Chronicle of Processes Leading to the 2018 Eruption at Mt. Etna As Inferred by Seismic Ambient Noise Along With Geophysical and Geochemical Observables

This work analyzes temporal variations of seismic velocities at Mt. Etna from August 2018 to February 2019. During this time period, a strong flank eruption accompanied by intense seismicity and ground deformation took place along a fracture that opened on 24 December 2018 at the base of the New South-East summit crater. Furthermore, two moderate earthquakes-the 6 October 2018 M-L 4.7 and 26 December 2018 M-L 4.8-associated with the volcanic activity were recorded. In this study, we computed cross-correlation functions (CCFs) between windows of seismic ambient noise to identify seismic velocity variations within the volcano edifice. We calculated daily CCFs at 16 stations for 120 interstation pairs using the vertical component in the 1.0-1.5 Hz frequency band. We observe that dv/v starts to decrease rapidly from the beginning of October 2018 and reaches approximately -0.45% in the pre-eruption period. The spatio-temporal distribution of seismic velocities shows that the reduction of dv/v mostly occurs in the vicinity of the summit and close to the flank area and is interpreted to be affected by magmatic intrusion at 0-3 km depth. To infer the source mechanism of this eruption, we compared these observations with volcano-tectonic earthquakes, volcanic tremor, volcanic degassing, gravity, and ground deformation data. Our study suggests that a relationship between magma intrusion and associated crack opening is responsible for the decrease of dv/v.