Seismotectonic setting and seismic hazard in the Etna region: constraints for the definition of the seismic input for microzonation

Abstract

This paper defines the seismotectonic and seismic hazard background used to establish the seismic input for Level 3 Seismic Microzonation (SM3) and Active and Capable Fault (ACF) studies in the Etna area. The analysis integrates over 400 years of historical macroseismic records from the Catalogo Macrosismico dei Terremoti Etnei (CMTE) with a robust instrumental dataset from the 2000–2026 period. The definition of the seismic input was based on the identification of the most impacting events for each municipality through a critical analysis of the "site seismic histories": i) regional scenario, for municipalities dominated by purely tectonic events, reference spectra were defined according to Italian seismic code NTC18 and considering  return period of 475 years; ii) volcano-tectonic scenario, for municipalities close to local seismogenic faults, a “volcanic” scenario spectrum was defined based on a representative M5 earthquake at R = 3 km, using the December 26, 2018, Fleri earthquake as a reference. The introduction of this spectrum was required because earthquake recordings in volcanic areas show distinctive spectral features, with larger amplitudes in the intermediate period range. Spectrum-compatible sets of seven accelerograms were selected using the REXELweb tool for three representative target spectra (NTC18), as well as for this additional scenario spectrum. In this way, the variability of the seismic input is better represented, as the selected accelerogram sets include recordings from both tectonic and volcano-tectonic events.