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Seismic footprints of shallow dyke propagation at Etna, Italy

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What happens in an active volcano during a shallow magma intrusion ? Effects at the surface are evident : ground deformation, fracturing, thermal changes precede and accompany the release of gases into the atmosphere until magma rises to the surface and the eruption starts. However, magma upraise may also yield other subtle evidence, potentially detectable only at seismic sensors nearby eruptive fractures.
This is the case of the station EBEL (Etna Belvedere, at an altitude of 2900 m), destroyed by lava flows on Etna volcano in February 2013. Before its breakdown, a dyke tip propagated nearby and under the seismic station (Figure). The opening of eruptive fractures deformed the ground, creating the pathway of magma to the surface. The seismic signals recorded at EBEL during its last operational days gave the rare opportunity to document these processes and to calculate the propagation speed of the magma-filled fractures (from 0.02 m/s to 0.46 m/s). The results of this study(*) were recently published in Scientific Reports, Nature, and describe what the authors called the “seismic footprints” of shallow dyke propagation.

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Cross sections (a, b, c) and map views (a’, c’, d’) illustrating the conceptual model of the main intrusion episodes occurred between 20 and 28 February 2013. The trace of the cross section corresponds to the NW-SE dashed grey line in a’), b’), c’). (a) dry fracturing close to EBEL station on February 20 (Vent “a” is not showed being out of perspective). (b) opening of Vent “b” and dry fracturing intensification on February 21. (c) opening of Vent “c” that issued the lava flow that destroyed EBEL on February 28. (a’), (b’), (c’) depict the opening of fractures and vent activation with the lava flows. Note that the graben structure in (c’) was totally buried by the lava flow of Vent “c”.

Such a kind of “seismic footprints” is important as it allows the future identification of similar processes at Etna as well as other basaltic volcanoes worldwide. In addition, it has implications in terms of volcanic hazard. Indeed, lateral eruptions - that is eruptions fed by dyke intrusions along the flanks of a volcano - are potentially able to open eruptive vents at low altitude, close to or within urbanized areas. They are therefore a menace to local populations. The data collected in this study, which was carried out in the framework of the MED-SUV project, contribute to increase the scientific knowledge in the early recognition of shallow dyke intrusions.

The link to the open-access paper is
http://www.nature.com/srep/2015/150715/srep11908/full/srep11908.html

(*) Falsaperla, S. and Neri, M. (2015), Seismic footprints of shallow dyke propagation at Etna, Italy. Sci. Rep. 5, 11908 ; doi : 10.1038/srep11908.

CONTACT : Susanna Falsaperla & Marco Neri (Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo, Sezione di Catania, Italy)

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