Unveiling Etna volcano flank dynamics from new paleoseismological findings along the Fiandaca Fault

Abstract. We present the first paleoseismological results along the Fiandaca Fault, the source of the 26 December 2018, Mw 4.9 Fleri earthquake. We excavated two exploratory trenches along the coseismic surface ruptures at the Collegio Fiandaca site. Analysis of trench walls allow identifying, besides the 2018 event, two historical surface faulting events. The youngest one occurred in the period 1281–1926 CE, and most likely during the1894 Fiandaca earthquake. The oldest one, previously unknown, occurred in the Early Middle Ages (757–894 CE). This paleoseismic evidence strongly suggest increased seismic activity along the Fiandaca Fault in the last centuries. In order to verify this hypothesis, we conducted detailed morphotectonic analyses and throw rate measurements along the Fiandaca and other capable normal faults in the Mt. Etna eastern flank. Throw rates mean values show an increase from 1.4 mm/yr during the 15–3.9 ka time interval to 3.4 mm/yr between 3.9 ka and the Greek-Roman period, with a further increase since the late Middle Ages, reaching 10 mm/yr. This trend suggests a very recent growth in flank instability, in agreement with current geodetic data but also with historical eruptive activity. These findings highlight an increase of the associated geological hazards along the inhabited eastern flank, emphasizing the need for further research and a multi-hazard approach to risk assessment and land planning for Mt. Etna and similar volcanic regions.