A powerful earthquake that killed almost 100 people in Mexico last year shouldn't have happened according to current understanding of what triggers quakes, scientists say.
The magnitude 8.2 struck in September 2017 just off the coast of the state of Chiapas, near Guatemala. Mexican President Enrique Peña Nieto called it "the biggest the country has seen in a century."
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It's now emerged the quake was so strong, it broke the 60km-thick tectonic plate completely in two in a matter of seconds.
"We don't yet have an explanation on how this was possible," said Diego Melgar of the University of Oregon, who led a study into the quake.
"We can only say that it contradicts the models that we have so far and indicates that we have to do more work to understand it."
The quake's epicentre was 46km deep in the Cocos ocean plate, which is diving under the North American plate. Quakes are known to happen in sinking plates, as they bend on their journey to the underworld.
"If you bend an eraser, you can see the top half being extended and stretched, whereas the bottom bit is squashed and compressed," Dr Melgar told National Geographic.
It's understood that stretching at the top can trigger a quake, as the plate splits apart - but never before have scientists seen a quake split an entire plate in two, right down through the compressed part at the bottom.
"These kinds of cracks, we see all over the world but we don't see them propagating all the way through the tectonic plate," Dr Melgar told IFL Science.
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The Cocos plate is also geologically very young, at only about 25 million years old, and warm. Scientists believe these types of quakes only happen when plates are old and have had time cool off and become harder, and more prone to snapping.
One theory is that seawater seeped into the plate, accelerating its cooling and making it more prone to breaking. If this is true, Dr Melgar says the entire west coast of the US and Canada is at risk.
"Our knowledge of these places where large earthquakes happen is still imperfect," Dr 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."
The study was published in journal Nature Geoscience.
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