A damper on quakes.

• Author: Hardman, Chris
• Position: !Ojo!

PROPESSOR CINNA Lomnitz of the National University of Mexico has lived through some of the most deadly earthquakes in the Americas. When he was fourteen, he survived Chile's 1939 earthquake that measured a magnitude of 8.3 and killed twenty-eight thousand people. Years later he felt the effects of Chile's 1960 earthquake that measured a magnitude of 9.1. He has never forgotten the cries of the wounded after the 1985 earthquake in Mexico City that destroyed four hundred buildings and killed tell thousand people. Perhaps that is why he has devoted his professional life to understanding earthquakes and how to survive them. One question he set out to answer is why do some powerful earthquakes do little damage and weaker ones do great damage? For example, the 2003 Colima, Mexico, earthquake had a magnitude of 7.4 but killed eighteen people. The difference, according to Lomnitz, might be a matter of mud.

Significant parts of Mexico City are built upon ancient mud flats that have a natural "pitch" of one cycle every 2.5 seconds. All objects vibrate at a specific frequency, and unfortunately Mexico City's mud vibrates at the same frequency as certain earthquake waves. The danger increases in buildings that are ten to fifteen stories tall because they, too, have a 2.5 second period. "It's like a tuning fork," Lommitz says. "They start vibrating with the earthquake. Since they lack damping, they don't just move once. They move worse and worse as the earthquake proceeds." Eventually the buildings sway out of control and collapse. "Something like that can happen in every earthquake," he explains. "However, in Mexico City it just specifically attacks buildings with that height. For example, in the downtown area you can find hundreds of old churches. Nothing happens to them. They're not tall enough. Tiffs is true for all of the old colonial buildings."

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