SACRAMENTO – The state today released five revised preliminary maps that identify parts of Los Angeles County – including the cities of Lancaster and Palmdale -- that must require new construction projects to take steps to minimize the potentially devastating affects of large earthquakes. The maps cover the Alpine Butte, Del Sur, Lancaster East, Lancaster West and Rosamond quadrangles in the Antelope Valley.
The Department of Conservation’s California Geological Survey produced these Seismic Hazard Zone maps, which improve public safety and impact planners, developers and real estate transactions.
The maps define zones where there is evidence that liquefaction or landslides are more likely to take place during damaging earthquakes, generally those with a magnitude of 5.5 or greater. Liquefaction occurs when water-saturated sandy soil is shaken and, much like quicksand, temporarily cannot support buildings or other heavy structures. Liquefied soil can cause the ground to crack and move, resulting in damage to structures, buried pipelines and utilities.
If property is located in a Zone of Required Investigation, 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.
“Knowing where these hazards are improves public safety,” DOC Director Darryl Young said. “Once you define the problem, you can take steps to minimize the danger.”
In many cases, it is cost effective to retrofit houses and buildings to minimize the effects of severe shaking, which causes most of the damage in big earthquakes.
Local public libraries have a number of publications that can be used as guides to make homes more earthquake-ready.
It is generally more efficient to build in safety features to minimize damage from liquefaction and landslides at the design stage of construction than to retrofit an existing building. Thus, design changes to better protect life and property during future earthquakes -- such as deep foundations in liquefaction zones and slope stabilization in landslide zones – are required before new developments are approved and built.
“It’s better for public safety, as well as easier and less expensive for builders, to incorporate design changes and make buildings more resistant to these hazards in the construction phase than to rebuild after liquefaction or landslide damage,” said Michael Reichle, Acting State Geologist for California.
The new preliminary Seismic Hazard Zone maps were originally released in February and April of 2003. Those maps showed extensive potential liquefaction zones and small areas prone to earthquake-induced landslides in the Lancaster area based on information about the area’s historic water table. Most of the potential liquefaction maps up to that point covered areas where groundwater recharge is periodically substantial. The City of Lancaster pointed out, however, that the groundwater conditions in the high desert are different – low recharge and high withdrawal -- prompting CGS to adjust its mapping parameters for the area.
“The City of Lancaster is pleased to be a part of the work that was performed by the Department of Conservation in establishing mapping to identify areas of potential liquefaction within our community,” said Dennis Davenport, acting city manager. “The preliminary maps that we reviewed over a year and a half ago created some concerns. New data and information was used in developing the new map, which appears to be more accurate in reflecting our expectations of conditions within our city limits.”
In the revised maps, five separate areas in the vicinity of Lancaster are delineated as Zones of Required Investigation for liquefaction. They are:
♦ A 6.5-mile long by up to 4,000-foot wide swath of low ground that originates in the area of the Los Angeles County Penitentiary and continues east into Lancaster. Storm-generated surface water collects within this wash and is funneled to Amargosa Creek, which drains towards Rosamond Lake.
♦ The channel of Amargosa Creek extending north from the San Gabriel Mountains through the City of Lancaster. Water seeping from the natural and artificial channels into the subsurface creates potential for liquefaction.
♦ The sewage treatment facility situated at State Route 14 and Avenue D, where water could saturate the subsurface in the immediate vicinity of the ponds.
♦ The water reclamation ponds at the county park situated at the east end of Fox Air Field. Pond seepage into the subsurface creates potential for liquefaction.
♦ The channel of Little Rock Creek. Storm-generated water seeping from the channel into the subsurface creates potential for liquefaction.
Three of the maps depict potential for earthquake-induced landslides.
The preliminary maps will become official after a six-month public review period.
DOC/California Geological Survey geologists use computer models as well as analyses of existing geological mapping and hundreds of engineering borings to produce the maps, which are drawn on a scale where one inch equals 2,000 feet.
The maps also can be viewed and downloaded on the Web here.
In addition to studying and mapping earthquakes and other geologic phenomena, the Department of Conservation regulates oil, gas and geothermal wells; ensures reclamation of land used for mining; administers agricultural and open-space land conservation programs; and promotes beverage container recycling.