sitelume · area guidesOpen the interactive map →

What is an Alquist-Priolo Earthquake Fault Zone?

If it appeared in your disclosure packet, here is what it means: an Alquist-Priolo zone is a regulatory strip California draws around known active faults, inside which new buildings for people must first prove they are not sitting on the fault itself. Being in the zone is a requirement to investigate — not a statement that your house is on a fault.

Where the law came from

In 1971 the San Fernando earthquake tore surface ruptures through neighborhoods built directly on the fault. California responded with the Alquist-Priolo Earthquake Fault Zoning Act (1972), which tasks the California Geological Survey with mapping regulatory zones around faults that are both sufficiently active — evidence of surface displacement within roughly the last 11,700 years (the Holocene) — and well-defined enough to locate on the ground.

What the zone does

  • Triggers a study: before a city or county can permit a new structure for human occupancy inside the zone, the developer must commission a geologic investigation demonstrating the building will not straddle an active fault trace.
  • Enforces a setback: where a trace is found, structures are generally kept at least 50 feet away.
  • Requires disclosure: A-P zone membership is one of the six statutory items on California’s Natural Hazard Disclosure statement (Civil Code § 1103), so buyers see it in every sale.

What it means if you are buying in one

For an existing home, the practical effects are mostly informational: the disclosure line, questions from some insurers, and rules that would apply to major additions or a rebuild. What the designation gives you is a precise question to ask — where exactly is the mapped trace relative to this lot? The official CGS zone maps show trace locations, and a records request to the city may turn up prior fault investigations on the parcel or its neighbors.

It’s also worth knowing what the zone does not cover: shaking intensity. Shaking from a Hayward fault earthquake will be strong far outside the A-P strip — the zone only addresses the narrower hazard of the ground surface breaking along the fault.

Bay Area context

The Hayward fault runs directly beneath the East Bay hills front, which is why Fremont, Hayward, Oakland and Berkeley all carry mapped A-P corridors through developed neighborhoods. On the Peninsula and in Marin, the San Andreas zone tracks the ridgeline valleys; in the North Bay, the Rodgers Creek zone passes through Santa Rosa. Each of our city pages shows the share of that city inside an A-P zone and the distance to the nearest mapped fault.

Check a specific address: the free sitelume interactive map shows which mapped zones any SF Bay Area address falls in — layer by layer, no sign-up.

Frequently asked questions

Does being in an Alquist-Priolo zone mean my house is on a fault?
No. The zone is a study buffer — typically about 500 feet on either side of a mapped active fault trace. Most parcels inside a zone are not on the trace itself. The designation means a site-specific geologic investigation is required before new human-occupancy construction, not that surface rupture will occur on your lot.
Can I remodel or rebuild a home in an A-P zone?
Generally yes. The Act applies to new structures for human occupancy and to additions or alterations over 50% of the building's value. Existing single-family homes are not retroactively affected. Projects that trigger the Act need a geologic fault investigation, and structures must typically be set back at least 50 feet from an active trace.
Is an A-P zone the same as a liquefaction or landslide zone?
No — different law, different hazard. Alquist-Priolo zones address surface fault rupture (the ground splitting along the fault line). Liquefaction and earthquake-induced landslide zones come from the separate Seismic Hazards Mapping Act and address ground failure caused by shaking. A property can be in one, both, or neither.
Which Bay Area faults have Alquist-Priolo zones?
The major zoned faults include the San Andreas, Hayward, Calaveras, Rodgers Creek, Concord-Green Valley, and Greenville faults, among others. Cities crossed by the Hayward fault — Fremont, Hayward, Oakland, Berkeley — have some of the most heavily zoned corridors in the state.

Related reading

Informational summary of publicly available government designations — not legal, insurance, or engineering advice. Zone boundaries and rules change; verify with the issuing agency before making decisions.