Earthquake Fault Zones vs Seismic Hazard Zones [Video]
Many California buyers assume "earthquake zone" is a single category. In reality, the NHD report checks two distinct seismic hazards: fault zones and seismic hazard zones. The video below explains the difference in 34 seconds.
Alquist-Priolo earthquake fault zones
The Alquist-Priolo Earthquake Fault Zoning Act identifies areas where active faults are capable of producing surface rupture during an earthquake. The California Geological Survey (CGS) maps these zones. A property inside an Alquist-Priolo zone may be subject to setback requirements and geologic investigation before new construction or substantial remodeling.
Seismic hazard zones
The Seismic Hazards Mapping Act addresses ground failure risks that occur during shaking, primarily liquefaction and earthquake-induced landslides. These zones are also mapped by CGS. Unlike fault zones, the concern here is not surface rupture but the ground turning fluid or sliding downhill under seismic stress.
The key difference
- Fault zones are about ground rupture along a fault trace.
- Seismic hazard zones are about ground failure from liquefaction or landslides during shaking.
A property can be in one, both, or neither. Both determinations appear separately in every Verity NHD report.
Both checked in every Verity NHD report
Verity NHD queries the latest CGS datasets for both Alquist-Priolo and Seismic Hazards Mapping Act zones, then explains each finding in plain language. For a broader overview of all California hazard categories, read our guide to California natural hazard zones.
Insurance and construction implications
Standard California homeowner's insurance does not cover earthquake damage. Buyers in either type of seismic zone should consider purchasing a separate earthquake policy through the California Earthquake Authority (CEA) or a private carrier. For properties within Alquist-Priolo zones, local jurisdictions may require a geologic investigation before issuing permits for new construction, additions, or significant renovations. Seismic hazard zones may trigger a geotechnical investigation to evaluate liquefaction or landslide potential. These investigations add cost and time to development projects, but they also protect buyers by identifying foundation-level risks before construction begins. Agents should advise clients in seismic zones to budget for both insurance and — if they plan to renovate — potential geotechnical review.
Key takeaways
- Alquist-Priolo zones address surface fault rupture risk; Seismic Hazard Mapping Act zones address liquefaction and earthquake-induced landslides.
- A property can be in one type of zone, both, or neither — the two determinations are independent.
- Standard homeowner's insurance does not cover earthquake damage; buyers should consider a separate CEA or private earthquake policy.
- Properties in Alquist-Priolo zones may require geologic investigation before new construction or major remodeling.
- Both seismic determinations are queried from the latest California Geological Survey data and appear separately in every Verity NHD report.