Why the Oaxaca earthquake made buildings sway hundreds of miles away

On the morning of June 23, at 10:29 local time, a strong earthquake gripped communities across Mexico. The magnitude 7.4 temblor rippled from just south of the coastal town of Crucecita, in the southern state of Oaxaca, and sent a jolt through the earth that caused buildings hundreds of miles away to sway like grass in the wind. An estimated half a million people felt the ground convulse under their feet, the U.S. Geological Survey reports. At least four deaths have been recorded by nightfall, and the extent of the damage is still being evaluated.

Mexico is no stranger to strong earthquakes, with multiple powerful events striking the nation in the last century. However, Oaxaca seems to be particularly prone to seismic shaking: A quarter of the country’s earthquakes have struck in that state, according to a report from Mexico’s National Seismological Service.

Memories of a massive 2017 quake are still fresh in people’s minds. That magnitude 8.2 temblor struck to the southeast of today’s earthquake, collapsed buildings and killed hundreds of people, and sparked a torrent of aftershocks. In 1787, an even bigger temblor—magnitude 8.6—rattled the same shores where today’s quake struck, creating a massive tsunami that flooded hundreds of miles of coastline. And hints of an even earlier strong quake, in 1537, also linger on land.

“The geologic scars of these events that happened in 1787 and then 1537 shows that this an area prone to large earthquakes and tsunamis,” says paleoseismologist María-Teresa Ramírez-Herrera of the National Autonomous University of Mexico.

The many strong temblors are caused in part by the area’s complex geology—a furious collection of tectonic forces that primes Oaxaca for intense seismic activity.

Restless Earth

A series of tectonic plates make up Earth’s surface, and are constantly jostling for position. Mexico rides atop the North American tectonic plate. Near its southern region, the North American plate collides with the Cocos plate, which is forced underground in what’s known as a subduction zone. This occurs because of the Cocos plate’s steady march to the northeast between 50 and 70 millimeters a year. While that advance might not seem very quick, it’s a gallop in tectonic terms, and it facilitates the rocky friction that causes quakes.

“It’s really, really rapid, which is why you get these quakes fairly regularly,” says Jamie Gurney, an independent geologist and founder of the UK Earthquake Bulletin.

The situation near Oaxaca becomes even more complex. In some subduction zones, sediments may be swept in along with the down-going plate. These sediments serve to lubricate the boundary region, helping one plate to slide against the other and limiting the buildup of stresses that causes earthquakes.

Oaxaca, however, sits near what’s known as the Tehuantepec fracture zone, where choppy crust forms a ridge that pokes up into the sediments along the seafloor. As the Cocos plate slides beneath North America, so too does this ridge. This may be causing one plate to stick against the other, increasing friction and the possibility of quakes, says paleoseismologist Ramírez-Herrera, who recently published a study suggesting just such a mechanism at work off the coast of Guerrero, Mexico. Other researchers have proposed similar mechanisms happening off New Zealand’s North Island, Gurney notes.

Much remains uncertain about this possible source of the region’s quakes, Ramírez-Herrera says. But she and her colleagues have identified subducting seamounts off the coast of Oaxaca—a tantalizing clue that there’s merit to the theory.

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The state of Oaxaca also lies just to the northwest of where the North American and Cocos plates collide with yet another crustal slab, the Caribbean plate. Tectonic maps of this region reveal a visible reminder of all this complexity: a kink in the fault as it hooks around the southern reaches of the country. With so many forces at play, Oaxaca is in the perfect position to host a plethora of powerful quakes.

Ground like Jell-O

The June 23 earthquake was felt widely throughout the area. On social media many videos showed the sickening sway of buildings throughout Mexico City, which lies some 300 miles to the northwest. The exceptional reach of the tremors is due to what lies beneath the bustling metropolis: sediment.

Mexico City stands on top of the remnants of an ancient lake that filled with sediments washed in from the surrounding terrain. These fertile soils were an attractive feature to early Aztecs, who built their capital city, Tenochtitlan, where Mexico City stands today. But the sediments are a problem when contending with a quaking planet.

When seismic waves hit solid rock, it shakes with a jolt. But once these radiating waves enter the basin under the city, they can get trapped, bouncing back and forth through the sediments. What’s more, this loose material causes the waves to slow and amplify, which intensifies the shaking felt at the surface, says Wendy Bohon, an earthquake geologist at the Incorporated Research Institutions for Seismology (IRIS). That means the people of Mexico City can feel nearly every major quake in the region, even if the epicenter is hundreds of miles away.

“Basically, Mexico City is sitting on, like, a bowl of Jell-O,” Bohon says.

The many videos of swaying buildings also highlight a particularly intriguing—and potentially deadly—feature of earthquakes, Bohon says. Just as holding down the strings of a guitar will make them vibrate at specific frequencies, buildings of varying heights sway back and forth at different time intervals, or resonances. When the length of the earthquake wave matches the building’s resonance, the quake sends the structure swaying with increased vigor, which causes maximum damage.

This effect was notable during a magnitude 8.1 earthquake in 1985, which caused intense damage throughout Mexico City despite the fact that its epicenter was more than 200 miles away. Thanks to the particular resonance of the seismic waves, the worst damage was seen in structures between eight and 20 stories tall.

The destruction from today’s event is still being assessed, but early reports suggest it is comparatively minor. Still, the danger has not completely passed. Such a hefty quake will produce a host of aftershocks; more than 140 already have rattled the region. “This is scary, but normal,” Bohon says via Twitter direct message. As aftershocks continue, people in affected areas are advised to stay away from things that can fall both inside and out, from bookshelves and lamps to power lines and building facades.

“The unpredictably of earthquakes makes them very frightening,” Bohon says. “But you can take back some control by being ready and having an earthquake plan.”