The borehole dilatometers network of Mt. Etna volcano: A powerful tool to detect and infer intrusive, explosive and effusive processes.

Bonaccorso A.

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IV - Geofisica e fisica dell'ambiente
GSSI Ex ISEF - Aula D - Mercoledì 25 h 16:30 - 19:00
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A network of 4 borehole dilatometers has been installed on Etna in two successive phases (2010--2011 and 2014). The borehole dilatometers are installed in holes drilled at depths usually greater than 100 m, and they measure the volumetric strain of the surrounding rock with a nominal precision up to $10^{-11}$ in a wide frequency range $(10^{-7}$ - $10^2$ Hz). We describe the characteristics of the network and the results of the in situ calibrations obtained after the installations by different methods. We illustrate short-term strain changes recorded during several lava fountains erupted by Etna during 2011--2013. We also show signal changes detected at all four stations during the lava fountain on 28 December 2014 and during the powerful explosive sequence of four episodes from Voragine crater (VOR) on 3--5 December 2015. Analytical and numerical computations constrained the eruptions source depth and also its volume change that is related to the emitted magma volume. In the case of eruptions characterized by small and repeated moderate explosive activity accompanied by overflows from the summit craters, the strain signals appropriately filtered showed unprecedented ultra-small changes ($\sim 10^{-8}$ -- $10^{-9}$), preceding and accompanying the 2017 modest explosive-effusive activity. In the case of dike propagation we report about the signals recorded during the last December 2018 eruption of Etna generated by a violent intrusion, and we show that the dilatometer network was able to detect the beginning of the propagation of the dike about 2.5 hours before the formation of the eruptive fissures. All these results, obtained through the borehole dilatometer network of Mt. Etna volcano, highlighted that this type of instruments can provide valid and advanced contribution to the understanding and interpretation of different eruptive activities, and also that it represents a powerful tool for surveillance purposes.

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