Big tremors with a gentle touch
How much do sediments amplify tremors during an earthquake? For a long time, this question could only be answered theoretically or through laboratory tests. A study evaluating data from a larger tremor shows that in some areas, large tremors are less amplified than small tremors. Sediments amplify movements in the ground during an earthquake, so structures built on ground rock more than those on bedrock. However, a new study shows that in the case of large earthquakes, these effects can be far smaller than feared by seismologists. The research, presented at the American Geophysical Union's annual conference and published in Nature on Dec. 11, has implications for the design of earthquake-resistant buildings. This is some rare piece of good news for the vulnerable Los Angeles area, says Edward Field, a researcher at the University of Southern California at Los Angeles.
Civil engineers have argued for years that sediments react differently to strong earthquakes than to weak ones. During small and moderate foreshocks, soft ground-sand, landfill, and loose earth-can amplify an earthquake by up to three times or more. However, laboratory studies indicate that the stronger the shock, the lower the amplification. Engineers took this non-linear response into account in their design codes. However, many seismologists have expressed concern that laboratory simulations have simply been transferred to the real world. The Northridge earthquake of January 1994 provided Edward Field and his colleagues with the data needed to settle this controversy.
Field's team reviewed earth motion records recorded by 21 seismic stations about the Northridge earthquake, the main event of which measured 6-7 on the Richter scale, and 184 aftershocks. In the course of smaller aftershocks, the average shaking intensity for stations on sediment was 1.4 to 3.1 times that for stations on rocky ground. During the main tremor, however, the tremors in the sediment were never greater than a factor of 1.9. "When the ground shakes less, sediments act like tiny springs," Field says, amplifying the waves. Intense shocks can disrupt the elastic forces between individual grains, and this can result in some of the wave energy being absorbed as they rub against each other.
This is excellent work, says geophysicist Thomas Heaton of the California Institute of Technology in Pasadena. Engineers correctly recognized that sediments reduce tremors in some cases, but not always. In fact, Heaton and Field warn against over-generalizing the results. The dry, stiff soil of the Los Angeles Basin behaves markedly differently than the mud around San Francisco Bay or the clay of Mexico City. Fields believes engineers should study the site on a case-by-case basis.