The Alquist-Priolo Earthquake Fault Zoning Act was signed on December 22, 1972 (effective March 7, 1973), in direct response to the 1971 San Fernando (Sylmar) earthquake — a M6.6 that killed 65 people, injured more than 2,000, and produced about 12.5 miles of surface rupture. It is codified at Public Resources Code §2621 et seq., with regulations at 14 CCR §3600–3606, and is administered by the California Geological Survey (CGS).
The Act addresses one hazard specifically: surface fault rupture — the ground breaking along a fault trace. (Shaking, liquefaction, and landslides are covered by a separate 1990 law, the Seismic Hazards Mapping Act.) It works in three ways:
1. Mapping Earthquake Fault Zones. CGS maps roughly quarter-mile-wide “Earthquake Fault Zones” along faults that are “sufficiently active” and “well defined.” The test for “sufficiently active” is recency — generally, evidence of Holocene displacement (within roughly the last 11,000 years) — not any earthquake magnitude.
2. Keeping structures off the trace. Structures for human occupancy may not be built across an active fault trace. Within a zone, a 50-foot area around any found fault is presumed underlain by fault branches unless a licensed geologist’s investigation proves otherwise (14 CCR §3603). For a Holocene-active fault, the only allowed mitigation is avoidance — you move the structure off the trace.
3. Seller disclosure. A property’s location within an Earthquake Fault Zone must be disclosed to buyers on the Natural Hazard Disclosure Statement (PRC §2621.9; Civil Code §1103.2).
The number 6.7 is not in the statute, the regulations, or CGS’s Alquist-Priolo guidance (Special Publication 42). It traces to the Uniform California Earthquake Rupture Forecast (UCERF), the state’s earthquake-probability model, which uses M6.7 as its threshold for a “large” earthquake — chosen to match the 1994 Northridge earthquake (M6.7). UCERF’s well-known headline is that there is roughly a 99% probability of a M6.7+ earthquake somewhere in California over 30 years. That is a probability forecast, not a zoning standard. Tellingly, CGS’s own Alquist-Priolo guide uses the 2014 M6.0 South Napa earthquake as its lead example of surface rupture — rupture is not limited to M6.7+.
If a property sits within an Earthquake Fault Zone, expect (a) a disclosure on the Natural Hazard Disclosure Statement, and (b) if you build or rebuild, a fault investigation by a licensed geologist before a permit is issued. You can check whether a property is in a zone using the CGS EQ Zapp lookup (maps.conservation.ca.gov). The practical point: Alquist-Priolo is about whether the ground itself might rupture beneath a building — it does not rate how large an earthquake a fault can produce, and it does not address shaking, which is the far more widespread risk. For the shaking risk at a specific address, see our seismic risk lookup.