PROFESSOR CINNA LOMINITZ of the National University of Mexico has lived through some of most deadly earthquakes in the Americas. When he was 14, he survived Chile’s 1939 earthquake that measured a magnitude of 8.3 and killed 28,000 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 400 buildings and killed 10,000 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 only 18 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 10 to 15 stories tall because they, too, have a 2.5 second period.
“It’s like a tuning fork,” Lomnitz 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. This is true for all of the old colonial buildings.”
Major metropolitan areas in other parts of the world — such as the San Francisco Bay Area and the waterfront district of the Japanese city of Kobe — also have been built on mud flats and lakebeds. Both of those cities suffered devastating earthquakes in the past two decades.
Professor Lomnitz suggests that buildings, like cars, be equipped with dampers to make them more earthquake-resistant.
“These dampers work exactly the way the shock absorbers do in the car,” he explains. He believes that not only should dampers be included in new buildings, but they should also be added to already-existing buildings.
According to Lomnitz, the technology is available, the process is relatively inexpensive, and even though dampers do add to the construction cost, they add to the value of the building because they help save lives.
Lomnitz points out that although a solution exists, dampers are not standard equipment in structures and their use is not yet widely recommended in building codes.
Mexico City now has one building that is equipped with dampers. At 55 stories high, the Torre Mayor is the tallest building in Latin America. It was built in 2003 and includes 96 dampers.
To educate others about the value of damping on soft ground, Lomnitz strives to be heard in the scientific community. A professor of seismology at the Institute of Geophysics, National University of Mexico, he is also a member of the Mexican Academy of Sciences, a chairman of the Committee of Earth Sciences in the National Research Council of Mexico, and a participant in UNESCO emergency field teams.
“My hope is that much more will be done about this peculiar problem to enable us to do away with earthquake risk on soft ground altogether.”