California's Bay Area could face bigger earthquakes—in the near future—than previously predicted, scientists warn in a new study based on the most comprehensive monitoring and analysis to date.

In particular, four faults in urban areas around San Francisco have now built up enough energy to cause a significant temblor at any time.

Most notably, the Green Valley Fault in the North Bay's Solano County "is likely to have a larger earthquake than people previously thought," says James J. Lienkaemper of the U.S. Geological Survey in Menlo Park, California.

Lienkaemper led a new analysis of the northern part of California's San Andreas Fault system that was published Monday in the Bulletin of the Seismological Society of America.

Using the most rigorous measurements made to date, the scientists found that the Green Valley Fault has stored up enough energy to produce an earthquake of magnitude 7.1. The fault accumulated additional stress when a 6.02 magnitude earthquake struck the nearby West Napa Fault in August, damaging parts of Napa and rattling awake thousands of sleeping Bay Area residents.

The scientists also found that three more faults have built up enough energy to produce earthquakes any day now: the Northern Calaveras Fault (magnitude 6.8), southeast of San Francisco Bay; the Hayward Fault (M 6.8), just east of the bay; and the Rodgers Creek Fault (M 7.1), which bisects Santa Rosa. Shaking in those areas would impact millions of residents.

When Will Big Ones Occur?

Exactly when an earthquake will strike along those faults is hard to predict, says Lienkaemper.

In the case of the Hayward Fault—which runs through Oakland, Berkeley, and other heavily populated East Bay locales—the team was able to look at the geologic record going back about 2,000 years and found that major earthquakes have tended to hit there about every 150 years. The last big one was in 1868, 146 years ago, so another one is expected at any time.

The Rodgers Creek Fault, which runs northwest from the bay through the wine country of Sonoma County, is about 90 percent "locked," meaning the rocks are stuck and the strain is building. "A large earthquake is likely there," says Lienkaemper, though he notes that because there hasn't been an earthquake on the fault in historical times or evidence of one available in the geologic record, the timing is hard to predict.

Similarly, because the Green Valley Fault is a complex, crooked system with an uneven seismic history, predicting exactly when the next big one will hit there is also difficult.

Geologists predicted back in April that in general there's a 99.7 percent chance of a "big one," of at least magnitude 6.7, striking California within the next three decades. Scientists have already warned that southern California is particularly at risk; Lienkaemper and his team focused on 1,250 miles (2,012 kilometers) of faults in northern California to fill out the picture.

With an estimated 37,000 tremors a year, California is among the most seismically active parts of the world. (See "Big Quake 'Guaranteed' to Hit California by 2037.")

The state's fault zone is also one of the best studied zones in the world, notes Roland Burgmann, a geologist at the University of California, Berkeley. Burgmann, who was not involved in Lienkaemper's study, calls the new paper "impressive."

Based on field measurements going back as far as 40 years, Lienkaemper's and his team's research "improves the precision of hazard determination," says Burgmann, and "shows these faults are still a substantial hazard." (See "7 Biggest Earthquakes in California History.")

View Images This apartment house in the Marina District of San Francisco was damaged by the October 1989 Loma Prieta earthquake, the strongest since the area's 1906 earthquake. Photograph by Grant Smith, Corbis

Measuring Fault Creep

Lienkaemper and colleagues, working in collaboration with San Francisco State University researchers, based their analysis on two sets of data collected along Bay Area faults. They used GPS measurements of changes along the faults, taken by equipment that had been deployed only in the past few years.

They also studied the areas on foot, using precise surveyor's scopes to measure movement to the scale of one millimeter.

Then they compared that physical data with historical records and geologic evidence of past earthquakes. The result was a more precise understanding of changes along the faults than has been available using any other method, says Burgmann. The main alternative is space-based radar systems, which can't provide as fine a resolution.

Lienkaemper's team was able to get the most data on the Hayward Fault, using surveying that's been done of 32 locations over the past 40 years. They found that the slow movement between the tectonic plates, known as creep, is more variable than previously thought. In some cases the creep was barely perceptible, and in other cases it was at the rate of two millimeters a year.

"The extent of fault creep, and therefore locking, controls the size and timing of large earthquakes on the Northern San Andreas Fault system," says Lienkaemper.

Faster creep helps dissipate the seismic energy, making an earthquake less probable. Plates that are locked are likely to release stored energy violently, through rupture and then shaking.

The scientists found that the main branch of the San Andreas Fault in northern California was only 20 percent (plus or minus 10 percent) locked in its central creeping section and 99 to 100 percent locked on the north coast.

The scientists are still trying to work out what factors determine the creep rate. They suspect that the composition of the rocks, especially how much of the lubricating mineral talc is present, plays a big role. Temperature and pressure may also affect the creep, as does the geometrical orientation of the fault—the more bends or breaks in the fault, the more it tends to stick.

View Images People walk along the streetcar tracks amid the rubble after the 1906 San Francisco earthquake, when more than three-quarters of the city's buildings were destroyed. Photograph by Casey & Clancy, Corbis

Future Work

More precise measurements are helping scientists better predict earthquakes and understand how faults behave, says Lienkaemper, who's training colleagues in the Philippines to use these techniques on similar slipping faults in their country.

Burgmann says additional funding would help scientists take more measurements on other faults around the world, some of which are poorly mapped and not well understood.

Few scientists, he notes, have the wherewithal Lienkaemper's team did to make measurements over such a long time: "It's especially hard to get funding to do something year after year."

He adds, "In many places we wouldn't know creep was happening without the measurements. This is really the best way to keep track of those faults."