New paleoseismological and morphotectonic investigations along the 2018 surface ruptures of the Fiandaca Fault, eastern flank of Etna volcano (Italy)

We present the first paleoseismological results along the Fiandaca Fault, source of the 26 December 2018, Mw 5.0 Fleri earthquake. This earthquake caused extensive damage and 8 km of surface faulting. We excavated two exploratory trenches close to the Fiandaca village, in the central segment of the 2018 coseismic rupture. Analysis of trench walls allows identifying, besides the 2018 event, two additional historical surface faulting events. Based on amount of displacement of dated stratigraphic units, including tephra from the 122 BCE eruption, these historical events were similar to the 2018 earthquake. The most recent one occurred in the period 1281–1926 CE, most likely during the1894 earthquake. The oldest one, previously unknown, occurred in the Early Middle Ages (757–894 CE). When compared with the available seismic catalogue for Mt. Etna volcano, this paleoseismic evidence might suggest increased seismic activity along the Fiandaca Fault in the last centuries. In order to test this hypothesis, we conducted detailed morphotectonic analyses and throw rate measurements across offset historical lava flows. In addition, we developed a trishear kinematic model that describes the fault zone and the morphological features of the scarps. Throw rates mean values show an increase from 3.3 mm/yr since the Greek-Roman period reaching 7.8 mm/yr since the Late Middle Ages. These findings highlight the needs of further investigations to evaluate the slip rates variations of other faults accommodating the flank instability. Our findings confirm that paleoseismological and morphotectonic studies are of critical value for defining surface faulting and seismic hazard in volcanic settings.