Italy Quakes: What Makes an Earthquake an Aftershock?

A magnitude-6.6 earthquake that rocked central Italy on Sunday (Oct. 30) was not only the strongest earthquake to strike the region in 36 years, but it was also the latest in a series of powerful tremors. Following the earthquake, Italy's geophysics and volcanology institution Istituto Nazionale di Geofisica e Vulcanologia (INGV) reported 560 post-quake tremors, which are typically referred to as aftershocks. While most aftershocks are minor, some can be as strong as the earthquake that preceded