3D seismic velocity models from local earthquake tomography furnish new insights on the Mount Etna volcano (Southern Italy)

We present a new seismotomography investigation providing a 3-D overall model of Vp, Vs and Vp/ Vs for Mt. Etna, the largest and most active volcano in Europe. We estimated and jointly evaluated P-and S-wave velocity patterns together with the Vp/Vs ratio, particularly useful to discriminate the presence of groundwater, gas, and melts and thus very precious for volcano investigations. We applied the LOTOS software to ~ 4600 crustal earthquakes that occurred in the Etnean area during the last 26 years, the longest time-interval ever analysed for Mt. Etna. This wide dataset has allowed us to characterize the volcano velocity structure getting over possible singularities due to specific eruptive phases. Our results further refined the high velocity body widely recognized in the southeastern sector of Mt. Etna by furnishing new clues on the possible former magma pathways. Moreover, the obtained 3D seismic velocity model depicted new anomalies revealing the presence of: (i) two shallow underground aquifers in the northern Etnean sector; (ii) a volume of strongly fractured rocks filled of fluids along the eastern flank; (iii) a quite deep region of probable fluid accumulation apparently not linked to the volcanic activity in the western sector. Seismic tomography based on arrival times of the P-and S-waves from local earthquakes is a powerful tool actively used for studying volcanic systems. For several volcanoes around the world, tomography allowed to successfully reconstruct the shallow-depth volcanic structure (see e.g., Refs. 1-4). Tomographic analyses have furnished, in particular, accurate pictures of the feeding systems and very precious constraints for modelling the volcanic processes also highlighting that each volcano has some peculiar features that makes it unique. In the last decades, tomographic studies have strongly benefited from the strengthening of seismic networks and computational progresses that, particularly for volcanic regions, allowed to carefully reconstruct 3D velocity models by furnishing P-and S-wave velocities and the Vp/Vs ratio, a key parameter to discriminate the presence of groundwater, gas, and melts (Refs. 5-7 , among others). On these grounds, in the present study we collected data from more than 4600 earthquakes recorded between 1997 and 2022 in order to perform a new tomographic inversion of Mt. Etna (South Italy) by applying the software LOTOS 8. Mt. Etna is the largest and most active volcano in Europe 9. It is a composite strato-volcano rising 3300 m above the sea level in eastern Sicily (Fig. 1). Mt. Etna is located at the intersection between several major structural units, where the Apennine-Maghrebian thrust belt, the Hyblean Plateau of the Africa foreland and extensional structures on the western side of the Ionian basin coexist (Fig. 1). Its activity primarily consists of nearly continuous degassing from summit craters, strombolian phases of highly variable intensity, and frequent basaltic lava flows, representing a main source of volcanic hazard in the area. Moreover, flank eruptions during which the magma bypasses the central plumbing system, intrudes as dikes and erupts along the volcano flanks, periodically occur. This kind of event produces a massive deformation affecting the entire morphology of the edifice (e.g., Refs. 10-12). In particular, in the last decade it has experienced a significant increase in eruption frequency together with the development of some of the most energetic paroxysmal sequences recorded at Mt. Etna in recent times 13. These include the three-year-long sequence of lava fountaining occurred between 2011 and 2013 14 and the very recent sequence of eruptions that took place between December 2020 and February 2022 15. Moreover, also during the writing of this paper, some episodes of explosions with ash emissions were recorded.