美国国家公共电台 NPR Mexico City Doomed By Its Geology To More Earthquakes

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ARI SHAPIRO, HOST:

Rescuers in and around Mexico City are still digging through rubble¹, searching for survivors² of yesterday's massive earthquake. It happened at 1 p.m. local time, in the middle of the school and work day. Buildings fell across five states in the country and in the capital. At least 225 people have died.

This is the second major earthquake in Mexico in less than two weeks, and it came 32 years to the day after another deadly quake. NPR science correspondent Christopher Joyce says Mexico City sits in one of the most quake-prone areas of the world.

CHRISTOPHER JOYCE, BYLINE³: Geologically speaking, Mexico City is not built in a very good place. Just to the west, a huge slab⁴ of the earth's crust called the Cocos Plate is grinding relentlessly⁵ toward North America, but it's running into an even bigger slab, the North American Plate. So the Cocos Plate is shoving itself underneath⁶ its northern neighbor. Lots of faults lie along and near the junction⁷ of these two plates like stitches in the seam of a baseball. When the faults slip from all that continental⁸ grinding, quakes happen. This is what surrounds Mexico City. But there's another problem, as geophysicist Gavin Hayes with the U.S. Geological Survey points out. The city sits on a dry lake bed, and that's not good.

GAVIN HAYES: So you've got a lot of soft sediments⁹. And when the energy from the earthquake comes into that, those basins really kind of amplify¹⁰ that shaking like a bowl of jelly shaking around. And it just keeps on reverberating¹¹.

JOYCE: Hayes also points out that shallow quakes in this region often create earthmoving waves at a frequency that's especially damaging to 10- to 20-story buildings. These buildings vibrate in sync with those waves like a tuning¹² fork.

HAYES: A lot of buildings in Mexico City are at that kind of 10- to 20-story level, and so they're just at the right height to be quite vulnerable.

JOYCE: The two quakes this month were on the same tectonic plate but several hundred miles apart. Hayes says it's not likely the first quake triggered a second one so far away, but it's possible. Mexico City has strengthened many buildings since the 1985 quake which killed more than 5,000 people, and it has designed an early warning system that detects the first vibrations¹³ from a quake to give people an earlier warning. But like many cities around the Pacific Rim¹⁴ - San Francisco, Tokyo, Manila - it's sitting on ground that's not going to stop moving. Christopher Joyce, NPR News.