SAN FRANCISCO--Sediments amplify ground motions during an earthquake, so that structures built on soil shake harder than those perched on bedrock. However, a new study shows that for large quakes, this effect may be far smaller than seismologists had feared. The research, presented here today at the annual meeting of the American Geophysical Union and in the 11 December issue of Nature, has implications for the design of quake-resistant buildings, and represents a rare piece of "good news" for quake-prone greater Los Angeles, says lead scientist Edward Field of the University of Southern California in Los Angeles.
Structural engineers have argued for years that sediments respond differently as the size of an earthquake grows. During small and moderate tremblers, soft ground--sand, landfill, and loose soil--may magnify a quake's waves threefold or more. Lab studies, however, suggested that the magnification is actually lower at high levels of shaking. Structural engineers incorporated this nonlinear response in their building codes, but many seismologists expressed concern about extending lab simulations to the real world. The January 1994 Northridge earthquake provided Edward Field and his colleagues with the data they needed to resolve this disagreement.
Field's team scrutinized records of ground motion from 21 seismic stations for the 6.7-magnitude main shock of the Northridge quake and 184 aftershocks. During the smaller aftershocks, average shaking intensities ranged from 1.4 to 3.1 times higher at the stations on sediments versus those on rocky sites. But during the main event, ground shaking in sediment was never more than 1.9 times higher. "At low levels of ground motion, sediments behave like tiny springs," amplifying the waves, Field says. But intense shaking may disrupt the elastic forces between individual grains, which can then absorb some of the wave's energy as they rub together.
"This is a nice piece of work," says geophysicist Thomas Heaton of the California Institute of Technology in Pasadena. "The engineering community correctly recognized that the role of sediments is to decrease the levels of shaking in some cases, but not all." Indeed, both Heaton and Field caution against extending the results too broadly. The Los Angeles basin's dry, stiff soil behaves quite differently from the saturated mud around the San Francisco Bay or the clay of Mexico City. According to Field, engineers still must evaluate sites on a case-by-case basis.