What Is a Seismic Hazard Zone?

After the 1989 Loma Prieta earthquake, California passed the Seismic Hazards Mapping Act of 1990. It directs the California Geological Survey to identify and map areas where earthquake shaking is likely to cause the ground itself to fail. CGS publishes Seismic Hazard Zone maps by quadrangle, and they define two zone types:

The zones are drawn from geology, groundwater depth, slope, and shaking models — not from distance to a fault. That is the key distinction from Alquist-Priolo Earthquake Fault Zones, which regulate surface rupture directly along an active fault trace. A property can be miles from any mapped fault and still sit inside a seismic hazard zone.

• Fault zone — the ground could split at the surface.
• Seismic hazard zone — the ground could liquefy or slide when shaken from a distance.

Inside a mapped seismic hazard zone, cities and counties must require a site-specific geotechnical investigation before approving most new projects, and they may condition approval on mitigation — ground improvement, deeper or reinforced foundations, retaining structures, or drainage work. That affects owners planning additions, ADUs, or rebuilds more than owners simply living in the home.

For a sale, the zone status is a required disclosure. It rarely stops a transaction, but it shapes buyer diligence: geotechnical review during the inspection period, questions about foundation history and past repairs, and earthquake insurance quoting, since standard homeowners policies exclude earthquake damage statewide. Disclosing the zone early keeps it a budgeting conversation rather than a late-escrow dispute.

Seismic hazard zone status is one of the six statutory determinations on the Natural Hazard Disclosure Statement. Your NHD report states whether the property lies within a CGS-designated seismic hazard zone, identifies which zone type applies, names the quadrangle map and release used, and time-stamps the determination for the file. Verity NHD includes a parcel hazard map, a plain-English summary of what the finding means, bilingual delivery, and built-in eSignature.