Quake split a tectonic plate in two, and geologists are shaken

On September 7, 2017, a magnitude 8.2 earthquake struck southern Mexico, killing dozens and injuring hundreds. While earthquakes are common enough in the region, this powerful event wasn’t any run-of-the-mill tremor.

That’s because part of the roughly 37-mile-thick tectonic plate responsible for the quake completely split apart, as revealed by a new study in Nature Geoscience. This event took place in a matter of tens of seconds, and it coincided with a gargantuan release of energy.

“If you think of it as a huge slab of glass, this rupture made a big, gaping crack,” says lead author Diego Melgar, an assistant professor of earthquake seismology at the University of Oregon. “All indications are that it has broken through the entire width of the thing.”

Such colossal fragmentation events have been observed before in a handful of places around the world, and all these epic earthquakes have one thing in common: No one really knows how they happen. This information gap matters, because huge populations from the western seaboard of the Americas to the eastern shores of Japan could be threatened by these enigmatic earthquakes.

For one thing, the deep quakes can induce strong shaking over a wide area that can level plenty of multistory buildings. One that took place beneath the Chilean town of Chillán in 1939, for example, killed at least 30,000 people. And when they happen near an ocean coastline, their destructive potential could be magnified.

“My real worry over these kinds of events is the tsunami,” Melgar says.

World’s most elusive earthquakes

Tectonic plates, also known as lithospheric slabs, are made up of the planet’s crust and the hot-but-solid upper mantle. They constantly move around Earth’s surface, either grinding side by side, crumpling up into one another and forming mountains, or descending under another plate in what is referred to as a subduction zone.

Along these various plate boundaries, you get earthquakes when friction generates stress that’s ultimately released. But quakes can also occur far from these plate boundaries, in the part of the slab that’s been pushed through a subduction zone and into the lower mantle. (Here’s what will happen when Earth’s tectonic plates grind to a halt.)

“If you bend an eraser, you can see the top half being extended and stretched, whereas the bottom bit is squashed and compressed,” Melgar notes. The same applies to these slabs. This bending can activate faults within the slab and trigger what are known as intraslab earthquakes.

To continue reading: https://www.nationalgeographic.com/science/2018/10/news-tectonic-plate-split-earthquakes-tsunamis-geology/?user.testname=none