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Several faults underlying urban areas in the San Francisco Bay Area may be primed for a major rupture that could lead to a magnitude 6.7 or larger earthquake, new data suggest.
Sections of the Hayward, Rodgers Creek and Green Valley faults are nearing or past their average earthquake recurrence intervals, according to the study published in the Bulletin of the Seismological Society of America.
The faults appear to be locked, unable to accommodate the inexorable movement of the Pacific Plate past the North American Plate, and that means it’s only a matter of time before one of those stuck segments gives way, producing a large earthquake, says the study’s lead author, Jim Lienkaemper, a research geophysicist at the U. S. Geological Survey.
In fact, there’s a nearly 70 percent chance that one of them will rupture within the next 30 years, Lienkaemper says.
Of the four fault segments most likely to break within the next 30 years, the Hayward fault is of most concern.
“The Hayward Fault is the one that affects the most people and infrastructure,” Lienkaemper says.
But a rupture on any of the others could also have devastating effects, potentially disrupting drinking water supplies by damaging the tunnel between the East and West Bay as well as between the northern and southern parts of the state.
"A large earthquake on the Hayward Fault would affect the entire San Francisco Bay Area — over 5 million people," says Greg Beroza, the Wayne Loel Professor of Earth Sciences in the Department of Geophysics at Stanford University, who was no associated with the study. "Damage would likely be most severe near the fault in the East Bay, but there are areas removed from the fault, including the entire periphery of San Francisco Bay, and the Sacramento Delta, that would be very strongly shaken as well."
A USGS report in 2008 suggested that a major earthquake on the Hayward Fault would cause "hundreds of deaths, leave thousands homeless, and devastate the region’s economy."
Lienkaemper and his colleagues combined information on the recent behavior of faults in the Bay Area with what is known about past earthquake histories in a model that spotlighted the segments most likely to rupture in the near future.
In a process earth scientists have dubbed “creep,” large sections of the San Andreas system grind quietly past one another without producing any ground shaking. But fault segments in the Bay Area appear to be stuck, displaying very little creep. The only way for these locked and slowly creeping segments to catch up with the movement on the rest of the fault is to break suddenly in a major earthquake.
Currently the Pacific Plate is moving 38 millimeters a year past the North American Plate. That movement will eventually push the Los Angeles region, which is on the Pacific Plate, up alongside San Francisco but that won’t be for millions of years. In contrast, three of the fault segments discussed in the paper are creeping at less than 1 mm per year.
“The movement on all these faults has to add up to the plate boundary rate,” Lienkaemper says. “The Pacific Plate is having its way with the North American Plate and each fault has an assumed slip rate. But the locked patches are resisting.”
Scientists have determined how often the faults have ruptured by digging trenches in the fault zones and cataloguing the size and timing of each historical event. Take the Hayward Fault, for example. A section ruptured in 1868, producing a magnitude 6.8 earthquake. Trenching studies have shown that section ruptures roughly every 160 years.
Lienkaemper and his colleagues have estimated the size of the next earthquake on the three segments that are near to or have exceeded their average recurrence time: the Hayward, a magnitude 6.8, the Rodgers Creek, 7.1 and the Green Valley, 7.1. For comparison, the Loma Prieta earthquake, which badly shook the Bay Area 25 years ago this month, leading to 63 deaths, was a magnitude 6.9.
No one knows exactly why certain sections of the fault lock up. It’s possible that bumps and crevices on the surfaces that need to slide by one another jam it up, says Arthur Lerner-Lam, a research professor and deputy director of Columbia University’s Lamont Doherty Earth Observatory, who was not involved with the study. “A lot has to do with whether there are bends in the fault trace” — zigs and zags that can hang the fault up until enough pressure builds to break through.
Experts say they hope the new study will energize individuals, governments and businesses to retrofit older structures to make them safer. Homes, for example, should be bolted to their foundations. Structures made from rigid substances like brick or concrete need to be reinforced.
“What [these researchers] find is that four faults are loaded to near failure,” says Ralph Archuleta, professor emeritus at the University of California, Santa Barbara. “What this tells you is that within your lifetime there is a high likelihood that you will experience a significant earthquake with shaking as violent as in Napa and perhaps more violent if one of these goes. And the duration of strong shaking will be longer.”
That’s because a longer length of fault will be “unzipped,” Archuleta says. The larger the area that slips in an earthquake, the longer the time it will generate seismic waves that rattle and roar through and across the ground.
“The purpose [of a study like this] is to further reinforce the notion that California is earthquake country,” Lerner-Lam says. “And proper precautions should be made for an eventual earthquake.”
The recent Napa Valley earthquake might have helped nudge Northern California residents out of their complacency.
But Archuleta worries that people will still ignore the new warning because of the forecast's long-range nature — 30 years — and procrastinate on the upgrades needed to make their homes more earthquake resilient.
People need to realize that this doesn’t mean you’re safe for 30 years, he says. “What they need to think about is that it could happen tomorrow or a week from tomorrow,” he explains. “We don’t know when, but we do know that inevitably there will be an earthquake.”