Fault rupture and aseismic creep accompanying the December 26, 2018, Mw 4.9 Fleri earthquake (Mt. Etna, Italy): factors affecting the surface faulting in a volcano-tectonic environment

On December 26, 2018 (2:19 UTC), during a volcanic eruption on the Mt. Etna eastern flank (Sicily, southern Italy), the largest instrumental earthquake ever recorded in the volcano ruptured the Fiandaca Fault, with epicenter between Fleri and Pennisi villages (hypocenter at ca. 300 m a. s. l., Mw 4.9). This was the mainshock of an earthquake swarm and it was accompanied by widespread surface faulting and extensive damage along a narrow belt near the fault trace. Few hours after the mainshock, an episodic aseismic creep event occurred along the Aci Platani Fault, a SE extension of the Fiandaca Fault, which caused several damages in the Aci Platani village. We surveyed and mapped the coseismic and aseismic ground ruptures, and collected structural data on their geometry, displacement, and fault zone fabric. We compared the mapped surface ruptures with topography, lithology, and morphology of the buried top of the sedimentary basement. We conclude that the geometry of the volcanic pile influenced the surface expression of faulting during the December 26, 2018 event. The top surface of the marly clay basement should be considered as a detachment surface for shallow sliding blocks. 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 1) understanding active faulting in the European largest onshore volcano, 2) modeling its complex dynamics, and 3) contributing to a more refined surface faulting hazard assessment at Mt. Etna. Results from this investigation might be useful for characterizing capable faulting in similar volcano-tectonic settings worldwide.