Submarine fluid escapes: their role on ground instabilities. Learning examples of RETURN project

The submerged structures associated with the term “Fluid escapes” are morpho-structural evidences of particular importance, linked to processes of fluid uprising, such as water or gas, from the seabed. Depending on their depth, the nature of the emitted fluid, their extent, and the area involved, both superficially and, especially, in the stratigraphy of the seabed, these effusions can play a significant role in the formation and triggering of submarine landslides. Studies conducted under the RETURN Project have allowed the analysis of these structures from geological, morphological perspectives, and to undertake an initial geo-mechanical approach. Analyzing the possible geomechanical variables associated with structures such as Pockmarks, Mounds, and Mass Transport Deposits (MTD) (Sultan et al., 2004) is indeed necessary to extract quantitative data and to study the geological evolution of these structures and the submarine landslides associated with them. The offshore of Palermo gulf and Sciacca gulf (Ag) are Sicilian marine areas where such structures are observed. In the Palermo gulf, Pockmarks are observed along the continental slope (Lo Iacono et al., 2011), and two large submarine landslides are connected to them. The formation of landslides is linked to the presence of unconsolidated sediment associated with these structures because the uprising fluids also impregnate the sediments of the flowed layers, thus causing the rupture of cohesive forces. In contrast to the pockmarks, which are depressions, the offshore area of the Sciacca Gulf (Servizio Geologico d’Italia, 2024) exhibits Mounds ([MM1] mud volcanoes). They are distributed along at least three lines of weakness, each about 2 km long and perpendicular to the coastline. The presence of these structures also indicates submarine areas with the active presence [MM2] of loose sediments susceptible to submarine landslide formation. Free gas accumulations (highlighted by enhanced seismic reflections) can be found below mass-transport deposits (MTD), whose seal capacity is attributed to its intensive consolidation in response to differences in capillary pressure and porosity. As demonstrated for the Pearl River Mouth Basin (South China), a retrogressive slope failure could be responsible for a lateral migration of free gas (Sun et al., 2017). Therefore, the identification and analysis of Pockmarks, Mounds and MTD lateral extensions comprise an important geohazard in terms of fluids escape structures prevention and control.