NR 2002-49
December 20, 2002

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


Mapping for Liquefaction, Landslide Potential Complete in Orange County

Editors: To view a JPEG file of a section of the Ojai Quadrangle (Ventura County), click here. To view a JPEG file of a section of the quadrangle containing Rancho Santa Margarita (Orange County), click here.

SACRAMENTO – Seven Seismic Hazard Zone maps – six for Ventura County (and a small portion of Los Angeles County) plus the 18th and final one for Orange County -- became official today. The maps, issued by the Department of Conservation’s California Geological Survey, impact local planners, developers, property sellers and real estate agents.

If property is located in a Zone of Required Investigation, where liquefaction or earthquake-induced landslides could occur during a large earthquake, the local building department must require geologic studies before projects are issued permits. Also, property sellers and real estate agents must inform buyers if property they're selling is in a Seismic Hazard Zone, as is the case when property is in a designated flood zone.

Orange County is the first county to be completely mapped for these types of seismic hazards. Each of the 18 maps for the county cover approximately 60 square miles. The maps affect 33 cities.

The first Orange County Seismic Hazard Zone Map, the Newport Beach Quadrangle, was issued on April 7, 1997. Since then, 76 official maps affecting six counties (San Francisco, Alameda and Santa Clara are the others) have been issued.

“The completion of this series of maps is a substantial advance in the geotechnical information available for Orange County, as well as a milestone for the Seismic Hazard Mapping program,” DOC Director Darryl Young said.                                    This is a section of the Ojai Quadrangle, Ventura County.

The latest maps include the communities of Rancho Santa Margarita and Mission Viejo in Orange County as well as the Ventura County communities of Fillmore, Ojai, Oxnard, Port Hueneme and Ventura.

Shaking causes most of the damage during earthquakes, and in many cases, it is cost effective to retrofit houses and buildings to minimize damage caused by severe shaking. Local public libraries have a number of publications by the Governor’s Office of

Emergency Services, American Red Cross and the Federal Emergency Management Agency that can be used as guides to making homes more earthquake-ready.

Seismic Hazard Zone maps show areas at risk from the secondary earthquake hazards of landslides and liquefaction. It is generally not as cost effective to retrofit an existing building for the impacts of liquefaction or landslides as it is to build in safety features at the design stage. Therefore, design changes to better protect life and property during future earthquakes are required before new developments are approved and built.

“Knowing where liquefaction and landslides are most likely to occur means that local officials can require special engineering steps on new construction to make people and buildings safer," Young said.

Color copies of official maps can be purchased through DOC's California Geological Survey at (213) 239-0878, (916) 445-5716, or (415) 904-7707. The maps also can be viewed and downloaded from the DOC Web site here

Following is a brief description of the seismic hazards shown in each map:

FILLMORE QUADRANGLE -- Two-thirds of this quadrangle consists of rugged, mountainous terrain in central Ventura County. The rest consists of lowlands along the Santa Clara River and Sespe Creek. Aside from the City of Fillmore, the quadrangle is unincorporated land.

A broad liquefaction zone extends east to west through the Santa Clara Valley area, covering most of the City of Fillmore. The area is defined by Sespe Creek to the west, and south of the Santa Clara River. A broad area also extends north of Fillmore into the Los Padres National Forest valley. Landslide zones occur north of Fillmore in the TopaTopa Mountains.

PIRU QUADRANGLE -- In east-central Ventura County, this quadrangle consists mostly of rugged, mountainous terrain except for the lowlands along the Santa Clara River valley and in Piru Creek Canyon. The rural community of Piru is the only residential area in this quadrangle.

Broad liquefaction zones occur in the southern portion of the map in the Santa Clara Valley, from the foot of the TopaTopa Mountains to south of the Santa Clara River. Most of the community of Piru is within the liquefaction hazard zone. Liquefaction zones also extend northward from Piru into the valleys of the TopaTopa Mountains. Most of the TopaTopa Mountains north of the liquefaction zones are zoned for landslide hazards. The Santa Susana Mountains south of the Santa Clara River are also are zoned for landslides.

VAL VERDE QUADRANGLE -- Homes in this area, about 37 miles northwest of the Los Angeles Civic Center, are concentrated in the small, unincorporated community of Val Verde and in Castaic, although residential, commercial, and industrial development is currently spreading westward along the Santa Clara River and Castaic Creek valleys.

Broad liquefaction zones occur in the southern portion of the map in the Santa Clara Valley, from the foot of the TopaTopa Mountains, to south of the Santa Clara River. Most of the TopaTopa Mountains to the north and the Santa Susana Mountains south of the Santa Clara River valley are zoned for landslide hazards.

OJAI QUADRANGLE – This quadrangle, in southern Ventura County, includes the eastern portion of the City of Ojai and the unincorporated community of Summit near the eastern boundary. Residential development is concentrated in the valley areas with scattered development in the canyons, on the hillsides, and along Sulphur Mountain Road on the crest of Sulphur Mountain.

Liquefaction hazard zones occur along Lion Creek in the Ojai Valley from Summit School west to Dennison Park, and from east of Ojai along Reeves Roard westerly along San Antonio Creek approximately to Camp Comfort Park on Creek Road. Liquefaction zones also cover about a 12-block area in northwestern Ojai. A mile-wide liquefaction zone just east of Ojai extends north from the foothills of the Black Mountains to the steep foothills north of Grand Ave. Landslide zones occur in most of the mountainous areas around the Ojai Valley.

OXNARD QUADRANGLE – This is a revised map for this quadrangle, which lies along the coastline in southwestern Ventura County. The on-land portion of the quadrangle consists of the coastal lowlands of the Oxnard Plain bordered by nearly 11 miles of beaches. Except for residential and commercial development in Oxnard and Port Hueneme, land use within the quadrangle consists mainly of farming.

A liquefaction zone covers almost the entire Oxnard Quadrangle except for the slopes north of the Santa Clara River in the northwest corner. No Zones of Required Investigation for earthquake-induced landslides exist in the quadrangle.

PITAS POINT QUADRANGLE -- The land portion of this quadrangle is mostly mountainous terrain bordering the Pacific Ocean in western Ventura County. Rincon Point, the westernmost land in the quadrangle is about 13 miles northwest of the City of Ventura along U.S. Highway 101. The small, rural communities of La Conchita, Seacliff, and Faria Beach are located along the coastline in this unincorporated area.

The liquefaction zone is restricted to the beach, the low-lying area between the beach and the base of the cliffs, and the bottom of Padre Juan Canyon and Los Sauces Creek. The zone follows the beach area from Rincon Point southeast to beyond Pitas Point. The deeply dissected mountainous terrain inland results in a landslide hazard zone that covers about 75 percent of the land area within the quadrangle.

SANTIAGO PEAK QUADRANGLE -- Most of the land in the Santiago Peak Quadrangle lies within the Cleveland National Forest. Only the southwestern quarter has been evaluated because residential development is underway in the communities of Rancho Santa Margarita, Trabuco Canyon, Coto de Caza, and Dove Canyon.

Liquefaction zones are restricted to portions of the bottoms of Arroyo Trabuco, Canada Gobernadora, Tijeras Canyon, Live Oak Canyon, Modjeska Canyon, Harding Canyon, Bell Canyon and the Oso Creek drainage area. The largest liquefaction zone runs northeasterly along Trabuco Creek through the northern low-lying parts of Rancho Santa Margarita. The mountainous areas of landslide hazards are mostly unpopulated.