The seismic swarm that accompanied the last eruption of Etna for days, the one that occurred last December 24, 2018, may have slowed the ascent of the magma, accumulated in a source about 5 km deep, limiting the extent of the eruption itself. This is the hypothesis advanced by a study by the researchers of the National Institute of Geophysics and Volcanology (INGV) Alessandro Bonforte, Francesco Guglielmino and Giuseppe Puglisi, recently published in the journal Terra Nova.
"The eruption of Etna on Christmas Eve represented a rather particular phenomenon", explains Alessandro Bonforte, co-author of the article, "in fact, although it was not a particularly significant event, it did record intense seismic activity which preceded and accompanied the event for days, even after the end of the eruptive activity”.
The eruptive fissure opened on 24 December spread from the South-East crater to the Valle del Bove and from there a flow flowed which was fed until 27 December, while the explosive phenomena had already ceased a few hours after the start. The seismic activity that accompanied the eruption activated all the faults affecting the sides of the volcano, with thousands of earthquakes recorded not only in the summit area where the fracture opened but also along the Ragalna, Pernicana faults and of Trecastagni. The largest event, of magnitude 4.9, occurred on December 26 along the Fiandaca-Pennisi fault system.
The data provided by the satellite images acquired by the Sentinel 1A and 1B satellites of the European Space Agency (ESA – European Spatial Agency), equipped with a particular SAR (Synthetic Aperture Radar) system, made it possible to measure the deformation of the Etna ground between 22 and on December 28, with the volcano split into two lobes with opposite motion.
“This deformation”, continues Bonforte, “was caused by an important ascent of magma from the depths and favored a radial fracture with respect to the summit craters which instead drained the already resident magma, which was stationed in the conduits just below the surface, at the below the southeast crater. The much more substantial mass of rising magma instead stopped below the volcano, approximately at sea level. The eruption observed on the surface would therefore have been only a "side effect", compared to the one that was being prepared. The hypothesis put forward in our study is that it was precisely the great energy dissipated in the seismic swarm that blocked the ascent of the magma from the depths. Naturally this hypothesis requires further calculations based on in-depth analyzes of the INGV multidisciplinary monitoring data in order to be verified and perfected”, concludes the researcher.
Image - Sentinel 1A/B interferograms for the December 22-28, 2018 passes, acquired in ascending and descending geometry. The colors correspond to the displacement of the terrain along the Earth-Satellite line of sight; one interferometric cycle (-pi, pi) corresponds to a displacement of 2.8 cm. The dashed blue lines indicate the main tectonic structures and the red star indicates the epicenter of the Mw = 4,9 earthquake of 26 December along the Fiandaca-Pennisi fault system. PFS = Pernican Fault System; FPF = Fiandaca-Pennisi Fault; BOF = Borrello-Each Fault; RFS = Ragalna Fault System.
Link to the article: https://onlinelibrary.wiley.com/doi/pdf/10.1111/ter.12403