NEW MAPS DEPICT SOIL-FAILURE THREAT IN FUTURE QUAKE; SOUTH VALLEY FOUND SUSCEPTIBLE.

Byline: Tony Knight Daily News Staff Writer

A broad swath of the southern San Fernando Valley from Woodland Hills to Studio City is at elevated risk for soil collapse - liquefaction - in an earthquake, according to new maps released Friday by the state Department of Conservation.

The new seismic hazard maps covering most of the Valley, Santa Clarita and Calabasas show dozens of square miles where the risk of landslide or soil collapse would be increased during a quake.

The maps - part of an unprecedented effort to document seismic hazards in quake-prone areas of the state - will be used to influence building code updates and to inform prospective home buyers of hazards.

Although ground shaking remains the primary cause of damage in a typical quake, damage from secondary effects such as landslides and a type of soil collapse known as liquefaction also can be very high, said Larry Goldzband, department director.

``By identifying areas where secondary hazards could occur, we can mitigate against them by helping local communities design and build better structures,'' Goldzband said.

Liquefaction occurs when strong shaking turns silty soil on top of high groundwater into a thick mush consistency that can't support buildings. The geologists mapped areas of silty soils where the groundwater comes to within 40 feet of the surface at least once a year.

The elevated liquefaction hazard is shown covering a two- to three-mile band across the southern Valley, roughly following the Los Angeles River. The area expands northward in the West Valley into Canoga Park, West Hills and Chatsworth.

The higher risk of landslides is shown on most of the sloping land of hillsides and mountains surrounding the Valley on all sides. The landslide zones were based on steepness of slope and rock strength data.

Virtually all of the canyons and low lying lands in Canyon Country, Saugus and north Valencia areas of Santa Clarita are marked at risk for liquefaction and nearly all of the slopes are in the landslide zone.

There was very little risk of liquefaction in the Calabasas area except along Malibu Creek, but the landslide zone is shown covering most of the slopes.

The areas outlined on the maps show locations that ``could be more susceptible to secondary earthquake hazards.'' But they say some areas at risk for liquefaction and landslides could be left out, and note that a single quake is not expected to cause liquefaction or landslides across the entire zone.

In fact, almost no liquefaction damage occurred in the Valley from the Northridge Earthquake, despite heavy damage from strong ground shaking in areas of Studio City and Sherman Oaks that are near the L.A. River.

``This type of map should not be interpreted as predicting mass, widespread destruction within the zone,'' said Ted Smith, a state geologist on the mapping project.

Los Angeles response

The maps were released Friday for a six-month review period after which they will become the official state maps of liquefaction and landslide seismic hazards.

Under state law, the maps will have two chief impacts on property owners:

Cities and counties must use them to establish codes for new construction in those areas.

Owners of existing homes in the zones now must disclose that fact to prospective buyers.

Although state law requires cities and counties to use the maps to establish new codes, it doesn't specify how the local jurisdictions must implement the program.

Los Angeles officials said they would not be in favor of imposing new code requirements across every square foot of hazard zone because the maps were too general.

``It's our opinion at this point that the maps are not as specific as we'd like them to be,'' said Dave Keim, spokesman for the L.A. Department of Building and Safety.

``We are reluctant to adopt new standards over such broad areas just based on these maps,'' Keim said. ``So we will probably require each builder in the zone to do additional soils analysis on his property and impose new standards on a case-by-case basis.''

New standards could add from $3,000 to $4,000 to the cost of a new home built in one of the zones, said officials with the Department of Conservation.

But a representative of the Building Industry Association said he thought the costs would be higher.

``I don't know where they came up with that figure,'' said BIA spokesman Charles Gale. ``What these maps set in effect is new building standards that local government will have to consider.''

Property values

It was not clear whether the maps would drive down the value of existing homes because of the disclosure requirement.

``You're required to disclose that fact only to the buyer,'' said Llewellyn Chin, senior council for the California Association of Realtors. ``You're not required to explain what it means.''

Maps showing the location of earthquake faults have been around for years with the same disclosure requirements, Chin noted.

``So far, property values haven't gone down because of that,'' he said.

State geologists are mapping liquefaction and landslide hazards in quake-prone areas throughout California under state legislation passed after the Loma Prieta Earthquake in the Bay Area in 1989.

The Seismic Hazards Mapping Act of 1990 was the result largely of liquefaction damage suffered by many homes in San Francisco's Marina District in the Loma Prieta quake.

These new maps are considered an adjunct to maps of fault zone's required under the Alquist-Priolo Earthquake Fault Zoning Act that was passed after the 1971 San Fernando Earthquake. Those maps show areas considered at risk for strong ground shaking, the primary risk in an earthquake.

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Map: NEW HAZARD MAPS