Earthquake Fault Zones in California: What Homebuyers Should Know

California's Earthquake Hazard Disclosure System

California sits on some of the most active fault systems in the world — the San Andreas, Hayward, San Jacinto, and dozens of others that crisscross the state. Unlike flood or fire hazards, where you can see the river or the brush, earthquake risk is invisible until the ground moves. That's why California has one of the most detailed earthquake hazard mapping and disclosure systems in the country.

Two separate state programs map earthquake risk for NHD disclosure purposes: the Alquist-Priolo Earthquake Fault Zoning Act and the Seismic Hazard Mapping Act. Each addresses a different type of seismic danger, and both are required disclosures under Civil Code §1103 for residential property sales.

Whether a property falls inside one of these zones — and what that means practically — is one of the most common questions that comes up when buyers review their NHD report.

Alquist-Priolo Earthquake Fault Zones

The Alquist-Priolo Earthquake Fault Zoning Act was passed in 1972, one year after the devastating San Fernando earthquake. It's one of the oldest hazard disclosure laws in California and remains one of the most consequential for property owners.

Under this act, the California Geological Survey (CGS) maps zones along known active faults — faults that have shown surface displacement within the last 11,000 years (the Holocene epoch). These zones extend approximately 500 feet on either side of the mapped fault trace, creating a buffer where surface rupture is considered most likely during a future earthquake.

What an Alquist-Priolo designation means:

• No new structures for human occupancy can be built across the trace of an active fault within the zone without a geologic investigation. A licensed geologist must prepare a fault study demonstrating that the proposed building site is not on an active fault trace before a building permit can be issued.
• Existing structures are not required to be retrofitted or demolished, but the disclosure is required when the property is sold. Buyers should understand that the property sits near a mapped active fault.
• The zone is about surface rupture, not shaking. Being outside an Alquist-Priolo zone doesn't mean a property is safe from earthquake damage. Ground shaking from a major earthquake can cause damage miles from the fault. The zone specifically addresses the risk that the ground will crack and shift directly beneath or very near a structure.
• NHD disclosure — A "Yes" for Alquist-Priolo on an NHD report means the property is within the mapped fault zone. This is one of the six statutory hazard disclosures required under Civil Code §1103.

California has mapped Alquist-Priolo zones along more than 150 active faults statewide, including major faults like the San Andreas, Hayward, Calaveras, San Jacinto, Elsinore, and Garlock faults.

CGS Seismic Hazard Zones: Liquefaction and Landslides

The Seismic Hazard Mapping Act of 1990 addresses two earthquake-related ground failure hazards that the Alquist-Priolo Act doesn't cover: liquefaction and earthquake-induced landslides.

Liquefaction occurs when loose, water-saturated soil loses its strength during an earthquake and behaves like a liquid. Buildings on liquefied ground can sink, tilt, or collapse — not because the structure fails, but because the ground beneath it does. Liquefaction is most common in areas with sandy or silty soils near bodies of water, in filled land, or in areas with high water tables. In California, liquefaction zones are particularly prevalent in the Bay Area, parts of Los Angeles, the Sacramento–San Joaquin Delta, and coastal lowlands.

Earthquake-induced landslides are slope failures triggered by seismic shaking. Hillside properties in earthquake-prone areas face compounded risk — the earthquake provides the trigger, and gravity does the rest. Mapped landslide zones tend to follow steep terrain in seismically active regions.

What a CGS Seismic Hazard Zone designation means:

• Site investigations required for new development — Before construction permits are issued within mapped liquefaction or landslide zones, a geotechnical investigation must evaluate the site-specific hazard and recommend mitigation measures.
• Mitigation may be required — If the investigation confirms the hazard, engineering solutions must be incorporated into the building design. For liquefaction, this might include deep foundations, ground improvement, or structural reinforcement. For landslides, it could include slope stabilization, retaining walls, or drainage improvements.
• Existing homes are not required to retrofit — Like Alquist-Priolo zones, the CGS seismic hazard designation is a disclosure and new-construction requirement, not a mandate to modify existing buildings.
• NHD disclosure — A "Yes" for seismic hazard zone on the NHD report means the property is in a mapped liquefaction zone, landslide zone, or both.

CGS has mapped seismic hazard zones for much of urban California, but mapping is not complete statewide. Areas without CGS mapping may still face liquefaction or landslide risk — the absence of a map doesn't confirm the absence of a hazard.

What a 'Yes' on Your NHD Report Means

When an NHD report shows "Yes" for an earthquake-related hazard zone, it's natural to feel concerned. But it helps to understand exactly what that determination means — and what it doesn't.

A "Yes" for Alquist-Priolo means the property is within the mapped fault zone — typically within about 500 feet of a known active fault trace. It does not mean a fault runs directly under the house, and it doesn't predict when or whether an earthquake will occur. Many thousands of California homes sit within Alquist-Priolo zones and have never experienced surface fault rupture.

A "Yes" for CGS Seismic Hazard Zone means the property is in an area mapped as having potential for liquefaction, earthquake-induced landslide, or both. Again, this identifies elevated risk, not a certainty. Many properties in these zones are built on soil conditions that, while mapped as potentially hazardous, may perform adequately in moderate earthquakes.

The practical impact of a "Yes" determination varies:

• For existing homeowners — The designation is primarily informational. No retrofitting is required, and earthquake insurance decisions are independent of zone status (earthquake insurance in California is voluntary for all properties, regardless of zone designation).
• For buyers — The disclosure allows you to make an informed decision. You may want to commission a geotechnical report to understand the site-specific risk, factor earthquake insurance into your budget, or evaluate the home's construction and foundation type.
• For new construction — A "Yes" triggers mandatory site investigations and may require specific engineering solutions before building permits are issued.
• For insurance — Unlike flood zones, earthquake zone status does not trigger mandatory insurance requirements. Earthquake insurance in California is available through private insurers and the California Earthquake Authority (CEA), and purchasing it is voluntary.

Earthquake Insurance in California

Unlike flood insurance in FEMA flood zones, earthquake insurance is not required for any California property — even those directly on top of an active fault. The decision to purchase earthquake coverage is entirely voluntary, regardless of zone status.

That said, earthquake insurance is worth serious consideration for any California homeowner. Standard homeowner's insurance policies specifically exclude earthquake damage. If your home is damaged in an earthquake, your regular insurance won't cover the structural repairs, foundation work, or contents replacement.

The California Earthquake Authority (CEA) is the primary source of residential earthquake insurance in California. CEA policies are sold through participating insurance companies and offer three components: dwelling coverage (the structure), personal property coverage (contents), and loss of use coverage (temporary living expenses).

Key things to know about earthquake insurance:

• Deductibles are high — CEA policies have deductibles ranging from 5% to 25% of the dwelling coverage amount. On a $600,000 policy, a 15% deductible means you'd pay the first $90,000 of damage before coverage kicks in.
• Premiums vary by location and construction — A wood-frame home in a lower-risk area might pay $800–$1,500 annually. A masonry home near an active fault could pay $3,000–$5,000 or more.
• Retrofitting can reduce premiums — Homes that have been seismically retrofitted (bolted to the foundation, braced cripple walls) may qualify for premium discounts. The CEA's Brace + Bolt program has provided grants for seismic retrofits in eligible areas.
• Zone status doesn't determine eligibility — CEA doesn't deny coverage based on fault zone or seismic hazard zone status, though location is one of the factors in premium calculation.

Major California Fault Systems

California's earthquake risk isn't uniform — it concentrates along the state's major fault systems. Understanding which faults are near a property adds context to the NHD disclosure.

• San Andreas Fault — The most well-known fault in California, running approximately 800 miles from the Salton Sea to Cape Mendocino. The San Andreas is a transform boundary between the Pacific and North American tectonic plates and is capable of producing earthquakes above magnitude 8.0. The southern section is considered overdue for a major rupture.
• Hayward Fault — Running through the East Bay from San Pablo Bay to Fremont, the Hayward Fault is often called the most dangerous fault in America because of the dense urban population living directly on top of it. USGS estimates a 33% probability of a magnitude 6.7+ earthquake on the Hayward Fault before 2043.
• San Jacinto Fault — The most seismically active fault in Southern California, running from the San Bernardino Mountains to the Imperial Valley. It produces frequent moderate earthquakes.
• Newport-Inglewood Fault — Running through densely populated areas of Los Angeles and Orange County, from Beverly Hills through Inglewood to Newport Beach. This fault produced the destructive 1933 Long Beach earthquake.
• Calaveras and Concord-Green Valley Faults — East Bay faults that run through rapidly growing suburban areas of Contra Costa and Alameda counties.

These faults are mapped with Alquist-Priolo zones, but earthquake shaking from a major event on any of them would affect a much larger area than just the mapped zones.

Seismic Retrofitting and Risk Reduction

For homeowners in earthquake hazard zones — or anywhere in California — seismic retrofitting can meaningfully reduce both risk and potential financial loss.

The most common and cost-effective retrofit for older homes involves two measures: foundation bolting (securing the wood framing to the concrete foundation with anchor bolts) and cripple wall bracing (reinforcing the short walls between the foundation and the first floor with structural plywood). These two measures address the most common failure mode in older California homes — the house sliding off its foundation during shaking.

Additional risk reduction measures include:

• Securing water heaters — California law requires water heaters to be strapped to prevent tipping during an earthquake. This is a basic safety measure that also prevents fire from broken gas lines.
• Soft-story retrofitting — Multi-story buildings with large openings on the ground floor (like apartment buildings with tuck-under parking) are particularly vulnerable. Several California cities have mandatory soft-story retrofit programs.
• Chimney bracing — Unreinforced masonry chimneys are among the first building elements to fail in an earthquake and are a falling hazard.
• Automatic gas shut-off valves — These devices detect seismic motion and shut off the gas supply, reducing fire risk after an earthquake.

The CEA's Brace + Bolt program has provided grants of up to $3,000 for foundation bolting and cripple wall bracing in qualifying homes and zip codes. Check the program's current availability when considering a retrofit.

Check Any Property's Earthquake Zone Status

Want to see whether a specific California property is in an Alquist-Priolo fault zone or CGS seismic hazard zone — along with flood, fire, and other natural hazard zones — before ordering a full NHD report? Use Verity NHD's free hazard preview tool. No account required, no credit card, no commitment.