'Hidden' earthquake faults underlie Peninsula

Beneath Peninsula residents' feet, multiple earthquake faults traverse the land

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.


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.


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."

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1 person likes this
Posted by Lives-In-A-Quake-Zone
a resident of Another Palo Alto neighborhood
on Feb 25, 2011 at 11:50 am

This is an interesting article. The USGS also has a lot of material on earthquakes on their Menlo Science Center web site:

Web Link

There are a number of troubling questions, though. For instance, what does a homeowner (or a potential homeowner) do with this kind of information/data?

or ..

Even if the USGS could predict earthquakes, what should the residents, local, county and state governments do to take advantage of foreknowledge of a pending earthquake?

Increasing the effectiveness of building standards/codes in earthquake-prone areas seems to be something that "government" should be doing. Hopefully, "government" is inspecting/rating all large structures for their ability to withstand seismic events.

It's nice that this information is available, but it's not clear that we (as a society) know what to do with it.

1 person likes this
Posted by look it up yourself
a resident of Adobe-Meadow
on Feb 25, 2011 at 12:06 pm

Don't blame you real estate agent. Look up the earthquake zones yourself. Lots and lots of tax money goes to create these maps and make the available to the public. Besides, the latest maps are probably a lot more accurate than what your agent may have known in 1967. The area between Foothill Expressway and the Pacific Ocean is laced with fault lines. And the area between Hwy 101 and the bay has lots of semi-stable landfill.

1 person likes this
Posted by look it up yourself
a resident of Adobe-Meadow
on Feb 25, 2011 at 12:11 pm

What can you do with the information? Talk to an engineer and stabilize and reinforce your own house without having the government nanny you. And talk to your insurance agent to get the coverage appropriate for your situation. Owners of large buildings are probably already doing this.

1 person likes this
Posted by Lives-In-A-Quake-Zone
a resident of Another Palo Alto neighborhood
on Feb 25, 2011 at 12:27 pm

> The area between Foothill Expressway and the Pacific Ocean is
> laced with fault lines. And the area between Hwy 101 and the
> bay has lots of semi-stable landfill.

So it should be zoned as "unstable..due to being earthquake prone" and residential construction disallowed?

> Look up the earthquake zones yourself

Hmmm .. most people can't read a simple road map. What are these sorts supposed to make of a seismic activity/fault zone map?

1 person likes this
Posted by get a grip when it's shaking
a resident of Professorville
on Feb 25, 2011 at 12:49 pm

Sue and Sara, please consult the online USGS fault maps for your edification. The Monta Vista fault in not "right near her house".
It's on the west side of 280, above Eastbrook and slices across Magdalena (it's the reason the San Antonio Hills are there from the uplifting thrust of eons ago. Ms. Siegel' Briarwood home is almost 2 miles east of the Monta Vista fault trace.
Sensationalism in media outlets only confuses people. Complacency sets in when the big one doesn't hit. Christchurch New Zealand was hit by a Thrust fault near the surface, but they were prepared because of the Sept. 2010 quake.
Yes, be prepared, reinforce your house with Simpson strong tie straps above and below, reinforce your brick chimney, have a fully stocked first aid kit, portable radio, flashlights, plenty of extra batteries, 3 days of drinking water, some propane for cooking and sterilizing water on your outdoor grill, $200. cash in the house as ATM's, Credit card machines may be down if power goes out. Inside, make sure water heater is well strapped, heavy bookcases and tall cabinets are secured to the wall with L brackets. Outside, make sure you have a gas meter shut off wrench ($10 at OSH)strapped near the gas meter,
Emergency communicatin plan, have a text (not cell) plan to reach loved ones. Cell towers will be overwhelmed with panicked folks. Texting is more reliable. Keep it short, have a rendevous point if possible, but don't gab with "OMG, DID YOU FEEL IT?, I'M SO FREAKED"
Keep the cell lines free for really important communication links to confirm safety of loved ones.

1 person likes this
Posted by homeowner
a resident of Another Palo Alto neighborhood
on Feb 25, 2011 at 1:42 pm

If you know this info, it must be disclosed in writing to any prospective buyers. A lot of stuff must be disclosed -- it is NOT "buyer beware" when it comes to residential real estate.

1 person likes this
Posted by narnia
a resident of Menlo Park
on Feb 25, 2011 at 2:57 pm

Certainly some precautions make sense, but Professorville speaks as if all live in single family housing (YOUR outdoor grill he/she says). The truth is that for many multiple units buildings /townhouses of which there are a huge number in the area, for renters and owners alike there is little they can do to reinforce a building for practical reasons. The renter needs the landlord to do it and the condo owners need others to agree to do it. Even the gas shut off valves are off the limits in large buildings. For short term visitors even those aware of the danger some things cannot be done. Sensible guidelines are issued only for single family units. Those guidelines should be made into law for medium to large multiple units building.

1 person likes this
Posted by Lives-In-A-Quake-Zone
a resident of Another Palo Alto neighborhood
on Feb 25, 2011 at 4:34 pm

> Keep the cell lines free for really important communication
> links to confirm safety of loved ones.

Laptops/Netbooks/etc. should be configured with Skype, or some sort of similar telephony software (google has a free outbound service at this time). This means buy a headset, and maybe setting up an outbound account with a few dollars balance. WiFi hot spots will be up where electricity and telephone service is up, so people could make telephone calls via VoIP in these hot spots-taking some pressure off cell phone service.

1 person likes this
Posted by B-Obama
a resident of Atherton
on Feb 26, 2011 at 8:40 am

Don't expect FEMA to rebuild your house if it is damaged in a quake either. Many people are under the misconception if a major quake hits the area and is declared a disaster zone that FEMA will pay to have your house rebuilt. They will not. They will only assist with living expenses and might offer low interest lows to rebuild your house. Only earthquake insurance will pay to rebuild your house. Yes I am an insurance agent. Just want to clear this myth and misconception many people have. Oh and don't forget if you have children brace all your tall, large or heavy furniture to the wall to prevent injury if they fall over in a quake or minor trembler.

1 person likes this
Posted by Local Observer
a resident of Los Altos
on Feb 26, 2011 at 9:40 pm

Another serious issue is liquefaction. Think what happened to the Marina in San Francisco during Loma Prieta; the Marina was some 70+ miles from the epicenter and had massive damage. My home was much closer to the epicenter and only some knick-knacks fell from a shelf and were damaged (and easy to repair).

I did some research earlier this week about this issue and found some interesting new websites with maps, one of which may make Palo Alto residents' spines tingle (see below URL aka web link).

I don't know if there's a message size limit here, so I'll keep this short and point only to top-level URLs from which you can easily find the PDF maps.


Web Link

Second, again from the USGS, "Maps of Quaternary Deposits and Liquefaction Susceptibility in the Central San Francisco Bay Region, California". Start at this URL:

Web Link

And now the spine-tingly ones, from the California Department of Conservation dated February 2009 (i.e., current). Start at this URL:

Web Link

The two most important files for Palo Altans are from the California Geological Society, Seimic Hazard Zones, Palo Alto Quadrangle, PDF map here:

Web Link 6.9MB

and the 70-page report "SEISMIC HAZARD ZONE REPORT FOR THE PALO ALTO 7.5-MINUTE QUADRANGLE, SAN MATEO AND SANTA CLARA COUNTIES, CALIFORNIA" accompanying the map which is available here:

Web Link 7.3MB

1 person likes this
Posted by Jocelyn Dong
editor of the Palo Alto Weekly
on Feb 28, 2011 at 11:00 am

Jocelyn Dong is a registered user.

A reader has asked for the link to the interactive USGS map that was printed on page 16 of the Feb. 25 edition of the Weekly (and also included in the online article, above).

Here's the URL for the Quaternary Faults Web Mapping Application: Web Link

1 person likes this
Posted by Steve
a resident of Menlo Park
on Feb 28, 2011 at 12:46 pm

Here's link to a guide to earthquake preparation that USGS, Red Cross, FEMA, and other agencies put out a few years ago. It describes the earthquake hazards in the Bay Area and what both homeowners and renters need to do to be prepared.
Web Link
Hard copies of this publication are sometimes available at Home Depot and Orchard Supply. They're always available at the map counter at USGS in Menlo Park.

1 person likes this
Posted by george
a resident of Charleston Gardens
on Feb 28, 2011 at 1:14 pm

There is a wealth of information on the following web site:
concerning what to do to prepare for an earthquake.

Like this comment
Posted by Faulty Info..
a resident of Midtown
on Feb 28, 2011 at 4:00 pm

Both the owner selling the house/lot and the person planning to buy the lot/home should be told about facts of the land by the realtor. But I'm not sure just what a realtor is truly responsible re: the structural parts of the house, the ground, etc.
Anyone know exactly how it should work?

Sorry, but further commenting on this topic has been closed.

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