- The epicenter of the Tehuantepec quake was deeper in the tectonic plate than expected.
- Quakes in that location usually only happen in much older and cooler subduction plates.
- It's possible the U.S. West Coast is susceptible to similar quakes.
A magnitude 8.2 earthquake that struck southern Mexico on Sept. 7, 2017, not only occurred where existing earthquake modeling said it shouldn't happen, it also broke a tectonic plate, according to scientists.
The Tehuantepec quake struck off the Pacific coast of Mexico's Chiapas state, which borders Guatemala. Nearly 100 people were killed and hundreds more were injured.
Geologists initially thought the earthquake occurred where the Cocos ocean plate is being overridden by a continental plate. Megaquakes generally occur near the top of where plates converge, an area called the subduction zone.
The epicenter of the Tehuantepec quake, however, was much deeper – about 28 miles deep in the Cocos plate – than earthquake models said it should be, according to a report in the journal Nature Geoscience.
The study also showed that the Cocos plate completely split apart, National Geographic writes. A tremendous amount of energy was released in seconds.
“If you think of it as a huge slab of glass, this rupture made a big, gaping crack,” Diego Melgar, lead author of the study, told the magazine. “All indications are that it has broken through the entire width of the thing.”
Earthquakes can occur far from a plate's boundary, but they usually only happen in older and cooler subduction zones, according to a news release about the study. An example of one of these tension quakes was the 1933 Sanriku, Japan, tremor. It generated a 94-foot tsunami that killed 1,522 people and destroyed more than 7,000 homes.
"This subducting plate is still very young and warm, geologically speaking. It really shouldn't be breaking," said Melgar, who is also an assistant professor of earthquake seismology at the University of Oregon.
Melgar's team suspects seawater got into the Cocos plate and possibly sped up the cooling, making it susceptible to tension earthquakes.
If that's possible, Melgar said, other areas – including the U.S. West Coast and from Guatemala southward in Central America – are susceptible to tension-zone earthquakes.
“Our knowledge of these places where large earthquakes happen is still imperfect,” Melgar said. “We can still be surprised. We need to think more carefully when we make hazard and warning maps. We still need to do a lot of work to be able to provide people with very accurate information about what they can expect in terms of shaking and in terms of tsunami hazard.”