Forecasting when a major earthquake will erupt — within a window of two to three years — could be possible, based on mathematical studies by researchers at UC Davis, Boston University and the University of Western Ontario, Canada.

The group, with colleagues at the National Aeronautics and Space Administration's Jet Propulsion Laboratory, previously forecast the likely locations of major earthquakes in a 10-year period.

From Jan. 1, 2000, to the present, 17 of California's 19 significant quakes -- those with magnitudes greater than 5 -- struck at identified "hot spots." Sixteen of those earthquakes occurred after the publication of the map of hot spots. The map appeared in the Proceedings of the National Academy of Sciences on Feb. 19, 2002.

The new approach narrows the time window, and takes in a broader geographic area. Major earthquakes are still most likely to occur on the hot spots identified previously, said John Rundle, director of the Center for Computational Science and Engineering at UC Davis, who heads the research group.

Graduate student James Holliday, working with Rundle and others, found that California earthquakes have been clustered in time, when the quakes have registered magnitudes greater than 6. The timing of large quakes has been associated with periods when "bursts" of small temblors, of magnitude 3 or less, have tended to be suppressed.

That is because small and large earthquakes are both related to the tectonic forces, or stressing processes, in the rocks below California, Rundle said. Suppression of the small earthquake bursts is associated with "spatial smoothing" of the stress field that produces the large quakes. A smoother stress field is capable of producing larger shocks than a rougher field, because earthquake ruptures can spread farther in a region of relatively even stress, he said.

Northern California is now in a period of suppressed smaller earthquakes. The opposite is true in Southern California, where bursts of smaller quakes are relatively larger. According to the methods described in the paper, Northern California is now at higher risk than Southern California for a major quake.