Surface Faulting of the 26 December 2018, M(w)5 Earthquake at Mt. Etna Volcano (Italy): Geological Source Model and Implications for the Seismic Potential of the Fiandaca Fault

At Mt. Etna (Italy), volcano-tectonic earthquakes produce impressive surface faulting despite their moderate magnitude (M < 5.5), with historically well-documented ruptures featuring end-to-end lengths up to 6-7 km. The 26 December 2018, M-w 5.0 earthquake represents the strongest event of the last 70 years, with ground ruptures extending for 7.5 km along the Fiandaca fault, a partially hidden structure in the volcano's eastern flank. Field data collected by the EMERGEO Working Group (INGV) are here integrated with high-resolution photogrammetric surveys and geological-morphological observations to enable a detailed structural analysis and to reconstruct the morphotectonic process of fault growth. The deformation zone develops in a transtensional regime and shows a complex pattern, consisting of brittle structures arranged in en-echelon scale-invariant overlapping systems. Offsets and kinematics vary along the strike due to a major bend in the fault trace. We reconstructed a prevailing right-lateral displacement in the northern section of the fault and a dextral oblique slip in the southern one (max 35 cm); the dip-slip component increases southward (max 50 cm) and overall resembles the along-strike pattern of the long-term morphological throw. The kinematic analysis indicates a quasi-rigid behavior of the two fault blocks and suggests a geological model of rupture propagation that explains both the location of the seismic asperity in the northern section of the Fiandaca fault and the unclamping in the southern one. These findings are used to propose a conceptual model of the fault, representing insights for local fault-based seismic hazard assessment.