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A much more damaging quake of magnitude of 7.5 or greater is at least 46 percent likely to hit on one of California's restless web of active fault systems within the same three decades, but probably in the southern part of the state, the team of federal and state earthquake scientists warned.
The new report by the team of federal and state geologists, seismologists and geophysicists does not significantly change the current probability estimates for future large quakes on the Bay Area's major faults that were calculated five years ago, but it does provide the first detailed forecasts for the odds of future quakes on faults in the Los Angeles area: on the southern San Andreas, on the San Jacinto and on the Elsinore faults specifically. "In our two major metropolitan areas where odds are high that a large quake is coming, people think a lot about quakes whenever even a smaller one shakes ... but ten days later most folks forget them, and they shouldn't," said David Schwartz, an earthquake geologist at the U.S. Geological Survey in Menlo Park who served on the scientific review panel that evaluated the new probability estimates.
The analysis was requested by the California Earthquake Authority, a public agency created by the state Legislature in 1996 and funded by companies throughout the state that offer limited quake insurance to all comers. The scientists used complex analytical tools that they have developed over many years and new computer programs to arrive at their new forecasts of earthquake magnitudes and the faults they may rupture.
By their calculations, the probability that a 6.7 magnitude quake will hit on any one of the faults in the Bay Area is now set at 63 percent, only a tiny bit higher than the 62 percent estimated in 2003. But the probability for that kind of severely damaging quake on the Hayward-Rodgers Creek fault was increased in the new forecast from 27 percent to 31 percent.
The analysis was the first the scientists done of probabilities for quakes on several Southern California faults. They calculated the odds of a 6.7 magnitude quake striking within 30 years somewhere in the greater Los Angeles at two-to-one, a probability of 67 percent, according to the report. The single fault in all California with the highest probability of a large quake occurring within the next 30 years is the Southern San Andreas, and the seismic odds-makers set the number for it at 59 percent.