"The take-home message is that the 30-year probability is significantly higher [than previous estimates]," Pollitz said at a session of the American Geophysical Union's fall meeting in San Francisco.
The most recent report by the USGS Working Group on California Earthquake Probabilities, released in 2003, predicted a 62 percent chance of a magnitude 6.7 quake or higher in the Bay Area before 2032.
"[Ours] is greater than predicted by the working group," he said.
James Lienkaemper is a scientist with USGS's Earthquake Hazards Team.
He has found physical evidence of the past 11 huge quakes on the Hayward Fault and discovered that events there occurred on average every 170 years over that period.
Pinpointing the next quake is not as easy as tacking 170 years onto 1868, he added—his analysis shows the event could be as late as 2105.
But he agreed that data based on the last five earthquakes does signal the next quake striking within a few years.
"Both projections are provocative interpretations of data," Lienkaemper said.
Favorable Orientation
The Hayward Fault has a notorious reputation because it allows the Pacific and North American tectonic plates to slide past each other, lining up more cleanly than they do at the better-known San Andreas Fault, Brocher said.
(See a map of Earth's moving plates.)
"[Because] Mother Nature likes to save energy, it tends to use faults that have more favorable orientation," he said.
Even more troubling, Brocher said, are new findings that show the Hayward Fault links to another nearby fault.
Scientists had previously regarded the two faults as separate structures.
But analysis of recent earthquakes, along with three-dimensional mapping research, has revealed that the faults are actually segments of one large system.
The longer the fault, the stronger the quake, Brocher said, so existing models may need to be altered to consider the possibility of larger quakes from a longer fault.
Much More Damaging
With the Hayward Fault raring to go, its location at the heart of the Bay Area could make for a devastating quake, Brocher said.
Two and a half million people—a hundred times more inhabitants than lived in San Francisco in 1868—are clustered along the fault lines, some of which run underneath San Francisco's public transportation system, the BART.
Four and a half million more people who live in outlying areas may also be affected.
Several scientists estimate the coming temblor at a magnitude 7—about the severity of the 1989 Loma Prieta quake, which was centered along the San Andreas in mountains 50 miles (80 kilometers) south of San Francisco.
Many city governments and nonprofits, such as the 1868 Alliance, are already encouraging the public to prepare for a catastrophe.
"Chances are excellent you're going to be a survivor," Brocher said, "[but] how comfortable you are after the quake depends on how well prepared you are."
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