Good News About Quakes For Southern California

December 10, 1997

The Los Angeles basin's sediments seem to moderate the type of ground motion that threatens single-story and low- rise buildings in a severe earthquake, a new study of data from the 1994 magnitude-6.7 Northridge quake has revealed.

University of Southern California seismologist Edward H. "Ned" Field, Ph.D., calls his team's research, which will be published in the Nov. 11 issue of the journal Nature, "good news" for Southern California.

Geologically, the Los Angeles basin is a valley filled with debris (sediments) that eroded from neighboring mountains over hundreds of thousands of years, Dr. Field notes.

For more than a century, scientists have known that such sediments usually amplify ground motion in earthquakes. But seismologists and engineers disagree as to whether the degree of amplification will change as the level of shaking increases.

Do all sediment-filled valleys shake like a bowlful of jelly in larger earthquakes, as they do during smaller quakes' Or do some behave like a bowlful of sand in which seismic energy is "absorbed" as the grains rub together and effectively reduce ground motion? (The jelly-like reaction of soils is called "elastic" behavior, while the latter is dubbed "nonelastic" or "nonlinear" behavior.)

Based on laboratory studies of sediments, engineers have argued for the bowl-of-sand theory and have designed structures on the assumption that amplification factors go down as the level of shaking increases -- that is, the shaking effects of a stronger earthquake aren't boosted as much as those of a smaller quake.

On the other hand, seismologists have traditionally argued for the bowl-of-jelly model. They have seen little evidence that sediment amplification is reduced, especially when the soil is of the stiff, dry variety found in the Los Angeles basin. They have therefore been concerned that some engineering designs may fail to account for the degree of seismic hazard that sediments actually pose.

The new study provides the first evidence based on large-scale measurements that the answer is closer to the engineering view than seismologists had thought. Sediment amplification in the Los Angeles basin would be significantly reduced during large earthquakes by the nonelasticity of the local soil, the researchers found.

Although the paper takes a step toward settling the debate, the researchers believe more work is needed to assess whether current engineering practices adequately reflect the degree of seismic hazard posed by the local sediments.

"This result will help us refine methodologies to assess seismic hazard," says Field, a research assistant professor of earth sciences in the USC College of Letters Arts and Sciences.

Collaborating with Field on the paper, titled "Nonlinear Ground-Motion Amplification by Sediments during the 1994 Northridge Earthquake," were researchers Paul Johnson of the Los Alamos National Laboratory; Igor A. Beresnev of Carlton University, Ottawa, Canada; and Yuehua Zeng of the University of Nevada, Reno.

The research was supported by the Los Alamos National Laboratory. The National Science Foundation provided additional funding through the USC-headquartered Southern California Earthquake Center.

-USC-





EDITOR: Dr. Field will be attending the meeting of the American Geophysical Union in San Francisco during the week of publication. Call him at the Westin St. Francis Hotel at (415) 397-7000.

University of Southern California

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