Surface Faulting of the 26 December 2018, M_w5 Earthquake at Mt. Etna Volcano (Italy): Geological Source Model and Implications for the Seismic Potential of the Fiandaca Fault

Abstract

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, Mw 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‐échelon 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.

Mechanically Consistent Model of the 2018 Christmas Volcano‐Tectonic Event at Etna

“…Eruptive fissure location and orientation are defined using the COSMO‐SkyMed ascending SAR amplitude image of 30 December (De Novellis et al., 2019), whereas the Pernicana and the Fiandaca faults traces are defined using the sharp displacement discontinuities visible in the Sentinel‐1 data (Figure 1). These discontinuities are in good agreement with field mapping of surface ruptures (Azzaro et al., 2022).…”

“…On the other hand, the southern segment determined is almost vertical, planar, and shallower. It accommodates a small amount of transtensive motion, consistent with field studies conducted soon after the 2018 events (Azzaro et al., 2022). Overall all the faults show a displacement consistent with the east‐southeast flank motion: when the fault is parallel to the flank motion, slip is right lateral; when it is perpendicular to the flank motion, it is transtensive.…”

“…Rather than assuming Fiandaca fault freely responds to the main intrusions, we assume stress had been accumulating, probably since the previous seismic event in 1984, and we invert for the amount of stress that was released in 2018. The Fiandaca fault is split into two segments in agreement with the two principal displacement zones identified during field mapping (Azzaro et al., 2022). For both segments, dips, bottom elevations, vertical curvatures, rakes, and shear stress drops are inverted (Table S5 in Supporting Information ), relaxing the hypothesis of a constant depth made for ΔCFF computation.…”