The Apennine fault system is a belt of active normal faults running along the mountainous backbone of the Italian peninsula, concentrated in the central Apennines around Umbria, Abruzzo and Lazio. Rather than compressing, the crust here is being pulled apart at a few millimetres per year as the range extends.
These are extensional normal faults: one block drops down relative to the other along fault planes dipping into the mountains, producing shallow but damaging earthquakes. The faults are numerous and segmented, and their shallow depth means even moderate magnitudes shake hilltop towns violently.
The system has caused repeated tragedies, including the 2009 L'Aquila earthquake (magnitude 6.3) that killed over 300 people, and the 2016 Amatrice–Norcia sequence (up to magnitude 6.5) that destroyed medieval villages across central Italy. The historic 1915 Avezzano quake, magnitude about 7, killed tens of thousands in the same region.