[Editor's note: This story was updated on Sept 22 to accurately state the years between the 1985 and 2017 quakes.]

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.
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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.”
So Mexico City began preparing for the next big one. Building regulations were aimed at securing buildings of specific sizes. And seismologists began looking at the potential source of a new large earthquake, a section of a fault near the coast called the Guerrero Gap. In 1980 Shri Krishna Singh at the National Autonomous University of Mexico (U.N.A.M.) had predicted that a section of underground fractures at coast could break soon, because they had been building up unreleased strain for a long time. That section indeed broke in 1985, which bolstered the idea of nearby  vulnerabilities, the most obvious one being the long-quiet Guerrero Gap.
But Tuesday’s earthquake did not happen at the Gap or even on the coast at all. Instead, the earth broke in the interior mountains, 35 miles below the surface, not far from where a large plate of crust has sunk deep down into molten rock. Allen Husker, also a seismologist at U.N.A.M., thinks his colleagues may have been looking for threats in the wrong place. “We have a bit of a controversy about whether or not the Guerrero Gap is the earthquake that we are all waiting for,” he says.
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Husker says there are two reasons to doubt that a section of a fault is “due” to break simply because it hasn’t broken in a long time. The first reason is that what humans consider “long” is meaningless in geologic time. While it might seem like a section hasn’t moved in awhile, we rarely have the full picture to make accurate predictions. (For example, Japanese earthquake records go back to the year A.D. 1000, but only in highly populated areas.) The second is he and other seismologists, notably David Jackson at the University of California, Los Angeles, have argued that the Guerrero Gap releases its energy over long periods through slow earthquakes—nearly invisible movements that relieve stress without causing damage.
Husker also says that although people were focused on a repeat of the 1985 coastal rupture, Tuesday was not the first time earthquakes have come from the deep crust under central Mexico. About seven quakes have struck this region over the past 100 years, although none of them damaged Mexico City. In fact, some scientists now say that rather than occurring in places where quakes have not happened for a long time, breaks tend to happen near the same spot they have before. It’s a similar situation to Japan and its Tokai Gap, which was predicted for decades to rock another metropolis—Tokyo. But it was a relatively ignored section of the fault that cracked in 2011 and caused the devastating Tohoku earthquake. “We were surprised by the Tohoku event in Japan and we were surprised by this earthquake,” says Melgar. “I think a little humility is good here. There are still a lot of unknowns in the planet, and we need to work a lot more.”
But quake causes and effects are not just academic issues to people in the city’s Roma district, who watched helplessly Tuesday afternoon as neighbors tried to dig people out of collapsed buildings. “Listen! We need water bottles, clothing and hydraulic car jacks. Yes, hydraulic car jacks!” said a harried young man who was running back and forth between a fallen building and the crowd that had assembled behind police tape. People immediately looked at one another and tried to figure out how far away their cars were and if their jacks were working. Behind the man, a small team of workers could be seen on the slumped roof of the building, hammering into the collapsed concrete and bent steel to find survivors.
The building was short and squat—not a taller structure that experts would have expected to fall in a slower, swirling 1985-style quake. It is, however, vulnerable to a closer quake with faster seismic waves, which early data suggests this was. (It is also possible that the damage was caused by reverberating waves bouncing across the valley and piling up to form a powerfully-vibrating wedge where the damage was worst, and experts in Mexico are studying this now.) A man who worked in the area said the building was old and poorly built. One cannot help but wonder if structures such as this were being ignored by safety experts while taller buildings down the street were being retrofitted for the next big quake from the coast.
“This idea of the Guerrero Gap, it shouldn’t be the focus any longer for Mexico,” Husker says. “Although we should worry about any earthquake on the coast, just remember that they can happen anywhere.”
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