Bay Area home to thrust faults / Some are hidden from scientists' view

Countless seismic faults just as dangerous as the one that triggered Monday's San Simeon earthquake are suspected to be lurking throughout the Bay Area.

Scientists have known about some of these faults for years and have mapped a few in detail. But many more are unknown, existing deep underground with no sign to betray their whereabouts -- or what potential damage they may someday prove capable of causing.

The 6.5 magnitude earthquake that rocked the Paso Robles region Monday was attributed to just such a fault. It's called a thrust fault, and in the Bay Area some can produce earthquakes just as severe as Monday's, according to geologist David Schwartz of the U.S. Geological Survey in Menlo Park, while the hidden ones pose hazards no one can calculate.

Most of the coastal Santa Cruz Mountains and the hills above the East Bay and west of San Jose are being slowly pushed upward by the action of these mysterious thrust faults. Every year, they can add a millimeter or more to the height of the mountains, which have been rising that way for millions of years.

Those geologic processes are very different from those operating along the more familiar San Andreas and Hayward faults, whose dangers are widely known. In thrust faults, one block of the Earth's crust moves up or down relative to the fault's other side; in faults like the world-famed San Andreas, the two sides of the fault move past each other side by side -- a motion scientists call "strike-slip."

"If you think of a floor that's warping and pushing a carpet upward into ridges," Schwartz said, "that's what a thrust fault does."

When strains build up beneath the ground, the motion at some point along a fault can occur abruptly with great violence. That's when a thrust fault earthquake strikes.

Russell W. Graymer, a research geologist at the USGS who produces digital maps of the Bay Area's faults, can name at least five of the Bay Area's known thrust faults whose locations are well documented.

There's the Verona fault, for example, which runs almost directly beneath a long-dismantled nuclear power plant in the low hills near Pleasanton. And although the Geological Survey's scientists can't estimate how large a quake it might produce if it ever ruptures, the unpredictable possibility of a strong temblor on that fault caused the government to order General Electric to dismantle its old nuclear reactor there in 1977. Scientists and engineers still conduct nuclear fuel research at the facility, however.

More familiar to East Bay residents is the Mount Diablo fault, which is well known to geologists even though it is known as a "blind" fault because ancient quakes thousands or millions of years ago have left no definite scars on the earth's surface.

The geology of the region, however, including the way Mount Diablo itself has been thrust upward, has enabled scientists to map the fault. And according to seismic researchers Thomas L. Sawyer and Jeffrey R. Unruh, the Diablo fault could produce a quake one day with a magnitude as high as 6.75 -- strong enough to cause plenty of damage in the densely populated East Bay suburbs near the mountain.

In the San Jose area, another blind thrust fault called the Shannon-Monte Vista could also generate a 6.5 magnitude quake similar to the one that hit the San Simeon region Monday, according to Graymer.

The Evergreen fault, east of San Jose, and the Moraga Fault in Contra Costa County are other examples of Bay Area thrust faults, although calculating the likelihood for strong earthquakes if those faults rupture is still being researched, according to Schwartz.

At the California Geological Survey, geologists research the seismic hazards that Californians face and prepare maps showing the potential for quakes' ground shaking, fault rupture, liquefaction and landslides. The state and federal agencies' geologists are now developing a new and detailed series of fault maps showing just where the newly detected thrust faults lie, and where other "blind" ones are suspected.

The devastating Northridge quake of 1994 that killed nearly 60 people struck on a blind thrust fault no one had ever suspected. So did the Coalinga quake of 1983, which caused no deaths but produced more than $30 million in damage.

More blind thrust faults probably lie well-hidden underground in many other Bay Area regions, according to Graymer. Oil companies drilling exploratory wells and blasting beneath the 116,000 acres of Suisun Marsh in the Delta have found geological evidence suggesting that thrust faults underlie the whole area.

And there is similar evidence of blind thrust faulting under the area of Pittsburg and Antioch, Graymer says.

Scientists from the Geological Survey continued their efforts Wednesday to find evidence that Monday's quake had actually ruptured the surface of the ground in the region -- evidence that would help them to map the length of the hidden fault and determine the direction it was moving.

They were working in the region's small towns where the worst damage occurred, and also trying to survey extremely rough terrain.