‘Hidden’ earthquake faults underlie Peninsula

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By Sue Dremann and Sarah Trauben

Embarcadero Media

Sunday morning, Dec. 19, 2010, started out with a bang for Los Altos resident Martha Siegel — and not the metaphorical kind. Sitting at her kitchen table, the earth shook beneath her.

Coffee sloshed in mugs and lights swayed in homes along Briarwood Court as the earthquake rattled residents out of bed at 9:28 a.m.

“It woke up my daughter,” Siegel said.

But Siegel, who works at SLAC National Accelerator Laboratory, located near the San Andreas earthquake fault, soon learned of something more jarring than the actual 3.1-magnitude quake. The temblor occurred on the Monte Vista fault (also called Monta Vista), and it is right near her home.

“I’ve been in Los Altos my entire life, and I had no idea,” she said of the fault’s existence, adding that a real-estate agent never disclosed her home’s proximity to the fault line.

Peter Roth has lived in the neighborhood since 1967 and has experienced “an uncountable number of earthquakes.” But “we were a little surprised when we heard where the fault line was. The USGS (U.S. Geological Survey) estimates it’s under my neighbor’s front door step,” he said, nodding toward the head of the street, “but the margin of error means it could be under my property,” he said.

The San Andreas fault might be the iconic expression of the shifting tectonic plates sliding deep beneath Bay Area residents’ feet, but numerous faults run up and down the Peninsula, some with a potential shaking power that has not been evident in what geologists consider “historical times” — since 1776.

Only a few have caused major quakes in recent memory: the San Andreas, Hayward and small portions of the Greenville and Las Positas faults, which are both located near Livermore, according to U.S. Geological Survey maps. But others are being watched.

In the Bay Area, Geological Survey scientists have predicted a 63 percent probability of a 6.7-magnitude quake or greater between 2007 and 2036. Seven faults contribute to that high risk. The San Andreas and the Hayward/Rodgers Creek faults in the East Bay carry the greatest risk, with 21 percent and 31 percent probability respectively.

Five other faults are cause for concern, however: San Gregorio, located near the coast and Half Moon Bay; and the East Bay’s Calaveras, Concord-Green Valley, Greenville and Mt. Diablo.

There are still other faults that could trigger quakes, according to Stephen Thompson, principal geologist with William Lettis and Associates, an earthquake-related, engineering geology and geotechnical services consulting firm in Walnut Creek.

Located between the San Andreas and the Santa Clara Valley floor, the Stanford (near San Francisquito Creek in Menlo Park through Stanford University to Page Mill Road), Shannon, Sargent and Berrocal fault lines are deemed potentially active with a magnitude estimate of 6.5 to 7.0, he said.

“Right now, they are considered active or conditionally active, based on their location and overall setting,” he said.

None of the faults are predicted to rupture on the magnitude of the San Andreas, which has been shown to produce quakes that are 8.0, plus or minus, he said. But they are capable of producing damaging earthquakes because of their locations and because they are under highly populated areas, he said.

Other active faults lie beneath our feet: the Foothills Thrust Belt (near Page Mill Road and U.S. Interstate 280), Pulgas (west of Palo Alto, Stanford and Redwood City), Pilarcitos, Zayante and Butano, according to Geological Survey maps.

And some are more ancient, not having moved for as long as 1.6 million years, including Palo Alto, La Honda, Ben Lomond and San Jose faults.

Some evidence is emerging that an ancient fault could produce a dangerous quake.

The Kern Canyon fault in the Sierra Nevada, which was previously thought inactive for 3 million years, was found to be active by California Institute of Technology scientists Elisabeth Nadin and Jason Saleeby. The fault moved as recently as 3,300 years ago. Geologists working for the Army Corps of Engineers in Sacramento have also determined that the Kern Canyon could trigger a magnitude of 6.5 to 7.5-magnitude quake.

Another previously inactive area, the Foothills fault zone, which last ruptured 1.6 million years ago, ruptured in 1975 on a branch within its fault zone near Oroville on what is now known as the Cleveland Hills fault, according to the earthquake-risk assessment plan for the City of Roseville.

But Thompson said the likelihood of a large quake occurring on ancient faults is distant when there is direct evidence that movement has not occurred for 1 to 1.5 million years.

“The chances of it rupturing are remote, but taken all together, we should be humble enough to acknowledge that a big earthquake could be on a fault we don’t recognize,” he said.

The 1989 Loma Prieta earthquake is a case in point. Scientists still debate whether the rupture occurred on the San Andreas or on another fault, he said.

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Carol Prentice, a Geological Survey research geologist, studies paleoseismology, the cycles of major quakes over time. Her current focus: the venerable San Andreas.

On a section of dirt road within the San Francisco Watershed, near the Crystal Springs and San Andreas reservoirs, Prentice and her USGS crew have dug a pit about 3 feet wide and 9 to 12 feet deep to get a first-hand look at the fault and where it has ruptured.

“A big earthquake fault will break all sediment layers on the surface. With more time, the rupture gets buried with new sediments,” she said.

Several feet down in Prentice’s trench, diagonal striations appear in the wall. These layers are filled with organic material that is different from the surrounding area — younger sediments that entered the rupture.

Using dating techniques, scientists can determine the ages of the sediments and how many years ago the rupture occurred, she said. The San Andreas has more or less timely ruptures about every 200 to 300 years.

The Hayward fault in the East Bay is of even greater concern — “its recurrence interval is about 140 years and is relatively constant,” she said. The last major earthquake on the Hayward fault was in 1868 — 143 years ago, she said.

In addition to digging trenches, Prentice uses LIDAR (Light Detection And Ranging) technology and aerial photography to study faults. LIDAR can digitally remove trees from the earth’s surface to allow scientists to see the faulted ground, she said.

“The real question about these faults is if they are part of the same structure or are separate structures,” Prentice said of the smaller faults.

If these lesser faults are connected to larger fault systems such as the San Andreas or Hayward, they could also slip if a major quake occurs on the larger faults.

Thompson said there is evidence the Shannon and Monte Vista faults slipped during the 1906 quake and also after the Loma Prieta quake. Such factors are taken into consideration when making engineering decisions, such as building dams or reconstructing water systems.

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The earthquake picture in the Bay Area is complex, researchers said, with different types of faults potentially affecting one another.

With vertical strike-slip faults, such as the San Andreas, the two sides move horizontally past each other. With thrust faults, one plate slides under the other at an angle and lifts up the earth. This is how the Santa Cruz Mountains were formed, Thompson said.

The interplay of thrust and strike-slip faults are being studied by scientists and could have broader implications for understanding how one fault line might affect a far distant one.

The Serra fault, which was recently found to have a branch parallel to the San Andreas under the city of San Francisco, has become a cause for concern among some scientists, who believe it could be activated by a large temblor on the San Andreas. The fault has moved within the last 10,000 to 11,700 years, ranking it as “active,” Thompson said.

Blind-thrust faults, which fracture deep in the earth, can also create potentially damaging earthquakes, but scientists might not know they are there because the fault is not evident at the surface, Thompson said.

A blind-thrust fault was responsible for the 1994 Northridge earthquake in Southern California.

A potentially dangerous blind-thrust fault is believed to run diagonally west near Bolinas Ridge east of Mt. Tamalpais and could contribute to seismic hazards throughout the Bay Area, according to a 2004 study by Penn State geoscientists. And a damaging 5.2 quake on a previously unknown thrust fault occurred Sept. 2, 2000, in Yountville, Napa County, causing an estimated $10 million in damage and displacing 70 people, according to the USGS.

Thompson said one should not think that the area is rife with undocumented faults, however.

Scientists, especially the Geological Survey, have amassed a large and accurate amount of knowledge regarding the area’s tectonics, he said.

“We have been mapping for decades — the geologic mapping is to a high level. The USGS has done an extremely diligent job to characterize what’s going on. Where the major faults are, the understanding is very high. To the first order, we understand the sources of future damaging earthquakes,” he said. “But there is always the humility that comes with a high-magnitude quake.”