Hi All. Just thought you would like to know what the scientists are thinking about the recent quakes in the North Bay. Take Care...Don in creepy town

Ground stress could be increasing
Scientists unsure if news is good or bad

A puzzling surge of recent earthquake activity in the northern Bay Area has left scientists wondering whether the modest tremors presage a major quake on the San Andreas Fault system as stress builds up deep underground.

But it is also possible, they say, that the smaller quakes could actually be relieving some of the strain and thus delaying the Big One that all earthquake experts say is virtually inevitable within the next 20 to 30 years.

The new concern comes on the eve of today's 96th anniversary of the great 1906 San Francisco earthquake, which claimed 3,000 lives and caused the fire that destroyed much of the city.

To mark the anniversary, the Earthquake Engineering Research Institute in Oakland announced yesterday a new four-year campaign, called "Quake06," to upgrade seismic retrofitting in homes, public buildings, schools, hospitals and industries throughout the Bay Area.

According to Chris D. Poland, president of the earthquake engineering group,

90 percent of California buildings do not fully meet current standards for seismic safety design, 50 percent need to be strengthened, and 10 percent "are just downright dangerous."

Among the most hazardous, said Peter Yanev, president of the group's Northern California chapter, are buildings made of precast concrete, wood- frame homes with garages on the ground floor, and hotels and public buildings with large ballrooms, lobbies or auditoriums on their ground floors.

Over the next four years, Yanev's group will work with city and county governments, construction firms, school districts and health care organizations to seek out hazardous structures and push retrofitting of dangerous buildings throughout the region.

Assessing probability of a major quake in the Bay Area, scientists at the U. S. Geological Survey in Menlo Park have set the odds at 70 percent that one with a magnitude of 6.7 or greater will strike somewhere in the region in the next 30 years -- "a near certainty," as Yanev put it yesterday.

Geologist David Schwartz of the Geological Survey said that concerns now focus on the Rodgers Creek Fault, which stretches from San Pablo Bay north through Sonoma county. Many scientists consider it to be a northern extension of the Hayward Fault, where scientists estimate the chances are one in three that a major quake will hit the fault within 30 years.

Surveying recent quake activity throughout the North Bay, Schwartz noted that the Rodgers Creek Fault has experienced 14 small tremors since 1970. The largest one, with a magnitude of 4.2, struck less than three years ago.

Near Cloverdale in northern Sonoma county, the Mayacama Fault has been extremely active recently, Schwartz noted, with five quakes recorded in the past five years -- including two with magnitudes of 4.2 and 4.3 in less than 10 days in January 2000.

And in September of 2000 a strong 5.2-magnitude quake hit near Yountville in the Napa Valley, causing nearly $60 million in damage throughout the area. It shook the ground severely, but caused no surface rupture at all.

Scientists located the epicenter of that quake beneath the summit of Mount Veeder. It struck on a wholly unknown fault that had never been mapped, Schwartz said -- proof that many strands of the broad San Andreas Fault zone have yet to be discovered and that their true dangers are unestimated.

"What all this means for the long range, we really don't know," Schwartz said, "but the trenching we've done on the Rodgers Creek Fault shows that major quakes, with magnitudes of at least 7, could have occurred there in 1670 and again in 1776."

The exploratory trenches seeking evidence of past quakes show that the average return time for major temblors on the Rodgers Creek Fault is about 200 to 225 years, Schwartz said. And if that is so, he said, then a truly big one may well be about due now, he said.

But there are still too many unknowns in earthquake science, Schwartz emphasized. "For the long range, we really don't know -- we can't tell from the recent quakes in the North Bay whether the stresses are building up, or whether they're being relieved."

E-mail David Perlman at dperlman@sfchronicle.com.