Most quakes occur at shallow depths, according to the U.S. Geological Survey. Italy's quake was very shallow, originating between 2 1/2 miles (4 kilometers) and 6 miles (10 kilometers) underground, according to Italy's geological service and the USGS. Seismic waves from deep quakes have to travel farther to the surface, losing energy along the way. Shaking is more intense from quakes that hit close to the surface like setting off "a bomb directly under a city," said Susan Hough, a USGS seismologist. The Italy quake devastated three towns, home to medieval structures built before there were building codes.