New Maps Specify Quake Hazard Zones

State geologists released the first seismic hazard maps for Topanga Canyon and a segment of the San Fernando Valley on Tuesday under a program requiring geologic investigations and mitigation measures for new building projects in zones of ground liquefaction and landslides in big quakes.

Homeowners who sell their properties will also be required to notify buyers that they are moving into a seismically hazardous zone once the maps become official in about six months.

The preliminary maps issued Tuesday were for widely scattered parts of the state. Maps for most of the San Fernando and Santa Clarita valleys will be issued next year, and the program will not apply in those areas until they become official.

Public comment on the maps issued Tuesday will be accepted by the Division of Mines and Geology in the state Department of Conservation until Jan. 7, and the official maps will be released in March, state officials said.

Liquefaction is a process by which water-saturated soils turn to jelly under heavy shaking, jeopardizing building foundations. It was a particular cause of heavy damage in the 1985 Mexico City quake and in the Marina district of San Francisco in the 1989 Loma Prieta earthquake.

Landslides can also be very damaging in the kind of hilly areas that mark the boundaries of the Valley.

Landslides were the major cause of the destruction that ravaged 3,500 homes in the Santa Cruz Mountains in the Loma Prieta quake, and they were an important factor in some areas damaged by the 1994 Northridge quake.

The maps released Tuesday show that a preponderant portion of Topanga Canyon and the small areas mapped in the San Fernando Valley fall within what scientists believe to be landslide zones.

There are so many twists and turns in zones shown for the area, and the maps are so comparatively small, that it would take an expert in many cases to say whether a particular resident lives in such a zone and is thereby subject to the law’s requirements.

Questions about where the maps can be obtained or viewed can be directed to the Los Angeles public information office of the Conservation Department at (213) 620-3560.

Charles Real, the supervising geologist for the project, explained that liquefaction areas, not prominent in the Topanga Canyon map released Tuesday, lie where sand and silt is common and water tables come to within 40 feet of the surface.

In areas of Orange County, such as Anaheim and Newport Beach, for which maps were released, the liquefaction areas, by contrast, comprise more than half the total territory.

The geologists caution that not all areas subject to the landslide or liquefaction process may be included in the map.

They also caution, “A single earthquake capable of causing (these damages) . . . will not uniformly affect the entire area.”

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Not shown on any of the maps released Tuesday were major faults where heavy shaking could cause extensive damage in areas outside the designated liquefaction or landslide zones.

Real said seismologists and engineers have as yet been unable to agree on criteria for bringing those zones under the designation as seismic hazard zones.

He noted, however, that for the most part shaking damage is regulated under building codes, but the designation of hazardous zones is designed more to educe dangers from underlying special geological conditions.

Being designated a hazardous zone triggers the state requirement that local governments require both geotechnical investigations and mitigation measures for any commercial project or any housing project containing more than four dwelling units.

Construction of just one housing unit will not be subject to the requirement, but single units will fall under the disclosure requirements at time of sale.

Real said that a common mitigation step in an area subject to liquefaction may be to strengthen foundations. He said that adding $1,000 to $2,000 worth of steel to a foundation could save money when a quake comes.

“You don’t have to restrict development under this program,” he explained. “You just have to know what to do to make things safe.”

He acknowledged, as is often the case with state mandates, that the investigations will have to be done at local expense, and he said that in most cases the cost is likely to be passed by municipalities on to developers.

There are no penalties written into the law for municipalities that do not adhere to the new requirements, but so far most local officials have shown a willingness to cooperate, he said.

And, he added, it will be highly embarrassing for localities in the event of a serious quake if damage is caused and it can be shown to be attributable to their failure to cooperate.