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GEOLOGICALLY, THE LOS ANGELES basin is a valley filled with debris – or sediments – that eroded from neighboring mountains over hundreds of thousands of years.
For more than a century, scientists have known that such sediments usually amplify ground motion in earthquakes. But engineers and seismologists disagree about 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 “non-elastic” 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.
Now, a new study of data from the 1994 magnitude-6.7 Northridge quake 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 non-elasticity of the local soil, USC seismologist Edward H. “Ned” Field and a team of researchers reported in the Nov. 11 issue of the journal Nature. This, according to Field, is “good news” for Southern California, although he adds that more work is needed to assess whether current engineering practices adequately reflect the degree of seismic hazard posed by the local sediments.
The research was funded by the Los Alamos National Laboratory. The National Science Foundation provided additional funding through the USC-headquartered Southern California Earthquake Center.
Stress Control
In a Jan. 30 Los Angeles Times article about coping at work while under great personal stress – the kind President Clinton may have been facing at the time – psychologist Jerald Jellison commented that evolution has prepared us to focus intently under stress by blocking out distractions. “If you have a simple picture of the world, you can take action,” he said. “When you see the complexities, there’s uncertainty, and the uncertainty can inhibit action.”
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