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The Geology Behind Italy’s Catastrophic Quake

A magnitude 6.2 earthquake shook the Apennine Mountains—and given the region’s geology, that’s no surprise

Rescuers search for survivors following the earthquake that struck central Italy early this morning. (AP Photo/Gregorio Borgia)
smithsonian.com

At least 73 people died in the mountains of central Italy this morning when a magnitude 6.2 earthquake and a series of at least 40 aftershocks shook the Umbria, Lazio and Marche regions. As affected areas in towns like Amatrice begin to dig out from beneath the rubble, they will doubtless uncover even more destruction. But why did the earthquakes strike in the first place? As Jonathan Amos reports for BBC, it’s a matter of plate tectonics: two of Earth’s plates collide at or near the site of the quake, making for a bumpy ride.

The Apennine Mountains, where the quake struck, stretch downward along nearly the entire “boot” of Italy—and they intersect with the boundary between the Eurasian and African tectonic plates near the quake’s epicenter that lies close to Norcia, Italy. As the United States Geological Survey explains, the mountain range was formed due to subduction, a process by which plates collide, pushing one beneath the other and forcing it down into Earth’s mantle. In this case, the mountain range piled up as an “accretionary wedge” as the top layer of the Adria microplate—a tiny plate wedged between the two larger masses—spilled over itself.

Though the region may have formed when Earth’s crust piled up on top of itself, now it’s being pulled apart. Amos explains that the Tyrrhenian basin, which is located beneath the western Mediterranean Sea, is opening up slowly—over a tenth of an inch each year. That spread is complicated by counter-clockwise movement in the Adriatic plate, which sits right at the boundary between the Eurasian and African plates.

“Italy is literally being pushed and pulled every way,” concludes Amos—and he’s not the only one. The USGS calls the area “tectonically and geologically complex,” which could be the understatement of the year. Since so much is going on at the plate boundary at one time, geological events like this morning’s earthquake can happen at any moment.

In this case, the USGS concludes, the earthquake happened because the Tyrrhenian basin is spreading more quickly than Eurasia and Africa are colliding. The region is no stranger to quakes—within the last 40 years, a series of catastrophic earthquakes have rocked the region, and a 1915 earthquake killed about 30,000 people.

All of that tectonic possibility can translate into chaos and death within Italy in the shift of a plate—and makes predicting earthquakes within an area known for its tightly-packed cities and historic buildings a high-stakes proposition. In 2012, a civic official and six scientists—three seismologists, two seismic engineers and a volcanologist—were accused of manslaughter for failing to accurately assess the risk of an aftershock to the deadly 2009 L’Aquila earthquake, a magnitude 6.3 shaker, which killed 309 people. The scientists were eventually acquitted, but the official was not cleared of charges.

Though six of the scientists were eventually exonerated, the uproar illustrates how vital it is that the experts get it right—and how outsized public reaction can be when they don’t. Of course, scientists can’t predict earthquakes, but they can calculate the probability of future tremors. The more they understand the complex regional tectonic forces at play, the more information they’ll have at their disposal when assessing future risk.

Update 8/24/2016: This post has been corrected to show that of the seven accused of manslaughter in the L'Aquila earthquake, only the six scientists were cleared of charges.

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