The 2016 Central Italy seismic sequence. A first look at the mainshocks, aftershocks, and source models

The 2016 central Italy seismic sequence consists so far of a series of moderate-to-large earthquakes activating within a few months along a 60-km-long and Apenninic-trending normal-fault system. Regrettably, the high vulnerability of the local infrastructure and the shallowness of the largest events (depth around 8 km) resulted in 299 casualties and more than 20,000 homeless, with great difficulties in the disaster management. The sequence evolved around its largest event (M-w 6.5, 30 October) that occurred right in the middle of a fault system already activated two months before with a first M-w 6.0 mainshock (on 24 August) located to the south near the town of Amatrice. Then, another M-w 5.9 mainshock occurred just four days before the largest mainshock (26 October) at the northernmost extent of the sequence, near the town of Visso. We analyze the space-time evolution of the first four months of seismic activity through the relocation of similar to 26; 000 earthquakes and the kinematic source models of the three mainshocks. All the main events nucleated at the base of a southwest-dipping normal-fault system segmented by the presence of crosscutting compressional structures. The presence of these inherited faults, separating diverse geological domains, appears to modulate evolution of the sequence interfering with coseismic slip distribution and fault segments interaction. Several secondary antithetic and synthetic faults are located at a shallow depth (< 4 km), both in the hanging wall and footwall. The whole normal fault system, confined within the first 8 km of the upper crust, is bounded below by a shallow east-dipping and 2-3-km-thick layer in which small events plus a series of large extensional aftershocks (approximate to M-w 4) occur, possibly decoupling the upper and lower crusts.