Linking the gap between seismic and geodetic observations during the 2020-2022 lava fountains at Etna volcano

Abstract

Lava fountains at Etna volcano are spectacular explosive eruptions consisting of powerful jets of gas and solid particles that cause a huge impact both to air traffic and urban areas due to ash dispersal and fall-out. The evolution of a lava fountain at Etna is usually a gradual process that progresses from a weak strombolian activity, passes to a typical lava jet and ends with the formation of a sustained eruptive column. Typically, lava fountain events last from tens of minutes to few hours. The interaction of the magma with the surrounding rock during such eruptive episodes induces tremor (0.5 - 5 Hz) and ultra-small deformation (10-9 to 10-7) of the superficial crust and generates geophysical signals at a wide ranges of amplitudes and time scales. Seismometers can capture only the higher frequency content of such signals by measuring volcanic tremor. Due to the small amount of related deformation, the traditional geodetic methods, such as GPS and InSAR, are not able to reveal significant variations because of the low precision (GPS resolution > 1 cm) or very low sampling frequency (InSAR temporal resolution limited by satellite passages). This limit is overcome by using Sacks-Evertson strainmeter that measures the volumetric deformation of the surrounding rock with the highest achievable resolution (10-10 to 10-11) and in a very wide frequency range (10-7 to > 20 Hz). Thanks to its characteristics, the strainmeter is perfectly suitable to explore the full spectrum and the very small amplitudes of the strain signal exerted by the lava fountain episodes at Etna, covering the frequency gap between the seismic and the common geodetic techniques. In this work, we analyzed the co-eruptive strain changes recorded by the borehole strainmeter network concurrently with more than 60 lava fountain events that occurred at Etna volcano between 2020-2022. We investigated all the lava fountain events, highlighting the main characteristics of the eruptive phases and their transition that characterize such eruptions, both in the high and in the low frequency band. These characteristics furnish fundamental constraints to improve the characterization of eruptive processes that lead to lava fountain events.