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Sediment Study 'Good News' for Quake-Prone Southern CaliforniaThe Los Angeles basin's sediments seem to lessen the ground motion that threatens single-story and low-rise buildings in a severe earthquake, a new study of data from the 1994 Northridge quake has revealed. The study was conducted through the National Science Foundation (NSF)'s Southern California Earthquake Center. University of Southern California seismologist Edward H. Field says that his team's research, which is published in this week's issue of the journal Nature, is "good news" for Southern California. "This work is an important step in our efforts to mitigate the damage caused by earthquakes," says James Whitcomb, program director in NSF's division of earth sciences, which funds SCEC. "Now, we must categorize the geological conditions and building types for which this phenomenon holds true. That's the cutting edge of future earthquake research." Geologically, the Los Angeles basin is a valley filled with debris (sediments) that eroded from neighboring mountains over hundreds of thousands of years, 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 bowl of jelly in larger earthquakes, as they do during smaller quakes? Or do some behave like a bowl of sand in which seismic energy is "absorbed" as the grains rub together, effectively reducing ground motion? 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, that the shaking effects of a stronger earthquake aren't boosted as much as those of a smaller quake. Seismologists, though, 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, the researchers found. Although the Nature 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.
Editors: Field will attend the meeting of the American Geophysical Union in San Francisco during the week of publication. He may be reached at the Westin St. Francis Hotel at (415) 397-7000.
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