Morpho-acoustic characterization of a shallow-water mud volcano offshore Scoglio d'Affrica (Northern Tyrrhenian Sea) responsible for a violent gas outburst in 2017

Abstract A violent gas outburst occurred offshore the Scoglio d'Affrica islet (Tuscan Archipelago, Northern Tyrrhenian Sea) on March 16th 2017, with local fishermen observing columns of dirty water rising up to 10 m above the sea surface. The integration of video footage and dissolved CH4 measurements collected 5 days after the event with high-resolution multibeam data collected 4 months later allowed us to characterize the source area of the outburst, corresponding to a shallow-water mud volcano. The mud volcano covers an area of ca. 170,000 m2, has a vertical relief of ca. 30 m with respect to the surrounding seafloor and an estimated volume of ca. 1 × 106 m3 based on bathymetric reconstruction. The elongated NNW-SSE shape of the mud volcano is compatible with local structural trends, indicating a tectonic control for its development. The mud volcano is made up of two mounds whose tops are located at a depth of ca. 10 m. The southern mound was responsible for the 2017 outburst, as testified by a 15–20 m wide circular crater on its summit where a large amount of mud breccia and diffuse seepage from small pockmarks were observed in video footage after the outburst. The flanks of the mud volcano are steep and characterized in the upper part by a hummocky morphology and multiple sediment flows on the western flank. The characterization of the mud volcano and the deposits associated with the 2017 gas outburst provides insight into seafloor-shaping processes linked to fluid seepage in shallow-water sectors. This is a particularly relevant issue considering both the paucity of studies on shallow-water mud volcanoes as well as the hazard associated with violent gas outbursts in such settings, as witnessed by the March 16th 2017 event.