Earthquake Zones in Portola Valley: Liquefaction, Landslide and Fault Zones by Neighborhood

The California Geological Survey maps three kinds of earthquake zones: Seismic Hazard Zones for liquefaction and for earthquake-induced landslides, and Alquist-Priolo Earthquake Fault Zones. A zone is a mapped area where a site study is required before new building. It isn’t a prediction. We checked the parcel center of every home inside our 8 Portola Valley neighborhood guides against the state’s zone maps and counted, guide by guide, how many fall in each zone. The zone maps and the parcel records are as published in October 2026. There are no addresses here.

The short answer

• Liquefaction zones: 7 of our 8 Portola Valley guides have homes in a liquefaction zone. The most are in Arrowhead Meadows & Stonegate (76 of 290 houses, 26%), Portola Road & Town Center (66 of 284 houses, 23%) and Woodside Highlands & Hayfields (29 of 166 houses, 17%).
• Landslide zones: all 8 of our Portola Valley guides have homes in an earthquake-induced landslide zone. The most are in Los Trancos Woods & Vista Verde (229 of 256 single-family lots, 89%), Woodside Highlands & Hayfields (65 of 166 houses, 39%) and Portola Valley Ranch & Blue Oaks (53 of 226 houses, 23%).
• Fault zones: 5 of our 8 Portola Valley guides have homes in an Alquist-Priolo fault zone. The most are in Los Trancos Woods & Vista Verde (142 of 256 single-family lots, 55%), Portola Valley Ranch & Blue Oaks (115 of 226 houses, 51%) and Portola Road & Town Center (56 of 284 houses, 20%).
• Every one of our 8 Portola Valley guides has at least one counted home in one of the three zones; shares run from 10% in Ladera to 89% in Los Trancos Woods & Vista Verde.

Neighborhood by neighborhood

Counted homes inside each guide’s outline whose parcel center is in each zone, sorted by the share in any of the three. A home can sit in more than one zone, so the zone columns can add up to more than “any of the three.” Some outlines overlap, so a home can count in two guides.

What the zones mean

• Liquefaction zones are areas where the state’s maps show ground and groundwater conditions, typically loose, water-saturated sand or silt near the Bay and along creeks, under which the soil can lose strength in strong shaking.
• Earthquake-induced landslide zones are hillside areas where the state’s maps show slopes that could slide in strong shaking.
• Alquist-Priolo Earthquake Fault Zones are drawn around faults the state considers active, such as the San Andreas and Hayward faults.
• What the zones require. Under the Seismic Hazards Mapping Act, cities and counties generally require a site-specific geotechnical report before approving new building in a liquefaction or landslide zone. In a fault zone, a geologic investigation is usually required before new construction, and new buildings for people generally can’t go across an active fault trace.
• A map, not a prediction. A zone marks where the state wants a closer look before building. It doesn’t rate a particular house, and the maps don’t show where shaking would be strongest.

What it means for buyers and sellers

• Sellers disclose it. California’s Natural Hazard Disclosure statement says whether a property is in a Seismic Hazard Zone or an Alquist-Priolo Earthquake Fault Zone. Ask for the report early in a purchase.
• Check the official map for the address. The California Geological Survey’s EQ Zapp map shows the zones for any address; this post counts by neighborhood, not by address.
• Ask your inspector and your insurer. A home inspector, and for the ground a geotechnical engineer, can speak to a specific house and lot. An insurance agent can quote earthquake coverage, which is a separate policy.
• Planning a project? The town’s building department can tell you whether a remodel or addition needs a geotechnical or geologic report.

How we counted

• Neighborhoods: the outlines drawn for our 8 Portola Valley neighborhood guides. They’re approximate, and some overlap, so the rows aren’t added into a town total. They cover our guides, not every part of Portola Valley.
• Homes: single-family houses as the parcel records identify them or lots zoned or coded single-family (these records identify the lot, not the house on it), from county assessor records published by the Town of Portola Valley, plus the San Mateo County parcel layer (use codes as of June 2020) for guide areas outside the town. Condos, townhomes and apartments aren’t counted.
• Zones: the California Geological Survey’s regulatory maps as published in October 2026: Seismic Hazard Zones for liquefaction and earthquake-induced landslides, and Alquist-Priolo Earthquake Fault Zones.
• The center of each parcel. A home counts in a zone when its parcel’s center is inside it, so a lot that only touches a zone isn’t counted. The zone describes the land, not the house on it.
• Maps change. The state revises its zone maps, so check the current map for a specific address.

Sources

• California Geological Survey, EQ Zapp: California Earthquake Hazards Zone Application (the official zone map by address).
• California Geological Survey, Seismic Hazards Mapping Act (liquefaction and earthquake-induced landslide zones).
• California Geological Survey, Alquist-Priolo Earthquake Fault Zoning Act (fault zones).
• Parcels: County assessor records published by the Town of Portola Valley, plus the San Mateo County parcel layer (use codes as of June 2020) for guide areas outside the town.
• The same counts are on our Earthquake Zones by Neighborhood page and in dataset PN-DS-29 in the Property Nerds Data Library.

Questions we hear

Is Portola Valley in an earthquake zone?

Parts of it are on the state’s maps. All 8 of our Portola Valley neighborhood guides have at least one counted home in a liquefaction, landslide or fault zone, though the share varies a lot from one guide to the next. The maps mark ground where the state requires a site study before new building. They don’t map shaking.

What does a zone mean for a home I’m buying?

It means the property is on a state map, so the seller’s Natural Hazard Disclosure statement will say so, and any new building on the lot would need a site study first. It doesn’t say anything about the house itself. A home inspector, and for questions about the ground a geotechnical engineer, can tell you about a specific property, and an insurance agent can quote earthquake coverage.

How do I check one address?

Look it up in the California Geological Survey’s EQ Zapp map, and read the Natural Hazard Disclosure report for the property. We’re glad to pull both for a home you’re considering.

Does a zone stop me from remodeling?

Not usually. The rules mainly apply to new construction and larger projects, which may need a geotechnical or geologic report first. The town’s building department can say what applies to a specific project.

Why isn’t there a town total?

Some guide outlines overlap, so a home inside two outlines counts in both. Adding the rows would count those twice, so we show each guide on its own. The table also covers our guide outlines only, not every part of Portola Valley.

Go deeper

• Earthquake Zones by Neighborhood has the same counts for all of our neighborhood guides, with a sortable table.
• Portola Valley Neighborhoods Compared puts these zones next to lot sizes and flood and fire zones.
• Atherton, Woodside & Portola Valley Neighborhoods by Their Lots.
• Flood & Fire Zones by Neighborhood counts FEMA flood and CAL FIRE fire zones the same way.
• Disclosure Decoder walks through the Natural Hazard Disclosure and the other seller disclosures.
• Earthquake Retrofit Grants explains the state’s Brace + Bolt program and which local ZIP codes are on its list.
• Insurance Lab covers which policies a California home needs, earthquake coverage included.

Buying or selling in an earthquake zone?

We’re the Property Nerds, the Boyenga Team at Compass, recognized as NextGenAgents by a national real estate portal. We can pull the zone map, the disclosure report and the recent sales closest to a specific home before you write an offer or set a price. Call (408) 373-1660 or (650) 383-8606, or book a strategy call. Commissions are negotiable, and we offer bundled services, so ask us what’s included.