Fault rupture during the December 26, 2018, Mw 4.9 Fleri earthquake (Mt. Etna): surface faulting in a volcano-tectonic environment

On December 26, 2018, the largest instrumental earthquake ever recorded in Mt. Etna (Sicily, southern Italy) shook the eastern flank of the volcano, with epicenter near the Fleri village along the right-lateral Fiandaca Fault (focal depth less than 1 km, Mw 4.9). The mainshock was accompanied by widespread surface faulting. We surveyed and mapped the coseismic ground ruptures and collected structural data on their orientation, displacement and fabric at surface. We compared the fault zone characteristics with near surface and deeper driving factors (topography and morphology of the buried top of sedimentary basement). The shallow geological layering underneath influenced the surface expression of faulting during the 2018 event: the top surface of the basement could be considered as a detachment surface for a shallow sliding block. The earthquake occurred on top of a depression of the sedimentary basement forcing the sliding eastward, causing at surface the re-arrangement of the fault strand pattern and deformation style, switching from shear faulting to a tensile failure. The Fleri earthquake therefore provides an unprecedented dataset for understanding 1) active faulting in the European largest onshore volcano, 2) the complex dynamics of this edifice, and 3) contributing to a more refined seismic hazard assessment.