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 . 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.