California Earthquake Faults by County
Open the USGS Quaternary fault map for your county, check the same area on the CGS Fault Activity Map, then use UCERF3 odds for your region.
Last updated November 2026
Contents9
- How California fault maps relate to counties
- How many faults California scientists map
- Check faults for your county in four steps
- Major fault systems by county group
- UCERF3 probabilities for California
- Why California concentrates U.S. earthquake loss
- Turn map results into household actions
- Los Angeles and other multi-fault metros
- Aftershocks and county-wide services
To see which earthquake faults cross your California county, open the USGS Quaternary fault map at county scale, then match those traces to the California Geological Survey Fault Activity Map. Use UCERF3 regional probabilities next so your household plan fits the fault systems near your address.
How California fault maps relate to counties
California counties are political boundaries. Faults cut across them, branch, and hide below young sediment in some basins. No single list replaces the official maps, but every county has at least one authoritative layer to check.
The California Geological Survey studies faults, publishes the Fault Activity Map of California, and prepares shaking and rupture hazard products. The USGS Earthquake Hazards Program hosts the Quaternary Fault and Fold Database and the interactive fault map used nationwide.
The earthquake kit guide groups supplies and response guides for shaken households. This page focuses on where faults run, not on what to pack.
How many faults California scientists map
The California Geological Survey states that there are hundreds of identified faults in California and that about 200 are considered potentially hazardous when judged by slip rates in recent geologic time, roughly the last 10,000 years. It also notes that more than 70 percent of the state's population lives within 30 miles of a fault where high ground shaking could occur within the next 50 years.
The USGS Quaternary Fault and Fold Database covers the United States and includes faults with evidence of movement in roughly the last 2.6 million years. California entries dominate the West Coast portion because slip rates and mapping are dense there. Counts depend on how small a trace is included and whether a feature is exposed at the surface or inferred at depth.
For household planning, the named faults under your county matter more than a statewide total.
Check faults for your county in four steps
- Open the USGS Quaternary Faults interactive map and search or zoom to your county.
- Note each fault name, trace color, and slip sense shown in the popup. Save the map view if your county has several overlapping systems.
- Open the California Geological Survey Fault Activity Map and turn on faults for the same area. Compare activity ages shown in the legend.
- Open the USGS National Seismic Hazard Map tools for expected shaking at your latitude and longitude, then read UCERF3 probabilities for your broad region.
If you are buying or retrofitting a home, also check whether the California Geological Survey has issued an Alquist-Priolo Earthquake Fault Zone map for your parcel. Those zones mark surface fault rupture hazard near traces with recent movement.
Major fault systems by county group
The table lists county groups and fault names that appear on UCERF3 fact sheet figures or the USGS Quaternary fault map for that broad area. It is a starting point for map cross-checks, not a substitute for parcel-level searches.
| County group | Fault systems named in the cited source | Primary source |
|---|---|---|
| San Francisco, Alameda, Contra Costa, Santa Clara, San Mateo | Northern San Andreas; Hayward; Calaveras (and Hayward–Rodgers Creek in UCERF3 narrative) | USGS UCERF3 fact sheet Figure 4 |
| Los Angeles, Orange, Ventura | Southern San Andreas; San Jacinto; Elsinore | USGS UCERF3 fact sheet Figure 5 |
| Riverside, San Bernardino | San Andreas; San Jacinto | USGS UCERF3 fact sheet |
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Several faults in southern California are blind thrusts, where the rupture surface does not reach the ground but still produces strong shaking. The 1994 Northridge earthquake is the magnitude 6.7 example UCERF3 uses when it ties forecast thresholds to a typical home mortgage horizon.
UCERF3 probabilities for California
These figures come from forecast models, not from a schedule of future earthquakes.
The USGS UCERF3 fact sheet gives a greater than 99 percent chance of one or more magnitude 6.7 and larger earthquakes somewhere in California within the next 30 years. The same fact sheet lists a 7 percent chance of magnitude 8 and greater within 30 years, a 50 percent increase over UCERF2.
For regions, the USGS FAQ on Los Angeles and San Francisco probabilities lists 30-year chances of 60 percent in the Los Angeles region and 72 percent in the San Francisco Bay region that an earthquake measuring magnitude 6.7 will occur. It also gives separate probabilities at magnitude 7.0 and 7.5 thresholds for each area.
Why California concentrates U.S. earthquake loss
Earthquake damage depends on shaking, building type, and population density. California combines fast plate motion, long mapped fault systems, and very large urban exposure.
FEMA P-366, published April 2023, estimates average annual earthquake loss to national building stock at 14.7 billion dollars per year, with about 65 percent, roughly 9.6 billion dollars per year, concentrated in California.
That share is a long-term model, not a prediction for any single season. It helps explain why high-hazard counties carry a large share of modeled U.S. building exposure.
Turn map results into household actions
After you identify nearby fault systems, shift to actions that do not depend on guessing when the next rupture will occur.
- Walk your home for tip-over hazards and strap tall furniture, as described in what to do during an earthquake.
- Write contacts and meeting spots in your family emergency plan.
- Keep a reachable kit, water, and a hand-crank or battery radio from the earthquake kit guide.
- Review what to do in an earthquake room by room so the drop-cover-hold-on response is automatic.
Los Angeles and other multi-fault metros
Los Angeles spreads across basins bounded by several fault systems. Many residents live within a few miles of multiple named traces, so parcel-scale map zoom matters more than county outlines alone.
A site in the San Fernando Valley can be closer to one mapped trace than another, while coastal addresses face different tsunami and shaking paths. The same rule applies in the Bay Area, where the Hayward–Rodgers Creek and Calaveras faults underlie dense housing.
UCERF3 treats some faults as ready for large earthquakes because centuries have passed since their last major rupture, while others show more recent surface breaks. Those details appear in fault popups and in USGS publications, not in county tax maps.
Aftershocks and county-wide services
A large earthquake anywhere in California can trigger aftershocks and secondary failures of roads, utilities, and slopes far from the main fault trace. County emergency managers use state and federal geologic products when they stage shelters and road closures.
Keep official information channels in your plan. Ready.gov earthquake guidance recommends signing up for earthquake alerts and following local news on a battery-operated radio.
For recorded events with dates and magnitudes, compare the United States disaster records catalog, which lists selected West Coast earthquakes with USGS sources.
Questions people also ask
How do I find earthquake faults in my California county?
Start with the USGS Quaternary Faults interactive map and zoom to your county. Open the California Geological Survey Fault Activity Map layers for named traces and activity ages. Compare both against the USGS National Seismic Hazard Map for expected shaking where you live.
How many potentially hazardous faults does California have?
The California Geological Survey counts hundreds of identified faults statewide. It lists about 200 as potentially hazardous based on slip rates in the last 10,000 years. Individual counties can sit on several named systems at once.
Which California regions face the highest shaking odds?
The USGS FAQ gives a 30-year chance of 60 percent for magnitude 6.7 in the Los Angeles region and 72 percent in the San Francisco Bay region.
Does Los Angeles sit on more than one fault?
Yes. Los Angeles County crosses more than one UCERF3 tabulated fault. Use the county-group table on this page, then zoom the USGS fault map to your address to see which named trace is closest.
How much of U.S. earthquake loss risk is in California?
FEMA P-366 puts about 65 percent of modeled average annual earthquake building losses in California, about 9.6 billion dollars per year of a 14.7 billion dollar national total. See the loss section on this page for the full Hazus citation.
What should I do after checking county fault maps?
Anchor tall furniture, agree on a household plan, and keep water and a radio reachable. The earthquake kit guide and family plan pages walk through supplies and contacts.