The Alquist-Priolo Act requires the State Geologist (CGS) to establish earthquake fault zones around the surface traces of active faults and to issue appropriate maps. CGS has identified earthquake fault zones in thirty-seven California counties, affecting more than one hundred cities. Here are a few ways for you to determine if a property is affected by an earthquake fault zone:
California Earthquake Hazards Zone Application (EQ Zapp). It's an interactive map that shows earthquake fault zones and seismic hazard zones in relation to any parcel in California.
CGS Official Maps of Earthquake Fault Zones by quadrangle. CGS has created more than 500 of these maps statewide:
You can download the quadrangles in Portable Document Format (PDF) from the
Regulatory Maps and Reports section of the CGS Information Warehouse web site.
As another option, paper copies of quadrangles are available for reference through your local municipality's planning department. The California Geological Survey distributes the maps to all affected cities, counties, and state agencies for their use in planning and controlling new or renewed construction. Counties are required to post notices at the offices of the assessor, recorder, and planning agency that advise the public of the availability and location of earthquake fault zones affecting that county (Public Resources Code 2621).
Paper quadrangles are also available for reference at the
CGS Library, at
several other libraries throughout the state, and at
our regional offices in Los Angeles and San Mateo.
How to interpret Official Maps of Earthquake Fault Zones
The California Geological Survey periodically issues official maps of earthquake fault zones, in compliance with the Alquist-Priolo Earthquake Fault Zoning Act. Two examples of earthquake fault zone maps are shown in the figure below. The newer map on the left shows an earthquake fault zone as a semi-transparent yellow polygon. The older map on the right shows an earthquake fault zone as semi-parallel lines connected to small circles where the lines change direction. Also on the older map, faults are depicted as thicker black lines running through the center of the zone. On newer digital maps, faults are not visible by default, but they can be made visible using the layer control feature integrated into each map.
In both examples, the zones are long and narrow. This is typical of earthquake fault zones, which can be miles long but average about one-quarter mile wide. Because they are relatively narrow, they will not exist throughout all of an affected city or county.
How to be certain a map is an Official Map of Earthquake Fault Zones
The following text appears on all official maps of Alquist-Priolo earthquake fault zones issued after June 1, 1995:
STATE OF CALIFORNIA
EARTHQUAKE FAULT ZONES
Delineated in compliance with
Chapter 7.5, Division 2 of the California Public Resources Code
(Alquist-Priolo Earthquake Fault Zoning Act)
OFFICIAL MAP/REVISED OFFICIAL MAP
[Signature of State Geologist releasing map]
NOTE: Alquist-Priolo earthquake fault zones were once known as "Special Studies Zones." Maps issued prior to June 1, 1995 will have "SPECIAL STUDIES ZONES" in the title block instead of "EARTHQUAKE FAULT ZONES."