The Bay Area's Big One will still be plenty big, but it might not be just one, according to a study released by US Geological Survey scientists.
A flurry of mid-sized quakes is more likely to strike the San Francisco Bay Area rather than a giant 1906-esque rupture, said David Schwartz, a paleoseismologist at the USGS's Menlo Park office and the lead author of the study, which appeared on Monday in June's Bulletin of the Seismological Society of America.
The study marks the first comprehensive history of the Bay Area's seismicity dating to 1600.
A quake cluster isn't necessarily good news, as it could keep communities constantly cleaning up the earthquake damage, several experts said.
"It presents a very different problem in how you respond and recover from earthquakes," Schwartz said.
After the 7.8-magnitude 1906 earthquake, the 20th century was abnormally stable, he said. Therefore, an earthquake cluster is overdue, the scientists said.
"Basically, what goes in, must go out," Schwartz said. The region's seismicity stems from the clash of two massive plates in the earth's crust. The Pacific Plate is sliding northwest, while the North American Plate is moving southeast.
Since 1906, the plates have moved about 13 feet in the Bay Area. Like a compressed spring, they're ready to burst.
In the Bay Area, the plate boundary fractures into a handful of fissures, all generally trending northwest-to-southeast. The well-known San Andreas Fault, which Schwartz calls the "master fault," is accompanied by the San Gregorio Fault, the Hayward Fault, the Calaveras Fault and the Rodgers Creek Fault in the North Bay, among others.
Future quakes are expected to spread out along these faults.
"These faults are being stressed by the plate movements ... and they all have to catch up," Schwartz said.
The various faults "talk" to each other, said Roland Burgmann, an earth scientist at the University of California, Berkeley. "The communicating family of faults sometimes tend to rupture together as a group or shut each other off."
The 1906 earthquake was likely a fluke, the perfect alignment of conditions that allowed 300 miles of the San Andreas Fault - from northern Mendocino County to San Juan Bautista - to release its pent-up pressure. This massive shaking kept the area unusually calm for a century, Schwartz said.
"Eventually, there should be more clusters," Burgmann said.
The scientists based their prediction on the historical record, which shows a cluster of quakes shook the Bay Area from 1690 to 1776. At least six earthquakes, ranging from 6.3 to 7.7 magnitude, rattled the region's major faults during that period, Schwartz said.
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