Main morpho-structural changes and eruptions of Etna in 2016-2019 captured by satellite observations

Persistently active volcanoes such as Etna, in southern Italy, are subject to frequent morphological and structural changes, especially at the summit. In recent decades, in particular, Etna has shown an evident increase in both summit and flank eruptive activity. This caused a striking transformation of the morphologies of its summit craters, which increased in number and size, also causing the formation of new small eruptive vents, fumarolic fields, fractures and crater collapses. Sometimes these morpho-structural modifications of the top of the volcano have been so rapid that they have not been all recorded accurately, or they have occurred in sequences so rapid as to overlap the effects of the eruptions, making some transitional events between an eruption and the next one. Eruptive activity during the period considered occurred mostly at the summit craters of Etna (May 2016: Voragine; February-April 2017: New Southeast Crater and fissures on its slopes; August and November-December 2018: New Southeast Crater). This was interrupted by the brief fissure eruption on the upper southeast flank of the volcano on 24-27 December 2018; renewed eruptions occurred at the New Southeast Crater and fissures on its flanks in May-July 2019. Finally, in September 2019, eruptive activity shifted to the Northeast Crater and Voragine, the latter feeding intermittent lava flows into the adjacent Bocca Nuova crater. In cases like this, satellite observations can complete terrestrial monitoring systems, providing a useful contribution of knowledge and detail of the eruptive activity and morpho-structural transformations of greater significance. In this study, we analysed the Mt. Etna activity using data from the Multispectral Instrument (MSI) and the Operational Land Imager (OLI), respectively onboard Sentinel-2 and Landsat 8 satellites, processed by means of the recently proposed NHI (Normalized Hotspot Indices) algorithm. The latter allowed us to identify thermal anomalies associated to main effusive and explosive activities as well as to the smaller eruptive events, revealing in some cases thermal phenomena several days in advance that can be interpreted as potential precursors. In addition, NHI also showed a fair sensitivity in grasping the incipient fracturing of the Etna summit area, an important phenomenon in the life of this volcano due to its close correlation with the slow lateral collapses that characterize its flanks, and which in turn can trigger lateral eruptions that are potentially dangerous for the Etnean populations.