State geologists have released new maps for Napa County that shows the location of the West Napa Fault — the culprit of the 2014 earthquake — and where it travels beneath the cities of Napa and American Canyon.
The maps feature thick yellow lines that represent the fault and its corresponding fault zone, which is the area where the surface is most likely to rupture and crack during an earthquake — and that poses “a potential hazard to structures,” according to the map’s legend.
In creating one of the maps labeled “Cuttings Wharf,” the California Geological Survey did not include all of American Canyon. But it does show some of the more vulnerable areas of town sitting within the fault zone.
Street names and urban landmarks were largely left off the map. Still, the map provides enough roadway and topographical information for someone to figure out what parts of the city have the fault.
Up until the release of the maps last week, the one location in American Canyon publicly revealed to have the West Napa Fault under it was Napa Junction Elementary School.
Following the August 2014 earthquake, which damaged some Napa Junction classrooms, the school district hired consultants to dig trenches on the campus to learn where precisely the fault was.
That study showed the fault zone runs diagonally through the school, forcing officials to find a new home for Napa Junction.
The fault continues its diagonal route through American Canyon, coming in from the northwest and past Oat Hill, then traveling in a southeasterly direction through town and across Highway 29.
Comparing this map with a Google street map of the city indicates some of the older neighborhoods are resting atop the fault.
It appears all of Theresa Avenue, located just south of Napa Junction Elementary, lies within the fault zone. So too does the eastern section of Eucalyptus Drive running from Theresa to Rio Del Mar.
All of Cassayre Drive and Melvin Road, and portions of Los Altos Drive also are within the fault zone.
The West Napa Fault zig zags as it approaches Highway 29 in the city between Napa Junction Road and Rio Del Mar. As a result the fault zone covers a portion of the highway through this section, and again a little ways south near the intersections with Donaldson Way and American Canyon Road.
Commercial properties residing within the fault zone include the 7-Eleven just off the highway and at least some of the businesses located near the northwest corner of Poco Way and Highway 29.
The areas on the west side of town resting within the fault zone were developed in the 1950s, long before the passage of the Alquist-Priolo Earthquake Fault Zoning Act of 1972.
This law, adopted after the 1971 San Fernando earthquake, requires the state to produce maps such as these to inform the public and developers about the location of faults.
Developers are not allowed to construct new homes on parcels that have a fault beneath them. But they can build them near faults, as close as 50 feet from them.
For existing houses within a fault zone, experts recommend bolting a home to the foundation and strengthening cripple walls, located between the first floor and the foundation.
The new fault maps can be found on the California Department of Conservation’s website at www.conservation.ca.gov/cgs/shzp/Pages/SHPMaps.aspx