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Abstract

Magma discharge through lava fountain generates slow deformation, associated with the deflation of the magma chamber, and tremors, associated to seismic radiation. Few studies have attempted to relate observations of seismic tremors with geodetic data in order to gain insights into magma supply and discharge rates. However, no simple relationships have been found. It is questionable if this difficulty may be imputed to limitations in the instrument accuracy and spectrum range of seismic and geodetic methods. The 2020-2022 explosive activity at Etna with 60 lava fountains offers the extraordinary opportunity to investigate the dynamics of these events using high precision strainmeter signals from Sacks-Evertson borehole dilatometer. This instrument measures the volumetric strain in a very wide frequency range (10^-7 to >20 Hz) and with the highest achievable resolution (10^-10 to 10^-11). Thanks to this characteristics, the dilatometer allows to fill the band gap between seismic and geodetic methods and provide a record to explore the full spectrum of the signal. By analyzing the 2020-2022 dataset at Etna, for all the events we recognize: (i) negative slow (from minutes to hours) step-like strain changes encompassing each lava fountain and (ii) strain tremors with peaks at around 1-2 Hz preceding and accompanying the event. We quantified the amplitude, the duration, the rate and the frequency content of the long-term changes and the strain tremors to explore their relation and the energy partition between slow deformation and seismic radiation.