A gradual underground shift that slowly twists sidewalks and cracks foundations may be lowering the likelihood of a major earthquake on the northern end of the high-risk Hayward fault.
A study by UC Berkeley earthquake scientists shows that the Hayward fault is moving millimeters per year both at the surface and several miles below, a surprising finding that could change how researchers look at the fault.
What it probably doesn't change is the risk for people living along the fault, which twists among the hills in Berkeley and Richmond before dipping into San Pablo Bay.
"We are clearly not off the hook," said Stanford University geophysics professor Paul Segall. "You can suffer a lot of damage from an earthquake that is not very close."
This gentle shifting of the ground, aptly called creep, stands in sharp contract to the sudden jolts residents foresee and fear as part of living in California.
As two massive continental plates move past each other, they scrape at the edges. In most seismic events, the plates stick and release in jerky motions that can cause massive destruction.
But creep is different. Instead of sticking, the plates glide past as if they are greased with oil. Creep leaves a different kind of destruction, tearing apart houses and businesses a few millimeters each year.
For years, scientists have known the Hayward fault is creeping at the surface -- as evidenced by the gradual destruction of the UC Berkeley football stadium and an entire neighborhood in downtown Hayward -- but scientists thought the lower layers were stuck. That concept was based mostly on evidence of several large earthquakes ripping through that area in the last 2,000 years.
"It seemed like a clever explanation that lets you have your cake and eat it too," said Bob Simpson, a U.S. Geophysical Union geophysicist and author of an article accompanying the Berkeley study.
The study, published Thursday in the prestigious research journal Science, is the first to say that a 12-mile section of the Hayward fault may not be sticking at all.
What this means for earthquakes is up to interpretation. It may mean faults that creep from top to bottom can't create their own large earthquakes, because there's no stress left to cause a quake.
Some scientists think that may be going too far. Instead, it could lengthen the time between big quakes or eliminate that possibility altogether.
Geology professor Roland Bürgmann and his colleagues used several new techniques to show that the area around the fault isn't twisting into the mountains and valleys as expected of a stuck fault. Instead, it seems to be sliding by without a hitch, an unexpected finding that has longtime fault followers doing a double take.
To show this, Bürgmann took two new looks at the fault, from high in the air and deep underground.
The team looked first at satellite radar data beginning in 1992. This high-flying look should have shown a buildup of stress around the fault from the plates sticking together, Bürgmann said. Instead, it show no stress at all.
For a second opinion, the team went to evidence of earthquakes so small that people could never feel them. The same size quakes repeated over and over in the same spot, which shows that small patches of the fault are sticking while the rest is sliding unrestrained, Bürgmann said.
"I think he opened our eyes and made us consider more seriously the range of interpretations" of creep, said Jim Lienkaemper, a geologist with the U.S. Geological Survey and longtime Hayward fault observer.
"It definitely has to make earthquakes a little bit smaller, but the devil is in the details."
What makes interpreting the study's results even more difficult is that scientists still aren't sure what causes creep in the first place. It might be the kind of rock involved, a likely suspect because California's state rock, a soft greenish stone called serpentinite, is often present when faults creep. Or it could be how water builds up inside the fault, lubricating the fault as its two sides slide past each other.
Either way, scientists have already taken the results seriously. A group of geologists incorporated the study's findings into Bay Area-wide earthquake probabilities released last year. In part because of the study's findings, the group decreased the probably of a large quake on the 12-mile-long northern Hayward fault.
"Loma Prieta was in the Santa Cruz mountains, and it knocked down bridges 60 miles away," said Tousson Toppozada, a senior seismologist at the state's Division of Mines and Geology. "Twelve miles is not a big shelter."
The overall probability of a quake on the southern segment of the fault, which has had massive quakes as recently as 1868, or its northern connector, the Rodgers Creek fault, remains high. And there is still a lot of research to be done before scientists really understand how faults work or how creep factors into that.
"The Hayward fault has been a really neat laboratory," Simpson said. "The fault is kind of talking to us and creeping along."
Andrea Widener covers science and the area's national laboratories. Reach her at 925-847-2158 or awidener@cctimes.com.
