September 23, 2021
SACRAMENTO – Future structures, and the people who occupy them, may be safer from earthquake hazards thanks to new maps from the Department of Conservation’s California Geological Survey (CGS)
for Contra Costa and San Mateo counties.
“Knowing whether a location is subject to landslides or liquefaction during heavy seismic shaking allows construction to incorporate design elements that protect lives and structures,” said Acting State Geologist Steve Bohlen, head of CGS. “Hazards management is fundamental to our work at CGS and the Department of Conservation.”
Released as a draft for public comment in February, these Seismic Hazard Zone maps became official on September 23. There are two maps for each county that establish zones where there is a significant likelihood of earthquake-induced landslides and soil liquefaction as a result of strong shaking. Liquefaction is a phenomenon in which seismic shaking causes soil to become saturated with groundwater and temporarily act like quicksand.
Within these zones, local governments must require site-specific geotechnical studies before new residential or commercial development occurs, ensuring the potential earthquake hazards are adequately identified and mitigated. Those who want to see whether their property is in a regulatory earthquake hazard zone can enter an address into CGS’ EQZapp tool.
The maps include parts of the Contra Costa communities of Clayton, Concord, Pittsburg, and Walnut Creek and the San Mateo County communities of Brisbane, Colma, Daly City, Half Moon Bay, Pacifica, San Bruno, and South San Francisco. Each map covers an area about 60 square miles in size.
In Contra Costa County, new liquefaction zones are primarily mapped in the Clayton and Mt. Diablo Creek valleys, as well as around the periphery of the Clifton Court Forebay Reservoir. New landslide zones in Contra Costa County are mapped along the north flank of Mt. Diablo and in upland areas northeast of Clayton.
Significant new liquefaction zones in San Mateo County affect areas adjacent to Colma Creek and coastal communities of the San Francisco peninsula, particularly San Bruno, Half Moon Bay, and Miramar. New landslide zones cover portions of San Bruno Mountain and the northern Santa Cruz Mountains.
Earthquakes of magnitude 5.5 or greater can trigger landslides or liquefaction. While shaking does most of the damage in a large earthquake, both liquefaction and landslides caused significant damage during the 1989 Loma Prieta quake. The Seismic Hazards Mapping Act mandating the regulatory maps was passed the year after Loma Prieta.
In creating Seismic Hazard Zone Maps, CGS uses Geographic Information System (GIS) technology, which allows the manipulation of three-dimensional data. Information analyzed in these studies includes topography, surface and subsurface geology, borehole log data, recorded ground-water levels, existing landslide features, slope gradient, rock-strength measurements, geologic structure, and probabilistic earthquake shaking estimates.
For real estate transactions, property sellers and real estate agents must disclose that a property is in a Seismic Hazard Zone.
The CGS Seismic Hazards Program currently is working on additional Seismic Hazard Zone maps for Contra Costa County and fault rupture zone maps along the Rodgers Creek Fault in Sonoma County and the San Andreas Fault on the San Francisco Peninsula.