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.

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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 United States.