Loma Prieta 25 years later: SF State project monitors Bay Area faults
It is rare that a day goes by without at least a small earthquake on one of the hundreds of faults that slice through California. Yet even when things are seismically quiet, the Earth's crust is on the move. Rocks on either side of a fault can slip past each other without any quake occurring, a process geologists call creep.
Since 1979, San Francisco State University's Creep Project has been measuring these millimeter-sized slips at nearly 90 sites along nine major faults across the Bay Area and Northern California. The program has yielded a meticulous record unlike any other in the world, researchers say -- one that can help scientists assess the strength and location of future earthquakes in the region.
Each year, University scientists and students fan out to the creep-measurement sites, which include traffic-clogged intersections and remote hillsides. Each site contains permanent markers -- everything from monuments set in concrete to nails driven into the street -- on opposite sides of the fault. Project volunteers use a surveying instrument called a theodolite to measure changes in the angle formed between the opposite markers. If the angle has changed, creep has taken place.
Stress is the reason to study creep, according to John Caskey, associate professor of geology and head of the project. Tectonic forces around a fault build up stress in the surrounding rocks. In faults where friction locks the rocks in place, stress relief comes when that friction is finally overwhelmed and that built-up stress is released in the form of energy: an earthquake. But in faults where friction is weak and rocks slide past each other more easily, creep acts as the main stress reliever.
"Most faults don’t creep, but in the Bay Area we have lots of serpentinite," said Caskey. The dark and slippery metamorphic rock, he explained, is "very, very weak, with low frictional properties, and when it gets saturated with water it gets even weaker."
The Creep project was created by SF State Professor Emeritus Jon Galehouse, with funding from the U.S. Geological Survey's National Earthquake Hazards Reduction Program. Galehouse and others, including USGS researcher James Lienkaemper, were curious to find out whether creep might affect the number or strength of earthquakes along a fault. The project has found some evidence connecting quakes and creep, Caskey said, although the relationship is a complex one.
"The area of the fault that may be creeping between earthquakes is not building up as much stress, so if an earthquake comes it's probably going to decrease the earthquake's size," he added. "But it may not reduce it by a lot, and the fault could still produce damaging earthquakes."
With more than 30 years' worth of observations on some faults, the Creep database gives scientists a better idea of what constitutes normal slip on particular faults, "and now if we see big deviations in their behavior sometime in the future, we're going to be alerted," Caskey commented. Read our main story about earthquake science and engineering.
Creep program manager Forrest McFarland saw dramatic evidence of a link between quakes and creep soon after joining the project in 1995 as a geosciences undergraduate. After a year and a half of virtually no creep detected at one station on the southern Hayward fault, McFarland was shocked to see a "strikingly large" 25-millimeter shift. "It had broken water mains in the street and tore the asphalt," he recalled. "Dr. Galehouse was so excited about it that he came out to measure it himself."
The huge change was probably a response to the 1989 Loma Prieta earthquake, the researchers later suggested. The Loma Prieta quake took place along the San Andreas fault, but its huge release of energy reduced stress around the adjacent southern Hayward fault too. It may have taken several years after Loma Prieta for enough stress to build up again in the crust and cause creep to resume on the Hayward, McFarland said.
Some faults creep rarely, some creep steadily and others stop and start abruptly. For instance, a Creep station near the West Napa fault had been so inactive over the years that the project had stopped taking measurements there. Recently, after the magnitude 6.0 quake there in August, the team placed five new stations along the fault that are now detecting strong after-slip movements.
The Creep team has added new sites over the decades, while cutting back visits to each site to once or twice a year as funding for the project has shrunk. The team's orange safety vests, tripods and tarps are a familiar sight at some stations, McFarland remarked. "We have sites where people get to know us, their dogs get to know us, and we get to see them year after year, see their kids grow up."
Creep's numbers are publicly available and used by everyone from seismologists to offices such as the Santa Clara Valley Water District, which keeps close tabs on creep that could put stress on their reservoirs' dams, he noted.
The project welcomes one or two new student members each year, and the work offers them an immediate gratification that can be unusual in the sciences, according to McFarland. "One of the really neat things for the students is that they can come up with the amount of change that happened right there in the field, and know instantly what the creep has been since last year," he said. "It’s pretty cool for a student scientist to be the first one to know something."
-- Becky Ham