Mexico City Doomed By Its Geology To More Earthquakes | KUOW News and Information

Mexico City Doomed By Its Geology To More Earthquakes

Sep 20, 2017
Originally published on September 20, 2017 9:20 pm

In Mexico City and surrounding areas, rescuers are still searching for casualties and survivors of Tuesday's earthquake. More than 200 people are believed to have died.

Geologically speaking, Mexico City is not built in a very good place.

This is the second big quake in Mexico in less than two weeks. It came 32 years to the day after another deadly quake. And there will be more in the future, though when is anyone's guess.

The reason is that just to the west, a huge slab of the Earth's crust called the Cocos Plate is grinding relentlessly toward North America. But it's running into an even bigger slab — the North American Plate. So the Cocos Plate is shoving itself underneath its northern neighbor.

Lots of faults lie along and near the junction of these two plates, like stitches in the seam of a baseball. When the faults slip from all that continental 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 made of clay, sand, silt and water. That's not good. "So you've got a lot of soft sediments," Hayes says, "and when the energy from the quake comes into that, those basins really kind of amplify that shaking, like a bowl of jelly shaking around, and it just keeps on reverberating."

Hayes also points out that shallow quakes in this region often create earth-moving waves — called surface waves — at a frequency that's especially damaging to 10- to 20-story buildings. A building will respond to or "resonate" with waves of a certain frequency depending on its height. If they are in sync, the building responds like a vibrating tuning fork and gets shaken more.

"A lot of buildings in Mexico City are at that kind of 10- to 20-story level," says Hayes, "so they are just at the right height to be vulnerable."

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 earlier this month triggered a second quake so far away, although it is 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 vibrations from a quake.

But like many cities around the Pacific Rim — San Francisco, Tokyo, Manila — it sits on ground that isn't going to stop moving.

Copyright 2017 NPR. To see more, visit http://www.npr.org/.

ARI SHAPIRO, HOST:

Rescuers in and around Mexico City are still digging through rubble, searching for survivors 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, BYLINE: Geologically speaking, Mexico City is not built in a very good place. Just to the west, a huge slab of the earth's crust called the Cocos Plate is grinding relentlessly toward North America, but it's running into an even bigger slab, the North American Plate. So the Cocos Plate is shoving itself underneath its northern neighbor. Lots of faults lie along and near the junction of these two plates like stitches in the seam of a baseball. When the faults slip from all that continental 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 sediments. And when the energy from the earthquake comes into that, those basins really kind of amplify that shaking like a bowl of jelly shaking around. And it just keeps on reverberating.

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 tuning 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 vibrations from a quake to give people an earlier warning. But like many cities around the Pacific Rim - San Francisco, Tokyo, Manila - it's sitting on ground that's not going to stop moving. Christopher Joyce, NPR News. Transcript provided by NPR, Copyright NPR.