​​​​​​​​​​​​​​​​​​The Alquist-Priolo Earthquake Fault Zoning Act

​​The Alquist-Priolo Earthquake Fault Zoning (AP) Act was passed into law following the destructive February 9, 1971 Mw 6.6 San Fernando earthquake, which was associated with extensive surface fault ruptures that damaged numerous homes, commercial buildings, and other structures.​  The AP Act provides a mechanism for reducing losses from surface fault rupture on a statewide basis.  The intent of the AP Act is to ensure public safety by prohibiting the siting of most structures for human occupancy across traces of active faults that constitute a potential hazard to structures from surface faulting or fault creep.

​​​​​​​​Wh​​at is the purpose of the Alquist-Priolo Act?

The Alquist-Priolo Earthquake Fault Zoning Act's main purpose is to prevent the construction of buildings used for human occupancy on the surface trace of active faults. The Act only addresses the hazard of surface fault rupture and is not directed toward other earthquake hazards. The Seismic Hazards Mapping Act, passed in 1990, addresses non-surface fault rupture earthquake hazards, including liquefaction and seismically induced landslides.

How doe​​s the law work?​

The law requires the State Geologist to establish regulatory zones (known as Earthquake Fault Zones) around the surface traces of active faults and to issue appropriate maps. ["Earthquake Fault Zones" were called "Special Studies Zones" prior to January 1, 1994.] The maps are distributed to all affec​ted cities, counties, and state agencies for their use in planning and controlling new or renewed construction. Local agencies must regulate most development projects within the zones. Projects include all land divisions and most structures for human occupancy. Single family wood-frame and steel-frame dwellings up to two stories not part of a development of four units or more are exempt. However, local agencies can be more restrictive than state law requires.

Before a project can be permitted, cities and counties must require a geologic investigation to demonstrate that proposed buildings will not be constructed across active faults. An evaluation and written report of a specific site must be prepared by a licensed geologist. If an active fault is found, a structure for human occupancy cannot be placed over the trace of the fault and must be set back from the fault (generally 50 feet).

What is an Earthqu​​ake Fault Zone?

Earthquake Fault Zones are regulatory zones around active faults. On the Earthquake Fault Zone Maps, the zones are shown as shaded yellow polygons that encompass mapped fault traces.The zones vary in width, but average about one-quarter mile wide. On older maps, the zones are defined by turning points (small circles with a center dot) connected by straig​​ht lines. Earthquake Fault Zones are plotted on topographic maps at a scale of 1 inch equals 2,000 feet. 

What is ​​a fault?

A fault is a fracture in the crust of the earth along which rocks on one side have moved relative to those on the other side. Most faults are the result of repeated displacements over a long period of time. A fault trace is the line on the earth's surface defining the fault. For the purposes of the Act, an active fault is one that has ruptured in the last 11,000 years.

What is "surface ru​​pture" in an earthquake?

Picture of faulting from the 1992 Landers earthquake, photo by Bill Bryant

Surface rupture occurs when movement on a fault deep within the earth breaks through to the surface. Surface ruptures associated with the 1992 Landers Earthquake, in San Bernardino County, extended for 50 miles with displacements of an inch to 20 feet. Not all earthquakes result in surface rupture. The Loma Prieta Earthquake of 1989 caused major damage in the San Francisco Bay Area but the movement deep in the earth did not break through to the surface.

Fault rupture almost always follows preexisting faults, which are zones of weakness. Rupture may occur suddenly during an earthquake or slowly in the form of fault creep. Sudden displacements are more damaging to structures because they are accompanied by shaking.

Fault creep is the slow rupture of the earth's crust. Examples of creep are well known along the Hayward Fault where it crosses highly developed areas in Contra Costa and Alameda Counties. Although the Hayward Fault ruptured suddenly in the 1868 earthquake, it also exhibits slow surface creep which offsets and deforms curbs, streets, buildings, and other structures that lie on top of the fault.

How can I tell if a property​​ is in an Earthquake Fault Zone?

Earthquake Fault Zone maps are available for download from the California Geological Survey’s website here, and are also available at local planning departments. These maps show most streets, drainages, and other features. Local government may have already transferred Earthquake Fault Zone boundaries to parcel maps, so the relationship of the Zone to each parcel can easily be determined.

Does the law require tha​​t all real estate within an Earthquake Fault Zone be disclosed as such before it is sold?

The fact that a property is located in an Earthquake Fault Zone must be disclosed to a potential buyer before the sales process is complete. The real estate agent is legally bound to present this information to the buyer. When no realtor is involved, the seller must inform the buyer directly. This is usually done at the time an offer is made or accepted.

Effective June 1, 1998, the Natural Hazards Disclosure Act requires that sellers of real property and their agents provide prospective buyers with a "Natural Hazard Disclosure Statement" when the property being sold lies within one or more state-mapped hazard areas, including Earthquake Fault Zones.

What does an Earthquake Fault Zone mean to me?

picture of an Example of a AP Map

​It means that an active fault is present near or within the land parcel and may pose a risk of surface fault rupture to existing or future structures.

If the property is not developed, a fault study may be required before the parcel can be subdivided or structures permitted. See the definition of "project" under "How does the law work?" Check with your local permitting agency for specific requirements.

If a property is ​developed, you will not need a geologic study unless you plan to extensively add onto or remodel an existing structure. See exemptions above and check with your local permitting agency.

You can learn more about the potential of fault rupture by:

  • Asking the property owner or real estate agent to see any geologic report prepared for the site.

  • Checking the files of local government for consulting reports for nearby sites. Also, most fault investigations required by the Alquist- Priolo Act are on file with the California Geological Survey.

  • Researching maps and data on active faults at technical libraries at the California Geological Survey, U.S. Geological Survey, and universities.

  • Hiring a licensed geologist to provide a preliminary assessment of the fault-rupture hazard for a specific site.