A new view of seismicity under Mt. Etna volcano, Italy, 2014-2023 from multi-scale high-precision earthquake relocations

Mt. Etna volcano in southern Italy, with a high rate of eruptive events and flank instability, poses a high hazard to surrounding, densely populated areas. Monitoring, hazard assessment and comprehension of Mt. Etna requires detailed knowledge of the structure and dynamics of the volcanic system across space-time scales. Here we relocate volcano-tectonic earthquakes from 2014 to 2023 at Mt. Etna with an enhanced location procedure which generates travel-time corrections to improve multi-scale precision and uses waveform similarity to improve fine-scale precision. Relative to previous studies, the relocated seismicity shows improved resolution of distinct patterns of organization and clustering at different space-time scales, and images novel features. Shallower than about 10 km depth, dense clusters dominate in the central part of the volcano, some concentrated within volumes of 0.5 km or less in extent, and mainly active for limited periods around the December 2018 and 2021-2022 eruptive episodes. These clusters likely indicate brittle fracturing due to stress changes from intrusive dykes adjacent to the clusters or from replenishment of a shallow, central magmatic reservoir preceding volcanic activity. Shallower seismicity surrounding the central clusters is composed of multi-scale, hierarchical clusters, bands and lineations of seismicity, some showing far-field response to intrusions over limited time periods and others active throughout the study periods and showing response to faulting and flank collapse in the volcanic edifice and to regional stress. Focal mechanism P- and T-axes for the shallower seismicity show clearly a previously identified radial symmetry, indicating pressurization strain sources concentrated under the summit area of the volcano. From about 10 to 25 km depth, diffuse and localized clusters of seismicity define an ~5 km or thinner, tabular surface plunging ~20° towards N30°W and characterized by NNW-SSE striking P-axes. This NNW plunging surface likely reflects thrusting below the volcanic cover due to NW oriented crustal convergence. Dense, elongated clusters of seismicity occur around 1-20 km depth, including two preceding an eruptive episode in 2021-2022 which may indicate magma movement and re-pressurization in the intermediate and deep crust.