What is a fault?
Faults are weaknesses in the earth’s crust that allow plates to slide past each other. Earthquakes occur when forces underground cause the faults beneath us to rupture and suddenly slip. If the rupture extends to the surface, we see movement on a fault (surface rupture). Because faults are weaknesses in the rock, earthquakes tend to occur over and over on these same faults. Strong earthquakes can occur when the fault rupture does not extend to the surface, and fault-related damage is rare when compared to shaking-related damage. More information from USGS.
Alquist-Priolo Earthquake Fault Zone Maps
Source: California Geological Survey from CD-ROM 2001-04 (2001), Official Map of Alquist-Priolo Earthquake Fault Zones.
Note: Fault zone maps for Alameda County and the cities of Hayward, Oakland, and San Leandro have been updated by CGS as of September 2012, but are not yet incorporated into the maps on this website. Please go directly to the CGS website to view these updated maps.
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The California Geological Survey publishes maps of the active faults in the Bay Area that reach the surface as part of its work to implement the requirements of the Alquist-Priolo Earthquake Fault Zone Act. These maps show not only the most comprehensive depiction of fault traces that can rupture the surface, but also the zones in which cities and counties must require special geologic studies to prevent the building of structures intended for human occupancy and in which the surface rupture hazard must be disclosed in real estate transactions.
Faults used in ABAG earthquake shaking maps (pdf) – this map also includes faults not not within the AP Earthquake Fault Zone. View shaking maps.
Vulnerability and Exposure of the Bay Area
Disclaimer: This map was created using digital files of AP EFZ quadrangles and is considered an electronic facsimile of the Official Alquist-Priolo Earthquake Fault Zone map. If there is any doubt or conflict with respect to the location of EFZ boundaries, the original clear-film overlay compiled by and on file with CGS is the official version of the map. Fault information in these digital files is not sufficient to serve as a substitute for the geological site studies required under Chapter 7.5 of Division 2 of the California Public Resources Code.