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NR 2001-45
May 17, 2001

Contact: Carol Dahmen
Mark Oldfield
Don Drysdale
Ed Wilson
(916) 323-1886


SACRAMENTO -- In an effort to make maps that impact land-use decisions more accessible, the California Department of Conservation is offering its large inventory of Alquist-Priolo Earthquake Fault Zone maps for the Central Coast and Northern/Eastern California as digital images on two CD-ROMs.

Basically, the maps from DOC's Division of Mines and Geology are screening tools that determine whether extra care should be taken to ensure that new construction does not take place on top of active faults. Locating new structures away from active faults can result in fewer injuries and lower repair costs in the event of an earthquake. By law, real estate agents and sellers must inform buyers whether property being sold is within one of the zones shown on these maps.

The Northern/Eastern CD contains 192 black-and-white images of maps with Earthquake Fault Zones highlighted in yellow. The digital images in this CD show the zones in Alpine, Butte, Inyo, Lassen, Mendocino, Modoc, Shasta and Siskiyous counties. The Central Coast CD contains 182 digital images including Alameda, Contra Costa, Fresno, Kern, Lake, San Luis Obispo, San Mateo, Santa Clara, Santa Cruz, Solano, Sonoma, Stanislaus and Yolo counties.

Viewing the digital images requires Adobe Acrobat Reader version 4.0, which is included in the CD for Windows/NT, Macintosh and Unix operating systems. The Earthquake Fault Zones are indexed by county and city. Users can select portions of map images and copy them to word processing or graphic software programs, where they can be annotated and printed.

"The main benefits are accessibility, cost and ease of storage," Department of Conservation Director Darryl Young said of the digitized maps. "A paper copy generally costs up to $7 per map. In this format, all of the maps for Northern and Eastern or Central California are available on one CD for $30, and users can print out whatever portion they need. You can move from county to county and map to map by clicking an arrow. It's as user-friendly as possible."

For information about purchasing either of the new CDs, call DOC's Division of Mines and Geology at (916) 445-5716. DOC's Division of Mines and Geology previously released a CD containing images of 235 Alquist-Priolo maps for the Southern California region.

The Alquist-Priolo Earthquake Fault Zoning Act addresses the hazard of surface fault rupture, which occurs when the movement on a fault deep within the earth breaks through to the surface. Not all earthquakes result in surface rupture. The earthquake must be large enough (generally greater than magnitude 5) in order to reach the surface. By definition, blind thrust faults that cause earthquakes, such as the Northridge Earthquake of 1994, do not break the ground surface and thus are not shown on Alquist-Priolo maps.

The Act was passed in 1972 -- a direct result of the 1971 San Fernando Earthquake, which caused extensive surface fault rupture that damaged numerous homes, commercial buildings, and other structures. Surface fault rupture -- generally the most easily avoided seismic hazard -- causes about five percent of the damage resulting from earthquakes. Ground shaking and related effects such as liquefaction and landslides account for 95 percent of the damage.

Each Alquist-Priolo map covers about 60 square miles, but the actual zones generally are about 1,000 feet wide. Before development can take place in those zones, the developer must perform a geologic investigation. Typically, a developer hires a state-licensed geologist to determine whether the project area is underlain by active faults (those that have caused ground displacement in the last 11,000 years). The developer submits the geologic report to the local planning department, which must agree with the report’s findings before the development can proceed. Structures for human occupancy generally are set back 50 feet from active faults.

There have been 27 earthquakes associated with surface faulting in California since the first Alquist-Priolo maps were issued, most recently the magnitude 7.1 Hector Mine quake last October. The last six of those occurred mainly within established A-P zones. Seven of those 27 quakes had surface displacement of greater than a foot.

In addition to studying and mapping earthquakes and other geologic phenomena, the Department of Conservation maps and classifies areas containing mineral deposits; ensures the reclamation of land used for mining; promotes beverage container recycling; regulates oil, gas and geothermal wells; and administers agricultural and open-space land conservation programs.