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.

On 6 April 2009, an Mw 6.2 earthquake struck beneath the city of L’Aquila, central Italy. The shock created significant damage and caused more than 300 deaths in the city and environs. The event followed a seismic sequence that started at the begin- ning of the year, with its largest shock of M w 4.2 occurring on 30 March. The 6 April earthquake became infamous worldwide because seven experts, who attended a Grandi Rischi Commis- sion meeting on 31 March, were convicted of failing to properly warn the public about the possibility of the mainshock and were sentenced to six years in jail. A second trial is in process. We do not wish to further discuss this important case here (in- stead see Marzocchi, 2012 and the website http://processoaquila .wordpress.com/; last accessed June 2014); however, it illustrates the importance of providing authoritative scientific information about earthquake probabilities to the public and other users and serves as a catalyst for the scientific developments now underway in Italy

We present the first high-quality catalog of early aftershocks of the three mainshocks of the 2016 central Italy Amatrice-Visso-Norcia normal faulting sequence. We located 10,574 manually picked aftershocks with a robust probabilistic, non-linear method achieving a significant improvement in the solution accuracy and magnitude completeness with respect to previous studies. Aftershock distribution and relocated mainshocks give insight into the complex architecture of major causative and subsidiary faults, thus providing crucial constraints on multi-segment rupture models. We document reactivation and kinematic inversion of a WNW-dipping listric structure, referable to the inherited Mts Sibillini Thrust (MST) that controlled segmentation of the causative normal faults. Spatial partitioning of aftershocks evidences that the MST lateral ramp had a dual control on rupture propagation, behaving as a barrier for the Amatrice and Visso mainshocks, and later as an asperity for the Norcia mainshock. We hypothesize that the Visso mainshock re-activated also the deep part of an optimally oriented preexisting thrust. Aftershock patterns reveal that the Amatrice Mw5.4 aftershock and the Norcia mainshock ruptured two distinct antithetic faults 3–4 km apart. Therefore, our results suggest to consider both the MST cross structure and the subsidiary antithetic fault in the finite-fault source modelling of the Norcia earthquake.

We investigate the temporal evolution of background seismicity rate in the Umbria-Marche sector of the northern Apennines that was struck by the 1997-98 Colfiorito seismic sequence. Specifically we apply the ETAS model to separate the background seismicity rate from the coseismic triggered rate of earthquake production. Analyzed data are extracted from the CSI1.1 catalog of Italian seismicity (1981-2002), which contains for the study area 12.163 events with ML > 1.5. The capability of the ETAS model to match the observed seismicity rate is tested by analyzing the model residuals and by applying two non-parametric statistical tests (the RUNS and the Kolmogorov-Smirnov tests) to verify the fit of residuals to Poisson hypothesis. We first apply the ETAS model to the seismicity occurred in the study area during the whole period covered by the CSI1.1 catalog. Our results show that the ETAS model does not explain the temporal evolution of seismicity in a time interval defined by change points identified from time-evolution of residuals and encompassing the Colfiorito seismic sequence. We therefore restrict our analysis to this period and analyze only those events belonging to the 1997-1998 seismic sequence. We again obtain the inadequacy of a stationary ETAS model with constant background rate to reproduce the temporal pattern of observed seismicity. We verify that the failure of ETAS model to fit the observed data is caused by the increase of the background seismicity rate associated with the repeated Colfiorito main shocks. We interpret the inferred increase of background rate as a consequence of the perturbation to the coseismic stress field caused by fluid flow and/or pore pressure relaxation. In particular we show that the transient perturbation caused by poroelastic relaxation can explain the temporal increase of background rate that therefore represents a fluid signal in the seismicity pattern