What Are the Alquist-Priolo and Seismic Hazards Acts in CA?

The Alquist-Priolo and Seismic Hazards Acts in California are two state laws that control where and how you can build on land exposed to earthquake dangers. The Alquist-Priolo Earthquake Fault Zoning Act, passed after the 1971 San Fernando earthquake, keeps buildings for human occupancy off the surface traces of active faults. The Seismic Hazards Mapping Act of 1990 covers the rest of what earthquakes do to the ground: liquefaction, landslides, and other ground failure. Together they map hazard zones, require geologic investigations before development inside those zones, limit or condition construction, and require sellers to disclose zone status to buyers.¹²

What Each Act Covers

The Alquist-Priolo Act (Public Resources Code Sections 2621 through 2630) addresses one specific hazard: surface fault rupture, where the ground physically tears along a fault trace during a quake. It is not a building code about shaking. It is about keeping structures off the spots where the earth itself splits.¹

Under the law, an “active fault” is one that has ruptured within the last 11,000 years. The State Geologist identifies qualifying faults and draws Earthquake Fault Zones around their traces. These zones are long, narrow bands that ordinarily cannot exceed one-quarter mile in total width, though the State Geologist may designate a wider zone when circumstances require it.³ In practice the zone boundary usually falls a few hundred feet on either side of the mapped fault, depending on how precisely the fault’s location is known.

The Seismic Hazards Mapping Act (Public Resources Code Sections 2690 through 2699.6) picks up where Alquist-Priolo stops. The Legislature found that strong shaking, liquefaction, landslides, and related ground failure account for roughly 95 percent of the economic losses caused by earthquakes.² The California Geological Survey analyzes soil, groundwater, and slope data and designates Seismic Hazard Zones for two main risks:

Liquefaction, where saturated, loose soil loses strength during shaking and behaves like a liquid, letting foundations sink or tilt.
Earthquake-induced landslides, where marginally stable slopes fail during shaking.

The act also authorizes mapping for amplified ground shaking, though designated zones so far have focused on liquefaction and landslides.⁴

How to Check Whether Your Property Is in a Zone

The California Geological Survey runs a free online tool called EQ Zapp (Earthquake Hazards Zone Application). Search any California address or parcel and it will show whether the property sits within an Earthquake Fault Zone or a Seismic Hazard Zone, overlaid on an interactive map.⁵

You can also view or download zone data through the CGS Information Warehouse and the Geologic Hazards Data Viewer. County recorder, assessor, and planning offices are required to post public notices identifying the official maps and their effective dates, so paper copies exist locally as well.¹

What Building in a Zone Requires

Before a city or county approves a qualifying project inside either type of zone, the property owner has to pay for a site-specific geologic investigation. Local agencies cannot skip this step. The investigation determines whether the mapped hazard actually shows up on the specific parcel and, if it does, what can be done about it.

The Geologic Report

In an Earthquake Fault Zone, the report must define and delineate any hazard of surface fault rupture on the parcel, meeting the criteria set by the State Mining and Geology Board.³ Investigations typically involve trenching across the suspected fault trace to expose subsurface soil layers and check for displacement.

In a Seismic Hazard Zone, a Certified Engineering Geologist or Registered Civil Engineer with competence in seismic hazard evaluation prepares the geotechnical report, and it must go through peer review by local agency staff or a retained consultant.⁴ The report analyzes soil samples, historical groundwater data, and slope conditions, then recommends specific engineering measures where the hazard is confirmed.

Once a city or county approves a geologic report for a parcel, a new report generally is not needed for later projects on that parcel unless new geologic data warrants it.³

Setbacks Along a Fault

The core Alquist-Priolo restriction is simple. You cannot place a structure for human occupancy across the trace of an active fault. State regulations define a “structure for human occupancy” as one used or intended for any use or occupancy expected to exceed 2,000 person-hours per year, which covers most homes, offices, schools, and commercial buildings.⁶ Small storage sheds and similar low-occupancy structures fall outside the rule.

The area within 50 feet of an active fault is presumed to be underlain by active branches of that fault. You cannot build a structure for human occupancy inside that 50-foot band unless a geologic investigation proves the ground is clear.⁷ Real projects often end up set back farther, because trenching reveals the fault zone is wider than the initial map suggested.

If a report confirms an active fault runs through a proposed building footprint, the local agency must deny the permit unless the project is redesigned to avoid the trace. There is no engineering fix that lets you build across an active fault. You move the building or you don’t build it.

Mitigation for Liquefaction and Landslides

Rules in Seismic Hazard Zones are less absolute. Construction can proceed if the geotechnical report identifies the hazard and recommends engineering solutions that reduce the risk to an acceptable level. Common approaches for liquefaction include soil densification (compaction grouting or vibro-compaction), deep foundations that reach past liquefiable layers to stable ground, drainage systems that let excess water pressure dissipate during shaking, and soil cementation using chemical or jet grouting. For landslide-prone slopes, mitigation typically involves regrading, retaining structures, or removing unstable material. The geotechnical report has to justify why the chosen method is adequate for the confirmed hazard.

Projects That Are Exempt

Not every project in a mapped zone triggers a full investigation. Both acts carve out exemptions.

Under Public Resources Code Section 2621.6, the Alquist-Priolo Act does not treat the following as “projects”:

A single-family wood-frame or steel-frame dwelling on a parcel where a geologic report has already been approved.
A wood-frame or steel-frame single-family dwelling of two stories or less, as long as it is not part of a development of four or more dwellings. Mobile homes wider than eight feet count as single-family dwellings for this rule.
An alteration or addition that does not exceed 50 percent of the value of the existing structure.³

The Seismic Hazards Mapping Act uses similar thresholds. An alteration or addition inside a Seismic Hazard Zone is not a “project” if it does not exceed either 50 percent of the value of the structure or 50 percent of the existing floor area, whichever applies.² Cities and counties can also exempt qualifying single-family dwellings, though they are not required to, and local agencies always have the authority to be more restrictive than state law.⁴ Check with your local planning department before assuming an exemption applies.

Disclosure When You Sell

California Civil Code Section 1103 requires sellers of residential property to disclose whether the property sits in state-designated hazard zones. The obligation applies to sales, exchanges, lease-option agreements, and ground leases with improvements on single-family residential property.⁸

The seller or the seller’s agent must deliver a Natural Hazard Disclosure Statement identifying whether the property falls within, among other zones, an Earthquake Fault Zone designated under Public Resources Code Section 2622 or a Seismic Hazard Zone designated under Section 2696. The disclosure goes to the buyer early in the transaction, before they are contractually bound.

Getting this wrong carries real consequences. Anyone who willfully or negligently fails to comply with these disclosure requirements is liable for the buyer’s actual damages.⁹ A missed disclosure does not by itself void the sale, but it opens the door to a lawsuit, and the disclosure statute does not limit any separate fraud or misrepresentation claims a buyer might bring.

Insurance and Financing

Standard homeowners insurance in California does not cover earthquake damage. If your property is in a mapped fault zone or seismic hazard zone, earthquake insurance becomes a more pressing question. The California Earthquake Authority, the state’s primary residential earthquake insurer, sets rates based on earthquake risk at the specific location, among other factors.¹⁰ Statewide, premiums for a typical home range from under $2,000 per year in lower-risk areas to over $4,500 annually in high-risk ones.

Financing can also get more complicated. Fannie Mae’s multifamily lending guide, for example, requires lenders to assess seismic risk and prohibits delivery of a mortgage loan when the property has both high peak ground acceleration and certain structural risk factors, such as an unreinforced masonry building that has not been seismically retrofitted.¹¹ That guide states that earthquake insurance does not mitigate seismic risk for lending purposes; a structural retrofit may be required instead. Those specific rules apply to multifamily loans, but they show the level of scrutiny lenders apply to seismically exposed properties. Expect questions about seismic exposure during underwriting regardless of the loan type.

What Compliance Costs

If your parcel is in a mapped zone and you want to build, the geologic investigation is your first major expense. A site-specific geologic or geotechnical report from a licensed professional runs from a few thousand dollars for a straightforward residential parcel to significantly more for complex sites that need extensive trenching, soil sampling, or slope analysis. The cost tracks the size of the property, the hazard being investigated, and how much subsurface work the professional needs to reach a defensible conclusion.

If the investigation finds a fault trace, you pay to redesign around the required setback, which may shrink the buildable area of your lot substantially. Liquefaction and landslide mitigation adds construction cost through deep foundations, soil treatments, or drainage systems. Actual numbers depend entirely on what the geotechnical report finds, but mitigation can easily add tens of thousands of dollars to a project budget. Once a hazard is confirmed, none of it is optional; the local agency will not issue a permit without approved mitigation in place.

1
California Department of Conservation. Alquist-Priolo Earthquake Fault Zones
2
Justia. California Public Resources Code 2690-2699.6 – Seismic Hazards Mapping
3
Justia. California Public Resources Code 2621-2630 – Earthquake Fault Zoning
4
California Department of Conservation. California Seismic Hazard Zones
5
California Department of Conservation. EQ Zapp – California Earthquake Hazards Zone Application
6
Cornell Law Institute. California Code of Regulations Title 14 3601 – Definitions
7
Cornell Law Institute. California Code of Regulations Title 14 3603 – Specific Criteria
8
California Legislative Information. California Civil Code 1103
9
Justia. California Civil Code 1103-1103.14
10
California Earthquake Authority. California Earthquake Authority
11
Fannie Mae. Fannie Mae Multifamily Guide – Seismic Risk