A new study deepens the knowledge of Silver Creek Fault, charting its precise route under San Jose. The emerging research suggests that if the fault erupts, the city could suffer high levels of ground shaking.

Seismic profiles of the earth underneath three streets near downtown San Jose - Empire Street, Mission Street and Gish Road - provide conclusive evidence of a shallow fault, according to a study by geophysicist Rufus Catchings of the U.S. Geological Survey in Menlo Park.

"Because the fault zone extends through downtown San Jose, and likely much of the East Bay, the fault may pose a significantly high seismic hazard to the region," Catchings said.

"The fact that it is directly under the Bay Area's biggest city makes it more of a concern," he said.

Geologists have previously mapped the so-called Silver Creek Fault in parts of Santa Clara Valley, and presumed it extended beneath downtown.

But the new research, reported Friday at a meeting of the Seismological Society of America in Monterey, more precisely identifies the location of the fault within the city of San Jose, running near Club Auto Sport, Cadence Design Systems and San Jose's Northside residential area.

The new evidence of young fractured rock near the surface of the earth suggests earthquake potential, posing greater risk of damage.

"This would be of concern because it would mean that parts of the San Jose urban area might be subject to high levels of near-fault ground shaking," said Thomas L. Holzer of the U.S. Geological Survey, who was not involved in the study.

The Silver Creek Fault is believed to be responsible for two major 6.1-magnitude earthquakes in the southeastern Santa Clara Valley in 1903. It is already known to geologists as a significant fault because of its impact on two unrelated phenomena: It influences how ground water flows and how earthquake shaking varies in the Santa Clara Valley.

But there is not enough evidence to predict the likelihood of another earthquake on the Silver Creek fault, noted seismologist Jack Boatwright of the USGS in Menlo Park, also not involved in the new study.

Without so-called recurrence data, it is impossible to forecast whether the Silver Creek Fault will rupture tomorrow - or thousands of years from now, he said.

"But it is not overstretching to say that this is an active fault - and even if there were only a 6 magnitude quake, there would be damage," said Boatwright. "It directly underlies part of San Jose."

Structural engineer Chris D. Poland of the San Francisco-based Degenkolb Engineers, which specializes in earthquake hazard work, said "the Silver Creek Fault sounds like one that we need to know about ASAP, although I'm not sure it will increase the overall intensity of expected shaking in the area, since there are so many faults that we consider now.

"Its location is very important" because it could influence the future of construction near the fault line, Poland said.

Catchings' team studied three high-resolution seismic profiles across the fault zone in 1999 and in 2007. These profiles corroborate previous views of the fault, acquired through magnetic and radar satellite imaging, yielding more definitive proof of the fault.

The geology of the Santa Clara Valley makes it especially vulnerable to earthquake damage, Catchings said.

Deep beneath the valley floor are two buried formations known as basins - enormous dips in the hard rock of Earth's crust that have filled with softer sediments, deposited by rivers, over millions of years. The Evergreen basin underlies the east side of the valley, and the Cupertino basin the west.

Because basins can trap and focus earthquake waves, they are sites of prolonged shaking. And the soft sediments can cause severe structural damage. It happened in the 1994 Northridge quake, where some neighborhoods shook three to four times harder than they otherwise would have, and in 1995 in Kobe, Japan, where shaking in some areas doubled.

In previous research, Catchings identified at least 210 earthquakes of magnitude 4.5 or less between 1967 and 1999 in the vicinity of the Silver Creek Fault.

"It is a concern, because it is under such a populated area,'' said Scarlett Li Lam, administrative officer for San Jose's Office of Emergency Services. The city has upgraded its building code and offers many classes in emergency preparation, so that San Jose residents know how to help themselves, their family and their neighborhoods in a disaster, she said.

"But we do not have to have a new fault to be more concerned. You never know which of our faults will become unstable," she added. "The important thing is to teach people not to panic, but get prepared - so that we're ready."