California Earthquake Fault Zone Maps and Disclosure Rules

California’s earthquake fault zone maps are the official Alquist-Priolo maps produced by the California Geological Survey, and they are the only earthquake maps in the state that carry legal weight. If your parcel falls inside one of these zones, that designation controls whether you can build on part of the lot, whether you need a geologic report first, and what you have to tell a buyer when you sell.¹

How to Check a Specific Address

The fastest way to find out whether a property sits inside a fault zone is EQ Zapp, the California Earthquake Hazards Zone Application. It’s a free CGS tool that takes an address or your current location and tells you whether the parcel is inside any mapped earthquake hazard zone. It also flags areas CGS hasn’t evaluated yet. The tool works on computers, tablets, and phones.²

Printed copies of the official maps sit at city and county planning offices for the jurisdictions they cover. If you’re buying property or planning to build, the local planning department can confirm the zoning status and tell you exactly what investigation, if any, your project will require.¹

What the Maps Show

Each Earthquake Fault Zone is a long, narrow corridor drawn around the trace of an active fault. Zones average about a quarter mile wide, though the width varies with the complexity of the fault and how precisely its location is known.¹

The maps address one specific hazard: surface fault rupture, where the ground itself cracks open along the fault line during an earthquake. They do not predict shaking intensity or ground motion, and they don’t cover liquefaction or landslide risk. Those hazards are addressed by separate CGS programs.

A fault only shows up on these maps if it qualifies as “active” under state regulations, meaning it has displaced the ground surface during the Holocene epoch, roughly the last 11,000 years.³ Older faults, even ones visible in the landscape, generally aren’t zoned. That’s why a fault you can see on a geology hobbyist’s map may not appear on the regulatory version.

What Happens If Your Property Is Inside a Zone

A parcel inside an Earthquake Fault Zone triggers a geologic investigation requirement before a city or county can approve most new construction. Local government must require a report identifying and mapping any surface rupture hazard on the specific site, prepared by a California-licensed geologist.⁴

If the investigation finds an active fault trace on the property, a structure for human occupancy cannot be built directly over it and generally must be set back at least 50 feet from the trace.¹ The 50-foot figure also appears in the state’s regulations, which treat the area within 50 feet of a known active fault as underlain by active branches of the fault unless a site-specific investigation proves otherwise.⁵

Once a geologic report has been approved or waived for a site, the local agency won’t require another one unless new geologic data warrants it.⁴

Exemptions for Single-Family Homes

Not every project inside a zone has to go through the full investigation. A single-family wood-frame or steel-frame home of two stories or fewer is exempt, as long as it isn’t part of a development of four or more dwellings. Mobile homes wider than eight feet are treated the same way. The exemption also covers a single-family home built on a parcel already covered by an approved geologic report from a prior subdivision.⁶

Disclosure When You Sell

Under California’s Natural Hazard Disclosure Act, a seller of residential property has to tell the buyer when the property sits inside an Alquist-Priolo Earthquake Fault Zone.⁷ The disclosure goes on a written Natural Hazard Disclosure Statement with a yes-or-no indication of whether the property is within an earthquake fault zone.⁸

When a real estate agent is involved, the agent is legally responsible for presenting the information to the buyer. Without an agent, the seller delivers it directly.¹ The disclosure is not a warranty, but buyers can rely on it in deciding whether and on what terms to buy.⁸ In practice, many sellers order a third-party Natural Hazard Disclosure report that checks all applicable hazard zones at once. These reports usually run $50 to $150.

Being in a Fault Zone Does Not Require Earthquake Insurance

A property inside a fault zone isn’t required to carry earthquake insurance. California doesn’t mandate the coverage, and mortgage lenders don’t either. Lenders require standard homeowners insurance, but earthquake coverage stays optional whether or not your parcel is inside an Alquist-Priolo zone.⁹

USGS Maps Are Not a Substitute

The U.S. Geological Survey publishes its own earthquake fault data, and some of it shows the same faults CGS has zoned under Alquist-Priolo. The USGS layer, however, is described by the agency as “intended solely as an educational tool” and does not include the regulatory zones that trigger building setbacks or disclosure obligations.¹⁰ USGS even cautions that its viewer lets users zoom in past the level of accuracy the underlying data supports.

For any decision about buying, selling, or building in California, the CGS maps through EQ Zapp are the ones that matter. USGS data can give a general national picture; it can’t tell you whether your parcel carries a building restriction.

1
California Geological Survey. Alquist-Priolo Earthquake Fault Zones
2
California Department of Conservation. EQ Zapp: California Earthquake Hazards Zone Application
3
Legal Information Institute. 14 CCR 3601 – Definitions
4
California Legislative Information. California Code PRC 2623 – Earthquake Fault Zoning
5
Legal Information Institute. 14 CCR 3603 – Specific Criteria
6
California Legislative Information. California Code PRC 2621.6 – Earthquake Fault Zoning
7
California Legislative Information. California Civil Code 1103
8
California Legislative Information. California Civil Code 1103.2 – Disclosure of Natural and Environmental Hazards
9
California Department of Insurance. Earthquake Insurance
10
USGS. Alquist-Priolo Faults