Just as a rhythmic tap can set a bell ringing, seismic pulses from an earthquake can make the land shake in deadly sympathetic motion. In 1985 a magnitude 8.1 jolt toppled 400 high-rises and killed 10,000 people in Mexico City. In contrast, last January's quake in Colima, Mexico, nearly as powerful, killed just two dozen. The difference, says seismologist Cinna Lomnitz of the National University of Mexico in Mexico City, is that the capital is built on an ancient lake bed that filtered and amplified ground waves into seismic pulses of just the right frequency to bring down skyscrapers— a "nonlinear interaction." Many other cities, including Los Angeles and Seattle, are similarly vulnerable.
All objects, from bridges and buildings to rock and soil, vibrate at an innate frequency. The muddy sediment in the Mexico City basin has an unfortunate beat period of 2.5 seconds, Lomnitz finds, identical to that of certain shallow earthquake waves. Buildings that are seven to 18 stories high also resonate on a 2.5-second cycle. The two effects reinforce each other, so ground waves can set the buildings swaying out of control. In high-risk regions, engineers can modify buildings to help protect them from these multiplicative effects— but only if owners understand the problem and are willing to invest in a solution. "All you need to do is put dampeners in the buildings, which are similar to shock absorbers in cars. We know how to make them, yet nobody is putting them in," Lomnitz says.