The earth beneath Italy's Apennine Range — where a magnitude-6.2 earthquake struck early Wednesday — is a tangle of fault lines and fractured rock.
The mountains, which run the length of Italy like the zipper on a boot, were formed about 20 million years ago as the African plate plowed into Eurasia, crumpling crust like a carpet. Now things are moving in the opposite direction. The crust on the northern side of the range is pulling away from the south at a rate of three millimeters per year, causing the earth to shudder along the spider web of minor fault lines that run beneath the surface.
That, in part, explains why Italy is so earthquake-prone, and why Wednesday's temblor was so destructive. At least 241 people were killed and dozens injured. The town of Amatrice, near the epicenter, was almost entirely reduced to rubble. Thousands of people were left homeless.
"Things are shifting around in complicated ways," said Susan Hough, a seismologist with the U.S. Geological Survey. "There's faults all along the Apennines that are fairly fragmented. They're capable of producing moderate and even large earthquakes, and it's kind of like throwing darts at a dart board — they just hit at different places over time."
Seven years ago, the target was L'Aquila, a city about 30 miles south of Amatrice. That earthquake killed more than 300. A century ago, it was Avezzano, where about 30,000 people died. Medieval Italians wrote of temblors that shook the mountain ranges and set church bells ringing as far away as Rome.
Earthquakes in this region are modest in magnitude — hundreds of 6.2 quakes happen around the world every year. Within hours of the Italian quake, a 6.8-magnitude temblor hit Burma. But that earthquake was much deeper, which means it was less destructive. According to Reuters, relatively few buildings collapsed, though three people were killed, including two children.
By contrast, quakes like those that hit L'Aquila and Amatrice were centered just below the surface.
"With deeper earthquakes, the waves have to travel farther, so we can have quite deep earthquakes that are not so damaging," Hough said. "But if it’s shallow, the energy released is quite close to the surface, so that’s an immediate punch."
Just as important as what the earth does, added seismologist Leonardo Seeber, is "what humans build on top of it."
Seeber, a research professor at the Lamont-Doherty Earth Observatory at Columbia University, was born in Florence and has studied the tectonic activity of the Apennine region for more than 35 years.
"Italy is an old country, and the houses are made of stone," he said. Closely packed medieval buildings, constructed before the emergence of things such as building codes and reinforced concrete, are vulnerable to shaking and much more dangerous when they collapse.
He compared the Italian temblor to the 2011 Virginia earthquake that shook the D.C. region exactly five years ago on Aug. 23. That quake measured a 5.8 on the Richter scale and was similarly shallow. But it happened in a more sparsely populated region, where most homes had resilient wooden frames. Not a single person died in that quake, and the property damage was relatively modest.
"It's tragic because these towns are like jewels," Seeber said of Amatrice and other hard-hit areas; they are centuries-old time capsules nestled in the mountains.
Their beauty is part of what makes them vulnerable. Italy got its gorgeous natural resources — craggy mountains, fertile soil, crystalline rivers — because of its tectonic activity. The collisions of plates and explosions of volcanoes account for some of what's best about Italy, Seeber said.
"As a seismologist, very often people ask me, 'I’m afraid of earthquakes, where should I go?'" he said. "And I tell them, 'You can go in the center of these plates, but you wouldn’t necessarily like it there."
This post has been updated to reflect the rising death toll of the quake.