  The scientific community’s consensus allowed for meaningful comparisons of earthquake probabilities in Los Angeles and the San Francisco Bay Area, as well as comparisons among several large faults.

Although earthquake probabilities for many parts of the state are in line with previous estimates, 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.  On average, major earthquakes occur about every 500 years along that zone, and the report calculated a 10 percent probability of a magnitude 8 to 9 quake the area during the next 30 years.

The 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.

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On the Net:

The full UCERT report can be viewed at http://pubs.usgs.gov/of/2007/1437/.