The magnitude 5.2 earthquake that occurred near Gilroy, Calif., on Monday was the fourth to have been correctly plotted on a forecast anomaly map developed by researchers at the University of Colorado's Cooperative Institute for Research in Environmental Sciences, or CIRES.
In a paper published in the Feb. 19th Proceedings of the National Academy of Sciences, CIRES scientists John Rundle and Kristy Tiampo, used a new method to forecast the occurrence of earthquakes greater than magnitude 5 in central and southern California that are likely to occur in the interval from Jan. 1, 2000 through Dec. 31, 2010.
Two such earthquakes have occurred within the margin of error near forecast locations since the paper was published. The first, a magnitude 5.7 event occurred on Feb. 22 in the Baja region of Mexico. The second, a magnitude 5.2 quake, occurred near Gilroy on Monday, May 13, at 10 a.m. PDT.
"These were the only two earthquakes of magnitude greater than 5 that have occurred within the central and southern California region since last Feb. 19, and can therefore be regarded as providing strong scientific support for the theory," Rundle said.
The first of the other two earthquakes, the magnitude 5.1 Big Bear quake of Feb. 10, 2001, occurred after the work was completed. The second of the other two events, the magnitude 5.1 Anza quake of Oct. 31, 2001, occurred after the paper had been accepted for publication.
Yesterday's Gilroy earthquake occurred on the edge of a forecasted anomaly, within the margin of error of the anomaly locations, plus or minus 11 kilometers.
The new earthquake forecasting method is a means of processing data, not a model, the scientists explained. The theme of the research is that seismic activity in a region such as southern California can be characterized by what is known as a "state vector." State vectors describe space and time locations of activity patterns.
Rundle and his team examined southern California earthquake patterns of magnitude 6 and larger. Since earthquakes of these magnitudes generally have fault lengths of roughly six miles and longer, the physicists used a corresponding grid of square boxes having side lengths of about seven miles at southern California latitudes, covering the entire study area.
The "grid process" yielded 3,000 boxes, areas for which the scientists established individual earthquake histories from 1932 to 1991. Using these histories of activity, or "time series," they then developed a scale of seismic activity for each 0.1 degree region.
The scales served as an index adequate to define a state vector that characterizes southern California's seismic activity. From the state vector, Rundle and Tiampo next developed a probability index that showed the seismic potential for the occurrence of large earthquakes.
Using the probability index, they computed and displayed regional anomalies in the state vector corresponding to changes in seismic potential of large events spanning the interval from Jan. 1, 1990 through Dec. 31, 2000 for southern and central California.
Rundle and Tiampo's real-time forecast of large earthquakes anticipated from 2000 to 2010 is available by calling Annette Varani at (303) 492-5952. Rundle, lead author of the study, also is a distinguished visiting scientist at the JPL and is an investigator in the Southern California Earthquake Center based at the University of Southern California.
University of Colorado at Boulder
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