The San Francisco Bay area has a 1 percent chance of a magnitude-7 or greater earthquake every year and a 25 percent chance that one will occur within two decades, according to a new computer forecast.

The Virtual California program was developed by John Rundle at the University of California at Davis with the help of scientists at NASA. It uses methods similar to weather forecasting, Rundle said.

The model considers 650 segments representing the major fault systems in California, including the San Andreas fault responsible for the 1906 San Francisco earthquake. The simulation takes into account the gradual movement of faults and how they interact with each other.

The researchers determine that major quakes in the Bay Area occur at an average interval of 101 years. The simulation data indicates a 25 percent chance of another such earthquake in the next 20 years, a 50 percent chance in the next 45 years and a 75 percent chance by 2086.

The forecast was published in Proceedings of the National Academy of Sciences.

In 1999, the U.S. Geological Survey issued a comprehensive report stating a 70 percent probability that one or more earthquakes of magnitude 6.7 or larger would strike the San Francisco Bay Area during the next 30 years. Such a quake would hold potential for widespread damage.

Earlier this year, another group of scientists calculated a 20 to 70 percent probability that Southern California will be hit by a large earthquake in the next 30 years.

Geologists caution that earthquake prediction is in its infancy.

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