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November 05, 1989

This map and the one on the following two pages are from earthquake scenarios drawn up by the state Division of Mines and Geology. The first, prepared in 1988, shows in general terms what might occur in various areas if there was a 7.0-magnitude quake in the Newport-Inglewood Fault Zone in Los Angeles and Orange counties. The second, drawn up in 1982, examines the possible impact of an 8.3-magnitude earthquake on the San Andreas Fault near Tejon. The maps are hypothetical and are not intended to predict that such earthquakes will actually occur. It is highly unlikely that a big earthquake, when it does come in Southern California, will conform to either of these specific scenarios. Even if it did, there would probably be variations from the intensities portrayed in these maps.

Still, the scenarios are useful for assessing the possible impact of a major temblor. Authors of the Newport-Inglewood Fault scenario describe their work as "a possible worst case for emergency planning" and, in discussing their purpose, remark, "Hopefully, increased awareness of specific damage effects will stimulate regional planning programs to cope with this eventuality."

In both scenarios, the authors were modest in assessing the reliability of their maps showing shaking intensities for specific neighborhoods expected from such temblors. A prominently displayed warning in each scenario booklets declares: "An earthquake of significantly different magnitude on this or any one of many other faults in the planning area will result in a markedly different pattern of damage."

The Newport-Inglewood scenario shows that a 7.0-magnitude earthquake centered within the Los Angeles-Orange County metropolitan area would be more destructive than an 8.3-magnitude earthquake centered on the San Andreas 40 or more miles away. The only areas excepted from the worst impact would be the Palos Verdes peninsula, the Santa Monica Mountains and portions of the Verdugo Hills and the San Gabriel Mountains. These areas would experience shaking levels slightly lower than in flatter areas.

Compared to the San Andreas, much less is known about the intervals between major earthquakes on the Newport-Inglewood Fault. The lead author of the Newport-Inglewood earthquake scenario, Tousson R. Toppozada, acknowledges that scientific convenience, rather than hard information about the likelihood of a big earthquake, was responsible for choosing that fault for the 1988 scenario.

"We already had a scenario for the San Andreas Fault, and we were looking for a fault that was more urban," Toppozada said. "The Newport-Inglewood Fault is the largest in the Los Angeles area. There are others, the Santa Monica-Hollywood-Raymond Hill fault systems, that are active, and we have no special knowledge that would bring us to fear an imminent quake on the Inglewood-Newport. But it has given us a 6 1/4 in the past, and none of the others have given us a 6 yet."

At least, Toppozada added, the scientists at the Division of Mines and Geology felt that "this (Newport-Inglewood) scenario would give the sort of impacts that would occur in the downtown (Los Angeles) area from whichever source a major earthquake on a local fault would come."

Legend for the Newport-Inglewood, showing four levels of intensity, slightest first.

Areas in white--Little or no structural damage to buildings of good design, considerable damage to poorly built or badly designed buildings. Considerable fall of plaster and window breakage.

Areas in light gray--Considerable structural damage in ordinary substantial brick buildings, partial collapse of wooden houses.

Areas in medium gray--More serious damage; fall of walls, chimneys, monuments, also factory stacks and towers.

Areas in dark gray--Most serious damage; considerable damage even in masonry structures built especially to withstand quakes. Shift of frame buildings off foundations. Occasional breakage of underground pipes.

Liquefaction zones:

Soil is subject to liquefaction. Under intense shaking, it becomes like gelatin or quicksand, often resulting in collapse of structures above or breakage of underground pipes.

Source: U.S. Geological Survey

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