Why the Italy quake was so severe

The combination of a shallow fault and old, unreinforced masonry buildings led to widespread devastation in the earthquake that struck central Italy early Wednesday.

The magnitude-6.2 quake killed more than 100 people and trapped many more in the rubble of collapsed buildings.

Like other villages and towns in the mountainous area, Amatrice, where the mayor lamented that “half the town no longer exists,” has stone churches and other buildings that were constructed centuries ago, when little if anything was known about earthquakes. Unless they have been reinforced in recent years, such structures are easily damaged or destroyed by shaking.

“Even 100 years ago, they didn’t know how to build structures to withstand earthquakes,” said David A. Rothery, professor of planetary geosciences at the Open University in Milton Keynes, England.

The earthquake was less powerful than many recent deadly quakes. The magnitude-7.8 earthquake that struck Nepal in April 2015, for instance, killing 8,000 people, released roughly 250 times more energy.

But the Italian quake was very shallow: According to the U.S. Geological Survey, it occurred about 6 miles below the surface.

“Shallow earthquakes cause more destruction than deep earthquakes because the shallowness of the source makes the groundshaking at the surface worse,” Rothery said.

Earthquakes are set off by the movement of the Earth’s crust, which is divided into large sections called tectonic plates. The Apennine Mountains, where the quake occurred Wednesday, are in an area where one plate, the African, is moving under another, the Eurasian.