Ferrarini, F.

New fault trace mapping and structural survey of the active faults outcropping within the epicentral area of the Campania-Lucania 1980 normal fault earthquake (Mw 6.9) are integrated with a revision of pre-existing earthquake data and with an updated interpretation of the CROP-04 near-vertical seismic profile to reconstruct the surface and depth geometry, the kinematics and stress tensor of the seismogenic fault pattern. Three main fault alignments, organized in high-angle en-echelon segments of several kilometers in length, are identified and characterized. The inner and intermediate ones, i.e. Inner Irpinia (InIF) and Irpinia Faults (IF), dip eastward; the outer Antithetic Fault (AFA) dips westward. Both the InIF and the IF strike NW-SE along the northern and central segments and rotate to W-E along the southern segments for at least 16 km. We provide evidence of surface coseismic faulting (up to 1 m) not recognized before along the E-W segments and document coseismic ruptures with maximum vertical displacement up to ∼1 m where already surveyed from other investigators 40 years ago. Fault/slip data from surface data and a new compilation of focal mechanisms (1980 – 2018) were used for strain and stress analyses to show a coherent NNE-directed least principal stress over time and at different crustal depths, with a crustal-scale deviation from the classic SW-NE tensional direction across the Apennines of Italy. The continuation at depth of the outcropping faults is analyzed along the trace of the CROP-04 profile and with available hypocentral distributions. Integrating all information, a 3D seismotectonic model, extrapolated to the base of the seismogenic layer, is built. It outlines a graben-like structure with a southern E-W bend developed at depth shallower than 10–12 km, at the hanging wall of an extensional NE- to E-dipping extensional basal detachment. In our interpretation, such a configuration implies a control in the stress transfer during the 1980 earthquake ruptures and provides a new interpretation of the second sub-event, occurred at 20 s. Our reconstruction suggests that the latter ruptured a hanging wall NNE-dipping splay of the E-W striking main fault segment and possibly also an antithetic SSW-dipping splay, in two in-sequence episodes.

We provide a database of the coseismic geological surface effects following the Mw 6.5 Norcia earthquake that hit central Italy on 30 October 2016. This was one of the strongest seismic events to occur in Europe in the past thirty years, causing complex surface ruptures over an area of >400 km2. The database originated from the collaboration of several European teams (Open EMERGEO Working Group; about 130 researchers) coordinated by the Istituto Nazionale di Geofisica e Vulcanologia. The observations were collected by performing detailed field surveys in the epicentral region in order to describe the geometry and kinematics of surface faulting, and subsequently of landslides and other secondary coseismic effects. The resulting database consists of homogeneous georeferenced records identifying 7323 observation points, each of which contains 18 numeric and string fields of relevant information. This database will impact future earthquake studies focused on modelling of the seismic processes in active extensional settings, updating probabilistic estimates of slip distribution, and assessing the hazard of surface faulting.

We present a 1:25,000 scale map of the coseismic surface ruptures following the 30 October 2016 M-w 6.5 Norcia normal-faulting earthquake, central Italy. Detailed rupture mapping is based on almost 11,000 oblique photographs taken from helicopter flights, that has been verified and integrated with field data (>7000 measurements). Thanks to the common efforts of the Open EMERGEO Working Group (130 people, 25 research institutions and universities from Europe), we were able to document a complex surface faulting pattern with a dominant strike of N135 degrees-160 degrees (SW-dipping) and a subordinate strike of N320 degrees-345 degrees (NE-dipping) along about 28km of the active Mt. Vettore-Mt. Bove fault system. Geometric and kinematic characteristics of the rupture were observed and recorded along closely spaced, parallel or subparallel, overlapping or step-like synthetic and antithetic fault splays of the activated fault systems, comprising a total surface rupture length of approximately 46km when all ruptures were considered.

New stratigraphic and biostratigraphic data arising from the realization of the Sheet No. 405 "Campobasso" of the new Geological map of Italy (1:50.000 scale - CARG Project) allowed, for the first time in this area, to stratigraphically and cartographically define all the ranges composing the basinal Sannio Unit Auct.. Structural analysis and the chronostratigraphic redefinition of siliciclastic deposits covering the Sannio Unit and the carbonate platform successions of the Matese- Frosolone Units, indicate two main evolutionary stages in the Miocene- Pliocene structuring of this portion of the Southern Apennines. In the first stage, starting before Serravallian times, E-verging contraction affected exclusively the basinal units together with their siliciclastic cover. During the second stage, beginning after early Messinian times, NE-verging compression involved both the basinal Sannio Unit and the Matese-Frosolone Units.

We provide field data of coseismic ground deformation related to the 6 April Mw 6.3 L’Aquila normal faulting earthquake. Three narrow fracture zones were mapped: Paganica‐Colle Enzano (P‐E), Mt. Castellano‐Mt. Stabiata (C‐S) and San Gregorio (SG). These zones define 13 km of surface ruptures that strike at 130–140°. We mapped four main types of ground deformation (free faces on bedrock fault scarps, faulting along synthetic splays and fissures with or without slip) that are probably due to the near‐surface lithology of the fault walls and the amount of slip that approached the surface coseismically. The P‐E and C‐S zones are characterized by downthrow to the SW (up to 10 cm) and opening (up to 12 cm), while the SG zone is characterized only by opening. Afterslip throw rates of 0.5–0.6 mm/day were measured along the Paganica fault, where paleoseismic evidence reveals recurring paleo‐earthquakes and post‐24.8 kyr slip‐rate ≥ 0.24 mm/yr.

We compare the seismogenic role played by two crustal-scale reverse-type shear zones located at the outer border of the Apennine- Maghrebian fold-and-thrust belt system: the westward-dipping Adriatic Basal Thrust (ABT) in eastern-central Italy and the northward-dipping Sicilian Basal Thrust (SBT) in southern Sicily. The epicentral and hypocentral distributions and, whenever possible, the kinematics of a merged dataset of pre-instrumental (1600 to 1900), early instrumental (1901 to 1980) and instrumental (1981 to 2005) earthquakes with moment magnitude ≥4.5 sited within the surface areas above the ABT and the SBT depth-contour lines (0-30 km) are analysed. They are both first-order crustal-scale seismogenic structures which strongly control reverse, reverse-oblique and strike-slip seismic activity. Relatively deep (10-to-30 km) seismicity occurring in the foothills region and in main- land Sicily may be associated with brittle shearing of the middle and lower crust ABT and SBT thrusts segments, whereas the shallow seis- micity (<10 km) close to the ABT and SBT surface tip line is associated with reverse, oblique and strike-slip shearing of the two upper crust thrust segments and their frontal and lateral splays. Despite that the seismic activity within the two studied areas, and especially in Sicily, is commonly considered minor, several highly damaging earthquakes have struck both areas. In the last four centuries, there is knowledge of 37 events with magnitude >5.0 in the Marche-Coastal Adriatic region, among which are the 1741 Fabrianese (Maw 6.08) and 1799 Camerino (Maw 5.93) events, and of 10 events with magnitude >5.0 in southern and mainland Sicily, including the 1818 Catanese (Maw 6.00) and 1968 Valle del Belice (Maw 6.12) events. Based on the integration of tectonic and seismological con- straints, the boundaries of two seismogenic compressional provinces located above the ABT and the SBT are defined and the associated values of yearly energy release per unit area evaluated. Similar val- ues are obtained for the ABT (~8E+14 erg/y/km2) and for the SBT (~4E+14 erg/y/km2). The recognition of close geometric, kinematic and seismotectonic similarities between the two compressional provinces, further supported by a comparable level of long-term seis- mic activity, has evident implications both for regional tectonic reconstructions and for seismic hazard assessment purposes.

This paper proposes a new seismogenic interpretation of mainland and central-southern Sicily, based on an integrated structural–kinematic–seismological approach. Through analysis of available structural and geophysical data, the tectonic setting, the major kinematic units and the crust geometry of the Sicilian southward-verging fold-and-thrust belt system are schematized in sections and in map form. The depth-contour lines of the outermost basal thrust plane, which dates back to Plio-Pleistocene times with some evidence of Holocene activity, are reconstructed. This plane, here named Sicilian Basal Thrust (SBT), emerges along the southward convex Sciacca–Gela–Catania front and reaches the base of the crust at a depth of about 30 km beneath northern Sicily. Additionally, an important regional upper crust splay, with the same arched shape, has been identified some kilometres northward. In order to detect any possible indication of seismogenic activity linked to ongoing deformation of the SBT and its splay, we analyzed the distribution and kinematics of the instrumental seismicity in the period between 1981 and 2006. Particular attention was given to the background seismicity (Ml up to 4.6) located beneath the sedimentary cover at Mt. Etna, as it allows individuating a northward deepening seismogenic volume undergoing an average N–S compression, which corresponds well with the SBT geometry in section view. A merged dataset of all the known major historical and instrumental events (moment magnitude ≥ 4.5) that occurred above the SBT 0-to-30 km depth-contour lines from 217 B.C. to 2006 was compiled and analyzed. Based on information from instrumental data and/or on speculations on the shape and extent of the historical earthquake macroseismic fields, two major ranges of hypocentral depths have been schematically identified within the merged dataset: an upper crust range (in average b~ 10 km) and a mid-to-lower crust range. Focal mechanisms available in the literature show prevailing reverse and reverse-oblique kinematics, compatible with a nearly average N–S shortening and with some field evidence of active fold-and- thrust deformation at the SBT front and along its inner splay. Several moderate earthquakes (nearly 40 in the last 400 years with moment magnitude ranging from 4.5 to 5.5) and a few more energetic events with moment magnitude up to ~ 6.0, such as the Mineo 1624, the Belice 1968 and the Catania 1818 events, can be attributed to an ongoing activity of the SBT. In previous studies, these events had been often attributed to isolated N–S strike-slip sources located within the Hyblean-Pelagian foreland, while all the interposed areas had been considered aseismic, as well as the SBT had been considered inactive. The identification in the SBT of a unique regional-scale seismogenic structure capable to connect the active deformation and the seismic activity of the western, central and eastern areas of mainland Sicily and those of central-southern Sicily evidently has strong implications in terms of seismic hazard assessment. In fact, it allows the definition of a new homogeneous compressional seismotectonic province, which extends between the SBT surface front line and the surface projection of the 25 km SBT depth-contour line. A subdivision in two sub-provinces, a shallow one above the 0–10 km SBT segment and a deep one above the 10-to-25 km SBT segment, is also considered and the energy released of the associate earthquakes evaluated. To conclude, the proposed model is discussed in comparison with other models from the literature and some important open problems are put forwards.