At a magnitude 6.2, the earthquake that struck central Italy this morning, killing at least 120, wasn't especially powerful. So why was it so devastating? Three factors: depth, geography, and age. Depth is pretty straightforward: The quake was a shallow one, at six miles down, which exacerbates the surface shaking. As for geology, it's complicated, report Live Science and the New York Times. The area where the quake hit is the site of a "complex interaction" of two tectonic plates, the African and Eurasian, whose movement is causing the basin of the Tyrrhenian Sea (part of the Mediterranean) to expand. That extension is causing tension in the Apennine Mountains, where the quake took place.
And then there's the age of buildings. New buildings are subjected to earthquake-related building codes that are actually "state-of-the-art in the field," an earthquake engineering expert tells Time, but retrofitting Italy's century- and even millennia-old buildings ranges from the costly to the impossible. Hard-hit Amatrice is home to many centuries-old stone buildings, and "even 100 years ago, they didn’t know how to build structures to withstand earthquakes," a professor of planetary geosciences tells the Times. And those earthquakes will keep coming. "It is expected for earthquakes of that size to occur" in that area, a seismologist tells Time. (Read more earthquake stories.)