Maps Showing Quake Hazards Released by State

The state Thursday released a series of Southland earthquake hazard maps in a step toward implementing mandatory safety studies for new apartment houses and commercial buildings in quake-endangered zones by the end of 1998.

State Geologist James Davis said that when the maps are made official, local governments will--under state law--have to require seismic hazards evaluations “to validate the level of hazard at the site and make appropriate recommendations for mitigation.”

The individual studies, which will cost several thousand dollars each, according to state officials, will be paid for by private developers.

The maps released at a news conference 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.

Liquefaction occurs during a powerful quake when soils with a high water table turn 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.

Davis and the new director of the State Department of Conservation, Elin D. Miller, emphasized that the maps are tentative and subject to public comment and revision before they become the official designations of the areas subject to the mandatory studies.

But during the news briefing, a leading seismologist, David Jackson of UCLA, observed that the maps seem to ignore other hazards and thus do not designate all hazardous zones.

Jackson noted that no fault zones are identified on the maps, nor are some of the areas closest to them that may be subject to the most intense shaking.

The maps released Thursday do not, in fact, show as hazardous zones all the areas that were severely damaged in the Northridge quake. Those left out include parts of Hollywood, Santa Monica and South-Central Los Angeles, including the Coliseum, which were subject to devastating damage.

Also, many parts of the northern San Fernando Valley that were most severely damaged are not in areas subject to liquefaction, so they do not show up on the maps as hazardous zones eventually requiring mandatory studies.

The liquefaction zones that are designated in the maps have expanded in some areas compared to maps released a few years ago by the state and the U.S. Geological Survey.

In the East San Fernando Valley, one map shows a liquefaction area to have expanded north several miles from the narrow band close to the Los Angeles River that had been designated several years ago.

Davis explained that the criteria for designating which areas would be subject to liquefaction have changed, accounting for some expansion.

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

A state briefing paper that was released cautioned that not all designated areas are equally hazardous.

The tentative, so-called “reconnaissance” seismic hazard maps shown Thursday “represent the earlier stage of analysis,” the briefing paper said. Later maps will be based on analysis of subsurface geotechnical data, the paper explained.

Asked why the state had released maps that might be revised substantially before becoming official, Miller said it was felt that the public deserved any information as soon as it became available.

Davis said the public should realize that areas shown to be hazardous are not zones that people should move from now, but zones where steps should be taken to make buildings safer.

Richard Andrews, director of the state Office of Emergency Services, said: “There are engineering solutions in nearly every case” to the hazards shown.

The state Legislature passed the law under which the mapping is done, and localities are required to enforce the mandatory safety studies undertaken after the Loma Prieta quake.

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Liquefaction Zones

State geologists released 16 tentative new earthquake hazard maps of parts of Los Angeles and Ventura counties, showing areas subject to liquefaction and landslides in heavy shaking. The maps, when made official by the end of 1998, will be used to designate areas where special geotechnical studies to mitigate quake effects will be required for new apartment houses and commercial properties. Here is one area where the liquefaction zone expanded considerably:

Expanded liquefaction zone

Original liquefaction zone