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 ∼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 (≈M_w 4) occur, possibly decoupling the upper and lower crusts.

A M_W 6.3 earthquake struck on April 6, 2009 the Abruzzi region (central Italy) producing vast damage in the L'Aquila town and surroundings. In this paper we present the location and geometry of the fault system as obtained by the analysis of main shock and aftershocks recorded by permanent and temporary networks. The distribution of aftershocks, 712 selected events with M_L ≥ 2.3 and 20 with M_L ≥ 4.0, defines a complex, 40 km long, NW trending extensional structure. The main shock fault segment extends for 15–18 km and dips at 45° to the SW, between 10 and 2 km depth. The extent of aftershocks coincides with the surface trace of the Paganica fault, a poorly known normal fault that, after the event, has been quoted to accommodate the extension of the area. We observe a migration of seismicity to the north on an echelon fault that can rupture in future large earthquakes.

We studied the anatomy of the fault system where the 2009 L'Aquila earthquake (M_W 6.1) nucleated by means of ~64 k high‐precision earthquake locations spanning 1 year. Data were analyzed by combining an automatic picking procedure for P and S waves, together with cross‐correlation and double‐difference location methods reaching a completeness magnitude for the catalogue equal to 0.7 including 425 clusters of similar earthquakes. The fault system is composed by two major faults: the high‐angle L'Aquila fault and the listric Campotosto fault, both located in the first 10 km of the upper crust. We detect an extraordinary degree of detail in the anatomy of the single fault segments resembling the degree of complexity observed by field geologists on fault outcrops. We observe multiple antithetic and synthetic fault segments tens of meters long in both the hanging wall and footwall along with bends and cross fault intersections along the main fault and fault splays. The width of the L'Aquila fault zone varies along strike from 0.3 km where the fault exhibits the simplest geometry and experienced peaks in the slip distribution, up to 1.5 km at the fault tips with an increase in the geometrical complexity. These characteristics, similar to damage zone properties of natural faults, underline the key role of aftershocks in fault growth and co‐seismic rupture propagation processes. Additionally, we interpret the persistent nucleation of similar events at the seismicity cutoff depth as the presence of a rheological (i.e., creeping) discontinuity explaining how normal faults detach at depth.

The characterization of fault zone structure and its evolution is essential for understanding earthquake mechanics and rupture evolution. Most of our knowledge about fault structure is derived from field studies of ancient faults. By using earthquake locations, we reconstruct the complex structure of a normal fault at a resolution directly comparable with field geological investigations. At the surface conjugate sets of faults are connected with the main fault plane, which with depth shows bending and dilational jogs. Parallel slipping planes occur at the base of the seismogenic volume, and minor synthetic and antithetic structures are widespread. Fault zone thickness ranges from 0.5 to 1.5 km, while the damage density decays exponentially away from the fault plane, with values comparable to those observed on fault outcrops. The strong similarities between seismological and geological images of fault structure indicate that earthquakes have a key role in the evolution of fault architecture

Summary. The NW-SE trending Val d’Agri extensional basin is one of the regions in Italy with the highest seismogenic potential. Field data do not univocally de