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The deadly earthquake that rocked Mexico City: what we know

Buildings have collapsed. And people are still trapped in the rubble.

Brian Resnick is Vox’s science and health editor, and is the co-creator of Unexplainable, Vox's podcast about unanswered questions in science. Previously, Brian was a reporter at Vox and at National Journal.

Frightening tragedy struck Mexico City Tuesday on the anniversary of a devastating 1985 earthquake that killed thousands in and around the same city.

At 2:14 pm Eastern time, the US Geological Survey reported a powerful 7.1 magnitude earthquake with an epicenter some 100 miles away from the Mexican capital, near Puebla.

At least 225 people were killed in the city and nearby states in the quake, according to the Associated Press. Among the victims: at least 21 children and four adults who were inside a school when it collapsed. Rescue efforts continued throughout Tuesday night and Wednesday, with crews often working in complete silence as to better hear victims calling out for help.

Thirty-two buildings collapsed in the city; 52 people have been rescued alive, Mexico City Mayor Miguel Angel Mancera said Wednesday. But people are still feared to be trapped inside the rubble.

The epicenter of the quake was less than 100 miles away from Mexico City.

Images and video from on the ground in Mexico City show buildings crumbling, thousands of people in the streets, and search and rescue efforts beginning in the immediate aftermath.

Some more hopeful stories have also emerged from the chaos: neighbors taking up the task of rescuing neighbors, crews working nonstop throughout the night to find victims in the rubble. There were also images of people holding their fists aloft signaling silence to listen for sounds of survivors buried in the wreckage.

On Wednesday, Mexican President Enrique Peña Nieto declared three days of national mourning for the victims. But the impact of this tragedy is likely to span weeks and months, if not years.

Even 100 miles or so removed from the epicenter, Mexico City felt 6.0 magnitude shakes.

Based on the size of the quake and its proximity to a high-density population center, the USGS automated earthquake reporting system predicted fatalities in the hundreds for this event. And costs could be in the hundreds of millions of dollars.

Why the earthquake was so damaging

Earthquakes are scored on a logarithmic scale of 1 to 10, so a magnitude 7 represents an earthquake with an amplitude 10 times greater than a magnitude 6. For every higher number on the scale, “the associated seismic energy increases by about 32 times,” the USGS explains.

But more simply: A 7.1 is a very powerful quake, capable of destroying whole buildings or ripping facades off others. Videos on social media show buildings collapsing.

“Big earthquakes are always possible in Mexico,” Susan Hough, a seismologist with the US Geological Survey, says in an email. And that’s because the country is near the boundary of three fault lines.

Fault lines are places where the Earth’s tectonic plates — the masses of crust that form the major continental regions of the planet — meet one another. As these plates push and pull against each other, they form massive areas of pressure. Occasionally the rocks in these high-pressure zones rupture or suddenly shift positions. The resulting release of energy forms a wave that propagates throughout the globe.

Tuesday’s earthquake occurred near where the Cocos Plate is pushing underneath the North American Plate (at a rate of 76 millimeters a year, according to the USGS.)

And then there’s the geography of Mexico City itself, which makes it particularly susceptible to earthquake damage. The city is built on a dried-up ancient lakebed. “These soft sedimentary clay deposits amplified the seismic waves, or they liquefied, destroying the foundation of some buildings,” the National Oceanic and Atmospheric Administration explains in a article about why the 1985 earthquake was so deadly.

“So far [Tuesday’s] quake has produced very few aftershocks,” Hough says. That makes her suspect the risk for continued damaging aftershock quakes is "relatively low."

Still, she stresses, “you never want to tell people that another earthquake won't happen -- especially people living along the Ring of Fire.” That’s the horseshoe-shaped region around the entire Pacific where tectonic plate activity makes volcanoes and earthquakes prevalent.

This quake comes just two weeks after an extremely powerful 8.2 quake struck off the Pacific Coast of Mexico, killing at least 90. Did that event cause this quake? It’s too early to say, reports volcanologist Erik Klemetti in Discover magazine:

There could be the chance that the previous earthquake added more stress to a fault that was close to rupture. However, it will take time to determine if this was actually the case.

And eerier: Tuesday’s quake comes on the anniversary of a devastating 1985 magnitude 8 event that left 10,000 people dead, 30,000 injured, and thousands homeless. Also hard to believe: Just hours earlier in the day, many buildings across the city participated in earthquake drills.

Photojournalists on the ground captured the following scenes.

People remove debris of a collapsed building looking for possible victims after a quake rattled Mexico City on September 19, 2017.
A security guard walks over debris of a collapsed building.
Hector Vivas/Getty Images