Earthquake fault zones and liquefaction zones in California

The California Geological Survey (CGS) maps regulatory earthquake zones where cities and counties must require a site investigation before most building. This guide explains the two kinds Outpost shows and how to check any California location.

Data as of October 23, 2026; refreshed monthly.

Screening only — not a legal or official determination. Verify with the issuing agency.

Alquist-Priolo Earthquake Fault Zones

The Alquist-Priolo Act followed the magnitude 6.6 San Fernando earthquake of 1971, which broke the ground surface and damaged many buildings. It deals with one hazard: surface fault rupture.

• The zones surround the surface traces of active faults, meaning faults that have ruptured in the last 11,000 years. They are long and narrow and average about a quarter mile wide.
• Buildings for human occupancy can’t be placed over an active fault and generally must be set back at least 50 feet from it.
• Before permitting land divisions and most buildings for human occupancy, cities and counties require a geologic investigation by a California-licensed geologist. Local rules can be stricter.
• Sellers must disclose when a property is in a zone.

Liquefaction zones

Liquefaction happens when loose, water-saturated sediment loses strength during strong shaking. The Seismic Hazards Mapping Act of 1990, passed after the Loma Prieta earthquake, has CGS map zones where it is more likely, mostly where groundwater is shallow in young sediments.

• Before permitting development in a zone, cities and counties must require a site-specific investigation by state-licensed engineering geologists or civil engineers.
• Some single-family homes are exempt from the state requirement, and local rules can be stricter.
• CGS is mapping California’s main urban and growth areas, so many places haven’t been evaluated yet.
• Sellers must disclose when a property is in a mapped Seismic Hazard Zone.

How to check a location on Outpost

1. Search the address on the map, or click the point you want to check.
2. The scorecard’s seismic row shows whether the point is in an Alquist-Priolo zone (with the fault’s name where CGS data has one), a liquefaction zone, or both.
3. If it is in neither, the row flags the nearest zone within half a mile, with its distance. Otherwise it lists the distance to the nearest mapped zone of each kind within about 10 miles.
4. To see the zones, turn on the Seismic Hazard layer and its Alquist-Priolo and Liquefaction parts.

Seismic zone colors on the map:

• Alquist-Priolo zone
• Liquefaction zone

What Outpost can’t tell you

• It doesn’t show shaking, which can damage buildings far from any zone.
• It doesn’t show earthquake-induced landslide zones or tsunami hazard areas. CGS’s EQ Zapp shows the landslide zones where they have been mapped.
• No liquefaction zone at a point may only mean CGS hasn’t mapped that area. CGS notes its maps may not show every area with the potential for these hazards.
• It isn’t a site investigation and can’t say how a particular building would perform.

Common questions

How do I know if a property is in an Alquist-Priolo zone?

CGS’s EQ Zapp map lets you search by address. Outpost shows the same CGS zones next to other hazards, with the fault name where CGS data has one. The local planning department can confirm.

Can I build in an Alquist-Priolo zone?

Often yes, after a geologic investigation. Buildings for human occupancy can’t sit over an active fault and generally must be set back at least 50 feet from it. Local rules can be stricter.

What is liquefaction?

Loose, water-saturated sediment losing strength during strong shaking, which can make the ground settle, spread or fail under buildings.

If a property isn’t in a zone, is it safe from earthquakes?

No. Strong shaking causes most earthquake damage and can reach far beyond these zones. The zones mark where ground failure is more likely.

Do sellers have to disclose these zones?

Yes. California sellers must disclose when a property is in an Alquist-Priolo Earthquake Fault Zone or a mapped Seismic Hazard Zone.

Official sources

• California Geological Survey: Alquist-Priolo Earthquake Fault Zones
• California Geological Survey: Seismic Hazard Zones
• California Geological Survey: EQ Zapp, the earthquake hazards zone map

Related guides

• Landslide susceptibility in California

  What the California Geological Survey’s deep-seated landslide susceptibility map shows, what it leaves out, and how to view it for any location.

• FEMA flood zones in California

  What FEMA flood zones A, AE, V, X and D mean, when flood insurance is usually required, and how to check a California location for free.

• California Fire Hazard Severity Zones

  What CAL FIRE’s Moderate, High and Very High fire hazard severity zones mean, SRA vs LRA, and how to check any California location for free.