The earthquake that struck Mexico on Tuesday, killing at least 225 people, owes its deadliness to its origin in the center of the country rather than its overall power.
Mexico is one of the most seismically active nations in the world, perched atop three clashing pieces of the earth’s crust. It was struck with two deadly quakes this month, including a magnitude 8.1 one that hit the southwest coast on September 8 and killed at least 90 people. (In local time, the earthquake struck a few minutes before midnight on Sept. 7.) Tuesday’s magnitude 7.1 quake struck about 76 miles southeast of Mexico City, according to the US Geological Survey. It produced strong motions felt by more than 12 million people, and noticeably swayed buildings in Mexico’s capital city, some of which collapsed.
The danger was no surprise to seismologists.
“Everyone in the earthquake business knows that Mexico City is built on pudding,” seismologist Max Wyss of the International Centre for Earth Simulation in Switzerland told BuzzFeed News. “It is uniquely vulnerable to earthquakes.”
When the Spanish conquistadors arrived in the Valley of Mexico in 1521, they found Tenochtitlan, the seat of the Aztec empire, a city built upon a lake. They rebuilt the city as Mexico City, draining and filling in the lake over the centuries. The city today rests on that soft lakebed atop a layer of clay, all of it cupped underneath by hard volcanic rock.
That peculiar geology explains Mexico City’s susceptibility to earthquakes such as Tuesday’s, which fell on the anniversary of a magnitude 8.0 quake in 1985 that lasted for three minutes and killed about 10,000 people in the city.
When earthquakes start, they unleash seismic waves that travel quickly through hard rock, but slow and spread out in soft clay like what Mexico city sits on. The soft ground magnifies shaking like a bowl of jello getting hit with a mallet: The bowl doesn’t move much, but the jello bounces a lot.
As well, many of the buildings in Mexico City have structural columns that resonate at the same frequency as these seismic waves, Wyss said. This matching of frequencies amplifies the shaking in vulnerable buildings, both steel and masonry ones. Many of the city’s buildings are older masonry structures that can collapse heavily on people, even if they have taken cover under tables or doorways.
“If a hut’s roof falls on you, you are injured. But if a stone roof collapses, you are killed,” Wyss said. The majority of the buildings that collapsed in the 1985 quake were 10 stories or less, built out of flat concrete slabs.
Monday’s quake was not particularly powerful, as Mexican earthquakes go, but its placement so close to the vulnerable city is what made it deadly. In terms of shaking the month’s other deadly quake was roughly 10 times stronger.
Earthquakes strike much more often along the Pacific Coast, part of the “Ring of Fire” encircling the Pacific Ocean, where the ocean floor plunges beneath North America. Its fits and starts on this dive into the earth are what trigger quakes and fuel volcanoes. The intrusion of a separate part of ocean crust, the Cocos Plate, into the mix from farther south adds to the seismic volatility of the region.
Building codes in Mexico have improved since the 1985 quake, but are still an area of concern, he added. The key statistic to watch is the number of people injured against those killed, since improved building codes lead to a higher injury-to-death ratio, said Wyss. “Of course, the poor often are living in buildings that are more vulnerable, so even with better building codes they can face a lot more danger.”
The larger magnitude 8.1 earthquake that struck southern Mexico on Sept. 8 struck much farther south. USGS seismologists told BuzzFeed News that the two deadly quakes this month appear unconnected seismically.
Zahra Hirji contributed reporting to this post.