Preliminary Review Maps of a Proposed Revised Earthquake Fault Zone
May 22, 2025 – The California Geological Survey released preliminary review maps and a report for a proposed revised Alquist-Priolo Earthquake Fault Zone affecting the city of Half Moon Bay and the counties of San Mateo and Santa Cruz. The index map below shows the locations of the quadrangles included with this release.
Index map showing outlines of the Earthquake Fault Zone maps in this release. Click an image to open a larger version in a new window.
Earthquake Fault Zones are regulatory zones that encompass surface traces of active faults that have a potential for future surface fault rupture.
These maps are released under the authority of the Alquist-Priolo Earthquake Fault Zoning Act (Alquist-Priolo Act) that was passed following the 1971 San Fernando earthquake. The Alquist-Priolo Act is a state law designed to reduce the hazard from surface fault rupture during an earthquake.
The maps and report are available as PDF files, which can be viewed using Adobe Acrobat Reader. To download these publications, follow the links provided below.
- Preliminary Fault Evaluation Report 275: The San Gregorio Fault Zone in the Montara Mountain, Half Moon Bay, San Gregorio, La Honda, Pigeon Point, Franklin Point, and Point Ano Nuevo 7.5-Minute Quadrangles, San Mateo and Santa Cruz Counties, California
Included with this release
- One (1) Fault Evaluation Report (FER 275);
- Five (5) Earthquake Zones of Required Investigation Maps (EZRIM).
Related Reference Material
- Reference Guide for Assessing Fault Rupture Hazards (Special Publication 42: Earthquake Fault Zones - A Guide for Government Agencies, Property Owners/Developers, and Geoscience Practitioners for Assessing Fault Rupture Hazards in California). This document contains technical guidelines for evaluating surface fault rupture hazards and for reviewing geologic reports.
Preliminary Review Maps of Proposed New Seismic Hazard Zones
May 22, 2025 – The California Geological Survey released preliminary review maps and reports of proposed new Seismic Hazard Zones affecting the cities of Sacramento, West Sacramento, Santa Rosa, Sebastopol, and Windsor, and the counties of Sacramento, Yolo, Contra Costa, Alameda, and Sonoma. The index maps below show the locations of the quadrangles included with this release.
Index maps showing outlines of the Seismic Hazard Zone maps in this release. Click an image to open a larger version in a new window.
Seismic Hazard Zones are regulatory zones that encompass areas that have a potential for future ground deformations caused by liquefaction or landslides.
These maps are released under the authority of the Seismic Hazards Mapping Act that was passed following the 1989 Loma Prieta earthquake. The Seismic Hazards Mapping Act is a state law designed to reduce the hazards from soil liquefaction and landslides during an earthquake.
The maps and reports are available as PDF files, which can be viewed using Adobe Acrobat Reader. To download these publications, follow the links provided below.
- Preliminary Seismic Hazard Zone Report 141: Seismic Hazard Zone Report for the Sacramento West and Sacramento East 7.5-Minute Quadrangles; Sacramento and Yolo Counties, California
Included with this release
- One (1) Seismic Hazard Zone Report (SHZR 141);
- Two (2) Earthquake Zones of Required Investigation Maps (EZRIM).
- Preliminary Seismic Hazard Zone Report 142: Seismic Hazard Zone Report for the Byron Hot Springs 7.5-Minute Quadrangle, Contra Costa and Alameda Counties, California
Included with this release
- One (1) Seismic Hazard Zone Report (SHZR 142);
- One (1) Earthquake Zones of Required Investigation Map (EZRIM).
- Preliminary Seismic Hazard Zone Report 143: Seismic Hazard Zone Report for the Sebastopol 7.5-Minute Quadrangle, Sonoma County, California
Included with this release
- One (1) Seismic Hazard Zone Report (SHZR 143);
- One (1) Earthquake Zones of Required Investigation Map (EZRIM).
Related Reference Material
- Reference Guide for Evaluating Seismic Hazards (Special Publication 117A: Guidelines for Evaluating and Mitigating Seismic Hazards in California). This document constitutes the guidelines for evaluating seismic hazards other than surface fault rupture, and for recommending mitigation measures as required by Public Resources Code Section 2695(a).
