Thursday, October 25, 2018

Mexico’s Huge 2017 Earthquake Split A Tectonic Plate In Two, Baffling Geologists

Mexico is no stranger to seismic activity. Located smack dab in the volcanic string known as the Ring of Fire, the nation experiences an average of more than 4,200 earthquakes each year with a magnitude of 4.5 or more.

When a larger-than-normal 8.2-magnitude quake struck off the coast in September 2017, killing dozens and injuring at least 200, scientists had a hunch something was up.
“Earthquakes aren’t rare in this part of Mexico, but this particular kind of large earthquake we have never seen before,” study author Diego Melgar told IFLScience.
Earthquakes occur all over the world when two plates rub against each other, suddenly releasing a wave of energy causing the seismic waves we feel as the ground shakes. But this wasn’t necessarily the case during the Tehuantepec earthquake. It occurred at the Cocos Plate rather than at the boundary with the North American plate.
Publishing their work in Nature Geoscience, researchers used a combination of geophysical data collected from a handful of observation networks in Mexico. The country has many earthquakes, but the team wanted to know what set this one apart. “It happened because as the plate enters and starts to sink into the earth it gets bent,” explained Melgar. “Anytime you bend something it’s prone to cracking. These kinds of cracks, we see all over the world but we don’t see them propagating all the way through the tectonic plate." 
Shallower cracks that occur at the top third or top half of a tectonic plate happen all over the world, but one that spits a plate “down its entire width” is an anomaly.

“What is rare about this one is that it sort of keeps going down the entire plate, and that is something we haven’t seen before,” said Melgar.
The crack measures 160 kilometers (100 miles) long, about 97 kilometers (60 miles wide), and goes to a depth of about 26 kilometers (16 miles) into the earth. “The tectonic plate that goes into Mexico is about 60 kilometers (37 miles) wide, and this earthquake just chopped it right through.”
The location of the quake was unique as well. Because the quake occurred between the trench (the deepest part of the ocean) and the land, this particular earthquake didn’t do as much damage. Had it happened further out to sea then it’s possible it would have resulted in a very large tsunami – but whether that’s possible or not, scientists aren’t sure.
“That is the $200,000 question,” said Melgar. “Why there? What is special about this little portion of the earth that allows this to happen there? We have some theories as to why this is possible, but we can’t say for sure whether it’s impossible anywhere else in the world or whether its unique to this part in Mexico.”
Determining the likelihood of a similar earthquake occurring at other places in the world could make the difference in life or death scenarios.          
“The biggest [risk] by far is the tsunami hazards. In this case, we were very fortunate that this particular earthquake occurred at the trench,” said Melgar. “If the conditions are similar to other places, then we want to know.”
“It shows that there is still so much we don’t know even though we’ve made gigantic leaps in our scientific knowledge. “The earth works at very long timescales so oftentimes we only see things that happen every few centuries.”
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