Scientists Question Seismic Threat

SAN FRANCISCO â€”  In a glimmer of seismic good news for the Los Angeles Basin, four scientists reported Thursday that, contrary to previous belief, sediments in the basin do not amplify shaking in large quakes as much as in small ones.

The results, according to at least one expert, suggest that Los Angeles may not be in quite as much danger from a huge earthquake of magnitude 7 or larger as many seismologists have suggested.

The scientists said that analysis of instrumental shaking readings from the 1994 Northridge earthquake and its aftershocks showed that enhancement of quake waves in larger shocks was less than expected. They reported their findings at the annual meeting of the American Geophysical Union here, and in an article appearing Thursday in the journal Nature.

In earthquakes, shock waves are amplified as they pass through loose sediments, as compared to when they pass through solid rock.

If amplification had been proportional between small aftershocks and the magnitude 6.7 main shock, Northridge shaking in much of the basin would have been twice what it was, and it is presumed that the same would apply in future quakes, the scientists said.

The findings apply to stiff soils, such as those found in most of the Los Angeles Basin, but not to areas subject to liquefaction under shaking, said Edward H. Field of USC, one of the authors of the study funded by the Los Alamos National Laboratory in New Mexico.

He added that low-rise buildings are principally affected by the findings. High-rises may be less affected, he said, because they are subject to so-called long-period quake waves that may act differently and were not the subject of the study.

Field noted in an interview that the findings tend to fortify views in much of the engineering community that a huge quake might not damage Los Angeles as much as many have feared.

Still, the results do not promise to end the controversy over high-rise building standards in the Los Angeles area.

Since the Northridge quake, Caltech engineering seismologist Thomas Heaton and others have contended that present building codes would not adequately protect mid- and high-rise buildings from a magnitude 7 earthquake.

Heaton, commenting Thursday on the new study, said that to say that Los Angeles Basin sediments do not amplify shaking by a large quake to the same degree as a small one still does not mean no amplification whatsoever.

“A big quake is going to be very damaging, both in the basin and on bedrock in the hills surrounding the basin,� Heaton said. “These matters are all relative.�

However, Field said, there was indication in some of the analysis that under certain circumstances basin sediments might damp shaking from a quake occurring outside the basin. But this does not go so far as to de-amplify the shaking, he said.

In a sense, he said, basin sediments might serve as a natural base isolation system to reduce the effect of the biggest earthquakes. Under base isolation, a building is protected to some extent from heavy shaking.

Joining Field in Thursday’s research report were Paul A. Johnson of Los Alamos, Igor A. Beresnev of Carleton University in Canada and Yuehua Zeng of the University of Nevada, Reno.

Field said that the most common view at the time of the Northridge quake was to picture the Los Angeles Basin as a bowl of jelly through which quake waves would reverberate back and forth, amplifying their original strength in direct proportion to the magnitude of the quake.

“So,� he said, “it has been presumed there would be a direct linear progression of amplification as quake waves passed through the basin.

“What we found was that there was a nonlinear progression. It does not build up as we expected.�

He said the researchers used information from 21 instrument sites, so they felt they had a comprehensive picture of what took place.