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Why the Mexico City Earthquake Shook Up Disaster Predictions

for Scientific American

Mexico City is badly rattled. On Tuesday—32 years to the day after a giant earthquake killed as many as 10,000 people in this very place—seismologists and city dwellers got another major shock. A rupture in a fault that had not worried building planners or seismologists caused heavy damage throughout the city and took the lives of more than 200 people. The implications of this new quake may shake the foundations of how people prepare for temblors not just in Mexico but also throughout the world.

The first thing to understand about Tuesday’s quake is that Mexico City was built on a lakebed, which makes quakes’ effects quite extreme. The area’s original inhabitants, the Mexica, built their capital on an island in the middle of sprawling Lake Texcoco, attached to the shore by a network of dikes and bridges. Over the next 300 years the Spanish and then Mexican governments filled in the lake, turning the island into a sprawling metropolis of some 25 million people.

Nowhere else in the world do you have landfill on this scale in dangerous proximity to a fault zone. The basin of the city has been compared with a bowl of JELL-O, which jiggles enough to topple buildings even after the quake that triggered it has all but petered out. But the jiggling has a pattern: In 1985 in the city’s affluent Roma and La Condesa districts, buildings between eight and 13 stories tall were hit especially hard. It turns out the deep columns of mud under Roma and Condesa shake at a frequency that resonates perfectly with buildings of that height. “It vibrates like a bell. It rings at only one tone,” says Diego Melgar, a seismologist at the University of Oregon who was raised in Mexico City. “That frequency was close to the natural frequency of these eight- to 13-story buildings. And that’s what led to most of the collapses.”