Crustal structure and last 15 years instrumental seismicity distribution in the MarzanoIrpinia area

The crustal structure in the area including the Picentini, Marzano and Alburni mountain ranges and the Irpinia hills (Campania-Lucania Apennines) was reconstructed using largely unpublished seismic reflection profiles (commercial lines and a re-processed segment of the CROP-04 deep crustal line), exploratory well logs and geologic-structural relations among the lithostratigraphic units. This area is one of the most seismically active in Italy and hosted destructive historical earthquakes. Several NW-SE striking normal faults have been considered active in this area. The best-constrained seismogenic structure is the Irpinia fault, which slipped during the Mw = 6.9, 1980 earthquake. The main rupture nucleated between 12-13 km depth and caused ~40 km long discontinuous surface faulting across the Marzano massif and in the NE part of the Picentini Mountains. We used a dataset of seismicity recorded in the Irpinia region complied by merging different sources, including the Bollettino Sismico Italiano-INGV (time range: 2009-2021) and the ISNet Network from 2007 to 2021. A portion of the BSI catalog events occurring between 2009 and 2014 were used as master events for the detection of small-magnitude seismicity through template-matching, over the same period. This technique allowed the retrieval of that portion of seismicity that usually falls below the detectability threshold due to the low signal-to-noise ratio. For all catalogs, earthquake locations have been determined with a common 1D average velocity model (mostly based on commercial sonic logs) and using the probabilistic, non-linear code NonLinLoc. The epicentral distribution of the final 2083 selected events (magnitude range between -0.3 and 3.7) on the depth converted seismic profiles shows that most of the 2007-2021 seismicity falls within a crustal volume broadly coincident with the hanging wall of the Irpinia fault and limited to the northeast by its antithetic fault. The deeper events could follow a curved projection of the Irpinia fault at depths between 8-13 km b.s.l. Few shallow events appear to follow the basal thrust of the Apenninic platform on the Lagonegro-Molise basin unit or are hosted within the latter unit between 3-5 km depth. Most of the events are host within the Apulian platform unit, where they tend to cluster downward. Based on deep well log data and rheological modelling, we infer that the increase in number and the clustering of event hypocentres beneath Mt. Marzano corresponds to the transition in the Apulia unit from the carbonate-evaporite sedimentary sequence to the Verrucano-equivalent layer and the crystalline basement starting from ~10 km depth.