Empirical amplification factors for seismic microzonation studies in volcanic regions: the study case of Mt. Etna (Italy)

Abstract

Quantifying local seismic amplification is essential for seismic microzonation (SM) and risk mitigation. Volcanic regions are particularly challenging due to complex stratigraphy, pronounced lateral heterogeneities, and topographic effects that strongly influence seismic wave propagation. We present an empirical characterization of site amplification across the Mt. Etna region (Sicily, Italy) that integrates two independent approaches and complementary datasets. The first approach derives site‑to‑site residuals (𝛿S2S) from a non‑ergodic Ground Motion Model (GMM) using 200 volcano‑tectonic earthquakes (ML ≥ 3.0, 2008‑2023) recorded by the permanent network installed on Mount Etna and its surrounding areas. This dataset is restricted to volcano-tectonic events over a long time span to ensure the repeatability of site effects. The second approach applies the Generalized Inversion Technique (GIT) to 125 events of both tectonic and volcano-tectonic origin (2022‑2024), combining permanent with a temporary network, deployed ad hoc on the volcano, and exploiting a denser dataset over a short time window corresponding to the operation period of the temporary network. We further compare these results with single‑station spectral ratios: namely horizontal‑to‑vertical ratios from earthquakes (EHV) and from ambient noise (NHV). The two main methods (𝛿S2S and GIT) provide broadly consistent amplification functions when referenced to a common site, while EHV and NHV provide complementary constraints on resonance features. Amplification factors (AF) are evaluated over selected period ranges relevant for engineering applications and seismic microzonation purposes and used to produce regional amplification maps. Results indicate generally low‑to‑moderate amplification with respect to the adopted reference site, with most AF values below 2 and locally higher amplifications observed at some stations. The integration of different datasets, temporary and permanent seismic networks, combined with these complementary approaches, enabled a robust empirical characterization of site amplification across the Mt. Etna region and can be applied to other seismic contexts, both volcanic and crustal.