Geologic Hazard Combining District

The Geologic Hazard Combining District includes areas in close proximity to active faults, which are defined by the California Geological Survey (CGS) as mandated through the 1972 Alquist-Priolo Earthquake Fault Zoning Act. The intent of the Act is to prevent the construction of buildings used for human occupancy where an active fault intersects the ground surface. In practice, the Act requires a specialized geologic study be conducted to define the location of active geologic faults in the vicinity of the building envelope. The scope of the required geologic study is described in California Geological Survey Special Publication 42 (PDF). A 10 to 1 benefit to cost ratio has been estimated for conducting geologic studies and locating structures away from active faults.

Official Alquist-Priolo Earthquake Fault Zones (EFZs) are prepared and published by CGS.  Sonoma County’s Geologic Hazard Combining District is intended to match the State approved EFZs. For properties located in the Geologic Hazard Combining District, California law and the Sonoma County Code limit the type of development that may occur within 50 feet of the surface trace of an active fault. The County Code requires that to develop property in the Geologic Hazard Combining District, a geologic report must be prepared that describes the hazards and identifies mitigation measures to reduce risks to acceptable levels. (See Sonoma County Code Chapter 26, Article 70.)

Note: There is a temporary inconsistency along the southern Rodgers Creek Fault Zone due to recent updates by CGS. On February 22, 2024, CGS released revised maps of EFZs for the southern Rodgers Creek Fault Zone in Sonoma County, in the Sears Point, Petaluma River, Glen Ellen, Cotati, and Santa Rosa quadrangles. The updated EFZs have not yet been incorporated into Sonoma County’s Geologic Hazard Combining District zoning. Geologic reports will be required of projects that are within EFZs, but outside of Geologic Hazard Combining District boundaries.

Alquist-Priolo Earthquake Fault Zones Map
Sonoma County Alquist-Priolo Earthquake Fault Zones, including Fault Evaluation Report(s)
 

The following recommendations are provided for those intending to apply for building permits within the Geologic Hazard Zone:

• Verify with map applications or Permit Sonoma staff that your proposed building envelope is within the G combining zone and specialized geologic study requirements apply to your project.
• Research past soils and geotechnical reports for your parcel, and provide these reports to the firm you select to conduct the geologic study.
• Consider working with neighbors when choosing a qualified firm to conduct the geologic study.
• Contact Permit Sonoma staff to discuss the geologic review process.

If you have additional questions or concerns please feel free to contact Robert Pennington, staff geologist at (707) 565-1352 or email at Robert.Pennington@sonoma-county.org