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Mexico's back-to-back earthquakes, explained by a seismologist

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The quake that hit Mexico City today comes less than two weeks after a magnitude 8.1 quake struck the country

Mexico City on September 19, 2017. A powerful earthquake shook Mexico City on Tuesday, causing panic among the megalopolis' 20 million inhabitants on the 32nd anniversary of a devastating 1985 quake. The US Geological Survey put the quake's magnitude at 7
Mexico City on September 19, 2017 after a powerful quake shook the city.
Photo by ALFREDO ESTRELLA/AFP/Getty Images

The second major earthquake to strike Mexico in less than two weeks has caused catastrophic damage in the country’s capital. The magnitude 7.1 temblor started at around 1:15PM — cracking highways, collapsing buildings, and, so far, killing more than 50 people.

Less than two weeks ago on September 7th (local time), a magnitude 8.1 quake struck roughly 400 miles southeast from today’s. It’s not common to hear of such strong earthquakes happening back-to-back so close to one another, says John Bellini, a geophysicist with the US Geological Survey. “Usually you don’t have large ones in the same general region right away,” Bellini says. “But in highly [seismically] active regions of the world, it can happen.”

Mexico qualifies as highly active. The country sits at the boundary of three pieces of the Earth’s crust that fit together like a jigsaw puzzle — called tectonic plates. Today’s quake originated on a fault within the Cocos plate, which is on Mexico’s western edge. “Whether or not faults rupture depends on the kind of stress that builds up,” Bellini says. The Cocos plate scoots rapidly under the continental crust of the North American plate, which “builds up the stress and strain at a faster rate,” Bellini says. “So you’re liable to have more frequent earthquakes because of that.”

Mexico sits at the boundary of three tectonic plates: the Cocos plate, the North American plate, and the Pacific plate.
Mexico sits at the boundary of three tectonic plates: the Cocos plate, the North American plate, and the Pacific plate.
Map by the US Geological Survey

Mexico City is especially prone to severe damage because of the ground it sits on — an ancient lakebed that quivers like jello, Bellini says. When earthquake waves pass through it, it jiggles, magnifying the vibrations. “So the reason that Mexico City seems susceptible to more damage is because of this amplification effect of the lake bed,” Bellini says.

At this point, a reasonable person might wonder whether the September 8th quake might have triggered today’s. But the epicenters of the two quakes are about 400 miles apart, and it’s unusual for such a strong aftershock to appear this long after a major quake. So it’s unlikely the two are related, Bellini says. “Usually to have something directly related, they happen within minutes of each other, whereas this has been a week and a half,” Bellini says. “But I’m sure somebody will study that to see if there’s any kind of relationship.”

There’s another coincidence, too: Today’s earthquake falls on the 32nd anniversary of the 1985 Mexico quake that left roughly 10,000 people dead. But Bellini says not to read into the timing too much. There’s no such thing as an earthquake season, and earthquakes can happen at any time: “There’s no particular time of year that earthquakes happen over another. They’re not weather related.”

Unlike most natural disasters, there’s no way to predict or forecast earthquakes. That makes preparation way more important, whether it’s through building codes or earthquake drills like the one Mexico City conducted earlier today. Though it is tempting to be lulled into complacency during seismically quiet periods, planning ahead is still the only defense for earthquakes.