The Silicon Valley liquefaction zones cover much of the Bay Area and Santa Clara County, but the risks are often not well understood or investigated. We know that this is earthquake country and tremblers are to be expected. But what difference does it make where you live or work – won’t the whole valley be shaking equally?
Well, we’re Realtors, not geologists or geotechnical engineers, but we can share some resources that may help answer these questions and provide avenues for further research on this topic.
What is liquefaction?
Liquefaction refers to the ground becoming liquified, or fluid. It takes 3 ingredients to liquefy land: loose sediment, water, and strong shaking. This loose, saturated soils when shaken to a certain point no longer behave like a solid and can slide, open, and swallow anything above it.
In 2010 and 2011, New Zealand experienced this and it made worldwide news. The Science Learning Hub website states that “During the Canterbury earthquakes of September 2010 and February 2011, liquefaction caused silt and fine sand to boil up and bury streets and gardens and caused buildings and vehicles to sink.”
But I never heard about that happening during the 1906 quake, or the 1989 quake – that means it’s not a risk here, right? Wrong. The USGS Liquefaction and Sea Level Rise explains that a relatively dry rainy season in 1906 lowered the liquefaction risk, and the 1989 quake happened near the end of the dry season when groundwater levels were at their low point. There was liquefaction from both of these events, but it could’ve been much worse.
Silty, sandy soil will respond very differently to bedrock and clay in the case of extreme shaking. So no, the valley won’t all be shaking equally in the case of a large tremor. Liquefaction hazard zones will likely get the worst of it. That’s why this designation matters so much.
What is a liquefaction zone?
After the Loma Prieta earthquake in 1989, the Department of Conservation, California Geological Survey created maps to make residents aware of areas in which there are increased risks from earthquake shaking due to landslides or liquefaction and to make sure that construction in those zones have extra investigational requirements to build safely. The liquefaction zones are noted by the state to be more susceptible to dangerous liquefaction in the event of an earthquake. You can learn more about these and related issues at the California Department of Conservation’s website. (more…)
When buying or selling a home in California, consumers can learn if the property is in a zone marked for potential flooding due to dam failure. The disclosure companies will not say which dam is causing the risk, or if it’s several since often there will be more than one dam along the same creek or river, as is the case along the Los Gatos Creek.
To clarify, the 100 year flood plains and 500 year flood plains are different from the flooding due to a breach in a dam or levee. If someone buys a financed home in a 100 year flood plain, the lender will require flood insurance.
Also relevant to the requirement for flood insurance: the 100 and 500 year flood plain inundation risks are considered to be “natural hazards” and the flooding from dam breaches are not, since humans have created the dams (or levees). Per FEMA: “a flood resulting from changes in river flows is a natural hazard, whereas flooding due to a dam failure is considered a manmade hazard, and therefore excluded from the National Risk Index.”
Approximately 25% of floods in the U.S. are outside of the flood plains. It may be advisable for those in other areas to consider buying flood insurance even if it is not required by the lender.
Map of flood risk from dam failure
Recently Clair sleuthed out the State of California’s Dam Breach Inundation Map Web Publisher. This interesting and helpful tool is an interactive map covering both most counties and most dams. I encourage our readers to check it out.
Click on the image to view the live Dam Breach Inundation Map to better understand the risk of dam failure
This Dam Breach website indicates which dams have their flood risk from failure marked in one of four colors to indicate the higher or lower risk levels. In Santa Clara County there are more than a dozen dams marked at the highest risk category, and some of them hold a large capacity of water. (Some of the dams appear to not be at lake-like reservoirs, but more like covered percolation ponds or holding tanks.)
The concern today, of course, is what happens if we experience a large earthquake and any of these dams fail.
Lake Anderson in South County is the largest reservoir in Santa Clara County. It is undergoing a 10 year seismic retrofit and until it’s completed is at only 3% capacity for safety reasons. You can view a short video about the retrofit here. With that in mind, take a look at the image below, which showcases what areas could be inundated should the dam there fail.
Click to view the live map
The map doesn’t show if the flooding goes beyond the county line to the south, and unfortunately not every county appears to be participating in this website and not every reservoir is covered. I was curious about the enormous San Luis Reservoir just beyond our county’s border, but there was no map for that one.
Earthquake faults and flood plains are of interest to home buyers throughout the Golden State and to their lenders, too. Part of the home sale and home buying process is to provide information on these risk factors so that consumers (and their lenders) can make an informed decision.
Natural Hazard Reports are included in the disclosures when homes are bought and sold here in Silicon Valley. Those reports will indicate whether or not the property is located in areas with known natural hazards, including
Flood Plains (100 and 500 year floods from heavy rainfall)
Just about every year California amends what sellers are required to disclose, and one change that I think we’ll be seeing a lot of is about home fire hardening. Many agents, including myself, will begin to use the fire hardening disclosure / document (which has already changed once in six months). The current one, as of June 2021, is the C.A.R. Form FHDS, 5/21 Fire Hardening and Defensible Space Advisory, Disclosure, and Addendum.
So what is in this document, who will have to use it, and how can it help buyers and current home owners?
The CAR Fire Hardening and Defensible Space form is a two page document completed by the seller of a residential non-commercial property to notify the buyer of fire hazard zoning, code compliance, and possible vulnerabilities and/or defensible features. Both the buyer(s) and seller(s) sign to acknowledge receipt and consent to comply with the appropriate terms in paragraph 4B.
Who Will Use This Form? Paragraphs 1 and 2: Prerequisites
This disclosure is required for homes (1-4 unit residential properties) in high or very high fire hazard severity zones when the seller must complete a Transfer Disclosure Statement (TDS form). Sellers for California real estate transactions falling within those criteria are obligated to provide specific information contained in this form to the buyer. If these properties were improved or were built before January 1, 2010 there are additional stipulations. However, use is not restricted to properties in these zones.
Owners of residences where the zone is unknown, or those outside of the designated fire hazard zones which are “in, upon, or adjoining a mountainous area, forest-covered land, brush-covered land, grass-covered land, or land that is covered with flammable material,” (Gov’t Code 51182 and 1C in CAR FHDS 5/21 – basically, homes in or near ample kindling) should also make these disclosures if they might be considered materially important. Even when it’s not legally necessary, any homeowner might voluntarily disclose using sections of this form. To show that a home is not in a designated high or very high fire hazard severity zone, sellers simply check the box indicating so in paragraph 2B.
Is the address in a high or very high fire hazard severity zone?
Not all homeowners know if their property is in one of these zones, but it’s the seller’s responsibility to find out! In paragraph 1B the form suggests that a natural hazard zone disclosure company could determine this information (and if you’re selling you may have already ordered a report that would contain those details), but it certainly isn’t the only resource.
Tanbark wood mulch separated by a dry creek bed, cement path, & brick retaining wall in a low-water San Jose yard.
May Day is a celebration of spring, but “April showers” were few and far between and it’s already starting to feel like summer! With another record-breaking hot year and the Bay Area in severe to extreme drought conditions homeowners concerned about water use and fire prevention are turning to gardening and landscaping for the solution. But a word of warning! Since updating my surprisingly popular post on mulch vs tanbark and the risk of termite infestation, I came across another reason to be cautious when applying it to your perimeter: fire.
Organic Mulches and Fire Hazard
Mulch can work wonders in a garden – it helps soil retain moisture, protects roots, reduces weeds, insulates the ground, can add nutrients and enrich the earth, adds visual appeal, and it’s affordable. It’s on every guide for landscaping water conservation (including Valley Water’s recommendations and San Jose Water’s tips)! Do a search and you’ll find it comes in a broad variety of materials. These can be divided into two groups: organic and inorganic. And organic matter can burn.
Example of a Santa Clara Valley home with a creek behind the house. Not every waterway is scenic.
Silicon Valley has a bad case of “urban sprawl”, unfortunately, but there are places in San Jose and nearby where creeks meander through neighborhoods, offering a little extra space between back neighbors. This extra breathing room is valued by homeowners with a creek behind the house. They often cite the pleasantly rural sounds of frogs and birds as an added bonus.
But some home buyers are a little spooked. Are there risks with buying real estate next to a waterway? Would the home flood in heavy rains? Is there an excess of unpleasant wildlife to worry about? One of my buyer clients was concerned that burglers would use the creek’s access path to steal things and get away unseen. Another was afraid of cougars or bobcats or other unwelcome visitors coming in from a creek or tributary.
When Jim and I were newlyweds, we lived in a townhouse on Neary’s Lagoon in Santa Cruz (a bird sanctuary) and I have sold several homes along creeks or ponds, so will make some comments based on my experience.
Creek behind the house: scenic or not?
In general, I would say that being next to or near a creek most often will improve the value of the home because creeks are scenic and also provide a space buffer between rear neighbors. They frequently have beautiful old trees framing their banks and are slightly curved, too, so these are usually quite pretty. I won’t say that living next to a waterway which looks like a Los Angeles flood control channel would be beautiful or enhance a home’s value much, though the space between neighbors would still be appreciated. Each case must be judged on its own merits.
Wildlife at the water’s edge
It is true that there will be more wildlife near water, whether it’s a creek, river, reservoir, pond, or percolation pond. Birds, reptiles and animals need water and will seek it out. If you love nature, you may welcome the sound of frogs and geese, and perhaps secretly hope to see a wayward deer! If you decide to live near water, it is very important to make sure that wildlife cannot enter your home (chimney, attic and crawlspace included) and it will require some ongoing diligence to keep them out because they will be drawn to the water over and over again. I’ve known people adjacent to water to have some challenges with birds, bats, mice, rats, and other creatures trying to make their way in. But that can happen anywhere. At our current home, which is not next to or near a creek, we had a squirrel try to claw its way through flashing on our roof to get into the attic. Another time we had a possum or raccoon get into the attic. Be clear that being away from the water doesn’t mean “no wildlife issues” – but if you are next to water, you will probably face them a little more often.
Floods and flood plains – what is the risk if there’s a creek behind the house?
Creekside locations do not all flood; this is perhaps the biggest misconception. When buying a home, you can check the flood plain status via the Natural Hazards Disclosure Report, which the seller provides. You can also check online at the CAL My Hazards Awareness site. And please know that there are different types and levels of flood plains – they are not all the same! The one which requires flood insurance is called a 100 Year Flood Plain and in those locations, water of up to 1 foot may be expected once every 100 years (so not that often). There are 500 year flood plains and areas which are “dam failure inundation” zones (if a dam were to break, water downhill would flood, of course).
Protected species that depend on the waterways
We have a number of protected species in California, including certain frogs and salamanders. If your home (or the one you want to buy) is in the habitat area of those animals, birds, or reptiles, you may have some constraints on landscaping near the creek or water. Most of the time it involves not placing a fence within so many feet of the creek and using only native landscaping in that area close to the creek too. Just know that having a creek behind the house may carry extra responsibilities and restrictions.
As for crime, I would have to say that you want to always check a site like CityProtect.com or similar sources to know what’s happening. We do have crime everywhere, and all kinds, to varying degrees. Most creeks do not have easy access to people’s homes or yards, and often the service road along the creek is a rough gravel, so I have a hard time picturing burglers trying to get in and walk their stolen loot a ways down that path. But check the reports. Realtors are not crime experts and we cannot make promises about any area or location.
Valley Water depth to first groundwater in Santa Clara County
In most of Santa Clara County, home owners do not own the right to drill a well and pump groundwater under their property. That will be clarified in the preliminary title report (similarly, oil and mineral rights are usually not sold with residential neighborhood parcels here). The depth of the water may be of interest, though, as a high water table may have possible risks.
High Water Table Risks
In come cases there could be problems with springs under the home. When that happens, the groundwater may percolate up under the house during times of heavy rains, causing water in the crawl space and possibly creating foundation damage later.
Another risk is underground water moving environmental hazards to your property. A plume of water could potentially move toxins released at a leaking underground storage tank (“LUST” site) to your property’s area, where you may be responsible for the cleanup! (For info on the leaking underground storage tanks, you’d want to read the JCP or similar report for natural and environmental hazards. That will tell you if there are any LUST sites within 1 mile of the property.)
Does your home, or the one you want to buy in the Santa Clara Valley, have a high water table? One way to learn is to check out the interactive map on the Valley Water website. Much or most of Campbell seems to have first groundwater at a depth of 50 to 100 feet. Much of the low-lying areas of San Jose and Santa Clara have water at a mere 0 – 10′!
The interactive map that I saw includes most, but not all, of the valley. Enjoy checking it out!
While there are many natural hazards that are commonly discussed in the San Jose area, such as earthquake faults, flood planes, and liquefaction zones, there’s one which comes up frequently on the east coast but is largely ignored here in the Valley of Heart’s Delight. The question is this: is there a radon risk in Silicon Valley homes?
First, though, what is radon?
Per the EPA website, “Radon (chemical symbol Rn) is an odorless, colorless, radioactive gas. It comes from the natural decay of uranium and radium found in nearly all rocks and soils. Radon can move up from the ground into buildings through openings in floors or walls that are in contact with the ground.”
If the idea that a radioactive carcinogenic gas can enter your home or workplace completely unnoticed spooks you, that’s understandable. Thankfully, it is uncommon to find radon at high enough concentrations for concern in Silicon Valley. It’s helpful in that regard that it’s a region with few smokers and few basements, both of which can increase the risk.
That being said, radon can be found all over the world, and similarly, homes high in radon can be found anywhere, though they are more or less common depending on where you are.
So how do you know your radon risk?
Santa Clara County is considered to be a moderate, and not high, radon area. Different geological conditions may make an area more or less prone to high levels of radon gas, though, so even here it is possible to have a radon risk.
If you would like to check your home’s radon risk levels, there are two tests you can use to measure indoor exposure: a short-term test and a long-term test, and both are affordable. The short-term test only takes a few days, might cost around $15, and is a less reliable way to get results quickly. Long-term measurements take a year and uses detectors placed one on each living level of the house (so approximately $25 for a single-story, $50 for a two-story home). These are much more accurate, so they are the ones preferred by researchers and home owners, but the results will take longer to reach and the cost is higher.
Worst-case-scenario, you have measured 4+ pCi/L (picoCuries per liter of air), the level at which the EPA recommends remediation. Now what? Prepare to spend a few thousand dollars (a great bargain to keep you and your family healthy). The standard treatment involves the installation of a pipeline and fan system which will pull the seeping gas from the ground beneath your house and redirect it outside where it can disperse safely in the air. The only apparent downside to this procedure (other than the time and cost of installation) is that your heat and air conditioning costs may rise slightly. Check with the experts, though, to get the full scope of both the risk and the remediation impact.
Since early July, fire danger signs have been out at Belgatos Park in Los Gatos (and I suspect at other parks throughout Santa Clara County too). To the right is the sign at the park’s main entrance. It admonishes the visitors:
“High Fire Danger No Smoking No BBQs”
To anyone who’s lived in Silicon Valley long, this is understood – the fire danger is quite high here in summer. Unlike most of the east coast, it does not rain here in summer (at least not often and not much), and our green grasses and plants of spring turn to kindling very quickly. One stray match, hot cigarette butt or one illegal firework can smolder into a flame which grows fast with the smallest amount of wind to destroy property, animal life and potentially human life, make breathing bad for days and leave a scar on the land.
This sign at the entrance may not feel very compelling to some as the lush green grass in the background would seem to contraindicate restraint. But venture to the park’s side entrance on Bacigalupi Drive (or hike up the trails) and you’ll understand immediately why this is nothing to take lightly.
Except for one little tuft of partially green grass, “cardboard hill” is entirely dry. So is the rest of this beautiful open space.
If you live close to or have open space in San Jose’s Alum Rock, Almaden, or other east foothill areas or the west valley places like Los Gatos, Monte Sereno, Sartoga, Cupertino or anywhere the foothills, your property is likely considered to be in a high risk fire hazard zone. If so, each year you are mailed information from Cal Fire reminding you of your obligation to provide clearance around your home and to cut down the dead brush.
Just outside of the main entrance to the park there’s a large and open lot which has a few trees, some prickly pear, and a lot of grasses and weeds in winter and spring. (It also had a rattlesnake it in by the prickly pear when my daughter walked past with our dog one day a month or two ago.) Below is a pan of two pics I took with my Blackberry and later stitched together – the park entrance is out of sight but is a little to the left of this photo.
These owners have done as needed and disked the field to help prevent fires or the spread of fires.
There are things you can do to “harden” your own home is you live near open space – that is, to make it more resistant to fire. Check out the whole list on the Cal Fire site, “Ready For Wildfire“.
Mary Pope-Handy, Realtor ABR, CIPS, CRS, SRES
Christie's International Real Estate Sereno, Los Gatos, CA 95030 408 204-7673 Mary@PopeHandy.com License# 01153805
Clair Handy, Realtor
Christie's International Real Estate Sereno 214 Los Gatos-Saratoga Rd Los Gatos, CA 95030 ClairHandy@sereno.com License# 02153633
Mary & Clair sell homes throughout Silicon Valley: Santa Clara County, San Mateo County, and Santa Cruz County. with a special focus on: San Jose, Los Gatos, Saratoga, Campbell, Almaden Valley, Cambrian Park.
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