State Releases Maps Showing Quake Hazard Zones in East Valley

The state Thursday released a series of maps that show an expansion in liquefaction zones signifying earthquake hazards in parts of the east San Fernando Valley, including an area from the San Diego Freeway east to Burbank.

The release of what were described as tentative maps--to be made official in the next three years--represented a step toward requiring by the end of 1998 costly safety studies for new apartment houses and commercial buildings in quake-endangered zones.

State Geologist James Davis said that when the maps become official, local governments must require seismic hazards evaluations to “validate the level of hazard at the site and make appropriate recommendations for mitigation.”

The evaluations will cost several thousand dollars and would be paid by private developers, according to state officials.

Liquefaction occurs during a powerful quake, when soil with a high water table turns jelly-like, weakening the underpinnings of buildings and other structures such as freeways. It was a serious problem in various areas during the 1989 Loma Prieta and 1994 Northridge earthquakes.

The initial mapping cost $12.5 million, 75% of which was paid by the Federal Emergency Management Agency. The aim of the program is to reduce eventual federal disaster costs by preparing buildings and residents to better withstand quakes without damage or casualties.

Davis said the zones have been expanded in part because state geologists used new criteria in making the evaluations.

Generally speaking, he said, the state will now designate areas as subject to liquefaction or landslides that it believes has at least a 10% chance of “exceeding our criteria” in a 50-year period.

For instance, in the east San Fernando Valley, the latest map shows the area said to be endangered by possible liquefaction to have expanded several miles north from the narrow band close to the Los Angeles River that had been designated by state and U.S. Geological Survey scientists.

There was much less expansion of liquefaction zones, compared with previous years, in the West Valley.

The maps released at a news conference at the Sheraton-Universal hotel showed areas subject to liquefaction or landslides in 16 areas of Los Angeles and Ventura counties, and represented the first of 38 areas to be mapped.

Davis and the new director of the state Department of Conservation, Elin D. Miller, emphasized that the maps are subject to public comment and revision before they officially designate the areas subject to the mandatory studies of new buildings.

The maps released Thursday do not show all quake-related hazards. Not so designated, for example, are parts of Hollywood, Santa Monica and South-Central Los Angeles, including the Coliseum, which were subject to devastating damage in the Northridge quake.

Many parts of the north Valley that were severely damaged are not in areas subject to liquefaction, so they do not show up in the maps as hazardous zones eventually requiring mandatory studies.

Nor are all areas marked as hazardous equally so, according to a briefing released with the maps. The tentative so-called “reconnaissance” seismic hazard maps shown Thursday “represent the earlier stage of analysis.”

The briefing continued: “Reconnaissance mapping methods are based mostly on surface geology and the resulting maps lack the confidence and detail of official zone maps, which are based on a more thorough analysis of subsurface geotechnical data.”

Davis said the public should realize that areas shown to be hazardous are not places they should leave, but zones where steps should be taken to make buildings safer.

“The aim is to reduce the level of seismic risk,” Davis said. “Without this kind of activity, it’s hard to see how we can reduce seismic risk in California.”

Richard Andrews, director of the state Office of Emergency Services, said the liquefaction hazards can be addressed by “engineering solutions in nearly every case.”

After the Loma Prieta quake, the state Legislature required the mapping and decided to mandate local safety studies after the project is completed. Under state law, localities must also incorporate the seismic hazard zone maps in future revisions of their general plans.

The city of Los Angeles has already begun requiring certain geotechnical assessments and mitigation plans for a wide array of new projects, whether or not they are regarded as being in an officially designated hazard zone, according to Bruce R. Clark, president of Leighton & Associates of Irvine. Clark said the new requirements have been accepted with little protest.

The schedule adopted by state geologists calls for the first five “preliminary” maps, subject to public comment before they become official six months later, to be released in July. These will be of east and west portions of Simi Valley, and Anaheim, Newport Beach and Topanga. Mapping will eventually cover all of Los Angeles, Orange and Ventura counties.