Fluid escape processes studied in Palermo offshore (NW Sicily). Quantitative analysis and modelling analysis of the factor of safety (FOS) of a submarine slope.

This study analyzes a submarine sector of Palermo offshore where the multiple cause-effect relationship between pockmarks and submarine landslides are studied by mean Multi Beam Echo Sounder (MBES) morphobathymetric data. This sector is characterized by freshwater continental origin pockmarks whose activity can be considered intermittent as associated with the recharge of the continental aquifer through meteorological events (Pennino et al., 2014). The integration of MBES data and Sparker seismic profiles allowed us to obtain quantitative measurements of: 1) thicknesses, areas and volumes of sedimentary body altered by upward migration of pressurized fluids; 2) the presence of preferential sliding surfaces (e.g. weak layers) likely responsible of pockmark-associated landslides; 3) tectonic alignments linked to recent geological structures (predominantly N-S oriented) along which the fluids escape structures are detected. We investigated the possibility that in the nearness of pockmark convergent reflectors, the pressurized fluids caused the loss cohesion resulting in sediments prone to movement. In the case of Palermo offshore, this hypothesis is strengthened by favorable boundary conditions such as: high slope values of the area, surrounding landslides and ongoing fluid escape processes. The identification of areas and volumes of altered sediments, together with a critical review of literature, allowed us to develop a qualitative model regarding the potential future retrogressive evolution of these areas and, also the computation of the pockmark related Factor of Safety (FOS, Noda et al., 2013; Sultan et al., 2004). Preliminary results highlight that the pore overpressure, in intra-pockmark area, strongly influence the equilibrium of the slope and, thus, can be considered as a predisposing factor for submarine gravitational processes. Moreover, the identification of potential landslide volume could allow to investigate possible landslide triggered - tsunami waves hazard scenario.