Seismic Hazards Shown in San Mateo, Contra Costa Counties

​California Geological Survey Releases Preliminary Maps That Could Impact Planning 

NR #2018-09
October 4, 2018

SACRAMENTO – The California Geological Survey (CGS) today issued five Preliminary Seismic Hazard Zone maps covering portions of Contra Costa and San Mateo counties. The maps identify areas where earthquake shaking could trigger landslides or liquefaction, and create zones within which geotechnical studies must take place before new commercial construction occurs.

“These maps designate areas where, during the planning stage, construction might have to incorporate design elements to protect life and property in the event of a large earthquake,” said Dr. John Parrish, State Geologist of California and head of CGS. “We know that, sometime in the future, large, damaging earthquakes will strike in a populated area. The hope is that the hazard information contained in these maps and zones will lead to newly built structures being better prepared to withstand those earthquakes than they would have been otherwise.”

CGS is releasing three maps covering parts of Contra Costa County, including all or portions of the communities of Pittsburg, Antioch, Concord, Brentwood, Oakley, and Bay Point. 

Also available for review are two new maps of San Mateo County, including all or portions of the communities of Woodside, San Carlos, Belmont, Burlingame, Half Moon Bay, Hillsborough, Millbrae, Redwood City, San Bruno, San Carlos, Portola Valley, and Pacifica.

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.  

Each map covers an area about 60 square miles in size corresponding to a USGS topographic quadrangle. The maps will become official after a 90-day review opportunity for local governments and geotechnical experts, followed by a 90-day revision opportunity for CGS. Printed versions of the maps can be seen at local planning and building departments.

Earthquakes of magnitude 5.5 or greater can trigger landslides or liquefaction, a phenomenon in which soil temporarily acts like quicksand and loses its ability to support structures. 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.

The maps establish Earthquake Zones of Required Investigation. For development within a zone, the local building department must require -- before permits are issued -- that licensed geologists and engineers investigate the site for evidence of liquefaction or landslide potential. If such evidence is found, design modifications must be made in the planning stage. Examples of these modifications include deep foundations in liquefaction zones or slope stabilization in landslide zones.

“It’s much less expensive and easier to take steps that can help minimize the risk of these seismic hazards before construction takes place than it is to retrofit,” said Tim McCrink, who heads the CGS zonation program. “In the Woodside Quadrangle, for example, the San Andreas Fault runs right through the area. So it’s certainly worth doing a geotechnical study before starting significant construction.”

Property sellers and real estate agents must inform potential buyers if property they're selling is in a seismic hazard zone, as is the case when property is in a designated flood zone.

For those in existing homes or structures, the Governor’s Office of Emergency Services, American Red Cross and FEMA have all released publications about becoming more earthquake-ready. Those publications are available online or at local public libraries.

A closer look at where hazard zones are proposed within each of the new seismic hazard zone maps shows:

  • Woodside Quadrangle (San Mateo County): Much of area is zoned for earthquake-induced landslides, due to the steep hilly terrain. There are liquefaction zones around streams near San Carlos in the northeast portion of the map, along the San Andreas Fault west of Woodside, and right through Woodside in the eastern segment.

  • Montara Mountain Quadrangle (San Mateo County): The San Francisco Airport is in a liquefaction zone that extends to the west of the airport nearly to the San Andreas rift zone. There are also liquefaction zones around Pacifica State Beach, the San Pedro Valley, El Granada, and the Half Moon Bay Airport. Three-quarters of the map – including most of the Golden Gate National Recreation Area and San Pedro Valley County Park – is zoned for landslides.

  • Antioch South Quadrangle (Contra Costa County): Parts of South Antioch, as well as several valleys (Lone Tree, Horse, Deer, Briones, and Cañada de Los Poblanos) are zoned for liquefaction. Much of the hilly western portion of the quadrangle is zoned for landslides.

  • Antioch North Quadrangle (Contra Costa County): There are extensive liquefaction zones along the shoreline, extending south through much of Antioch and Pittsburgh. There is a significant unzoned area south of the Southern Pacific Railroad tracks and west of Somersville Road that includes the County East Shopping Center and Marchetti Park. 

  • Honker Bay Quadrangle (Contra Costa County): Only the Contra Costa County portion of this area, not the Solano County portion, was evaluated for these earthquake hazards. Much of Bay Point and part of Pittsburg are within liquefaction zones. Those zones extend as far as the Southern Pacific Railroad line, with some fingers reaching farther south. In Pittsburgh, the high school, Parkside School, and civic center are shown as being outside of the liquefaction zone. There are small landslide zones in the hilly section in the southern portion of the map.

Don Drysdale
(916) 323-1886