3D Local Earthquake Tomography provides new insights on Mount Etna velocity structure

We present a new 3D overall model of Vp, Vs and Vp/Vs for the Mount Etna (southern Italy), the largest and most active volcano in Europe. We applied the LOTOS code (Koulakov, BSSA 2009) to a dataset of ~4600 crustal earthquakes that occurred in the study area during the last 26 years (Totaro et al., SciRep. 2024). The selected dataset, representing the longest time-interval ever analyzed for Mt. Etna, allowed us to characterize the volcano velocity structure getting over possible singularities due to specific eruptive phases. We estimated and jointly interpreted P- and S-wave velocity patterns together with the Vp/Vs ratio, particularly effective to discriminate the presence of groundwater, gas, and melts and thus very precious for volcano investigations (Kuznetsov et al., Geosciences 2017; Vargas et al., SciRep. 2017; Totaro et al., SciRep. 2022). The obtained 3D seismic velocity patterns allowed us to add further details on already known anomalies and to identify new previously undetected ones. Focusing on the latter, at the shallowest layer we highlight the presence of two high Vp/Vs volumes, located in close correspondence with low resistivity areas (Siniscalchi et al., JVGR 2010, JGR-SE 2012), that can be associated to underground aquifers generated by meteoric water penetrating the volcano edifice. Moreover, a high Vp/Vs anomaly characterized by intense seismic activity has been clearly detected along the eastern flank of Mt. Etna representing a volume of strongly fractured sedimentary rocks through which a large amount of fluids may rise. Finally, on the western side, a high Vp/Vs area with very low seismicity is detectable. The achieved velocity patterns may suggest fluid accumulation, probably not associated to the volcanic activity, even if further investigations are necessary to better solve and understand this previously unknown anomalous region. In conclusion, our study furnished a comprehensive velocity model that, encompassing specific volcanic phases and allowing a joint interpretation of Vp, Vs and Vp/Vs patterns, provides a more complete modelling of the main features of Mt. Etna.