GNGTS 2021 - Atti del 39° Convegno Nazionale

GNGTS 2021 S essione 1.1 16 WNW-ESE oriented, between 7 and 18 km depth, with the strongest earthquakes occurring in the upper part of the section. Such a distribution is consistent with a right-lateral slip on a vertical plane as suggested from the focal mechanisms in Fig. 3, while the stress field in this region is generally ongoing through a NE–SW extension (Mariucci and Montone, 2020). We estimated the focal mechanisms from the P-wave first-motion polarities using the FPFIT algorithm (Reasemberg and Oppenheimer, 1985). FPFIT is a grid search routine that searches for the double-couple fault plane solution that best fits of a given set of first-motion polarities observed for an earthquake. We rejected the earthquakes with less than six polarity readings and those mechanisms with multiple solutions. Based on the analysis of the nearby well and seismic reflection profiles, the 2019-2020 swarms were confined in the Apulia Carbonate Platform’s thrust-sheets and involved the underlying Apu- lia crystalline basement. Our study shows the existence in the Benevento area of a WNW-ESE striking fault plane, or shear-zone, with right-lateral strike-slip kinematics, quite different from the NW–SE-striking active normal faults responsible for moderate to large earthquakes (up to M7) in the Southern Apennines, as the fault systems cropping out in the nearby Matese and Irpinia regions. The fault structure activated during the 2019-2020 swarms measures 6 km along strike (at least), and it, therefore, represents a potential source of M>5 damaging earthquakes. With its WNW-ESE strike, this fault may describe a transverse strike-slip structure separating NW- SE trending unknown major normal fault systems and accommodating differential extensional deformation. Fig. 2 - A) Map view of Benevento area; the 2019-2020 epicentral locations are colored according to the legend. B) Vertical (N30°E) cross sections of earthquake locations: hypocenters within ±0.5 km distance from the sections are projected. Stars show M≥3.0 earthquakes.