Possible correlation between pockmarks and gravitative instabilities in Palermo Gulf

Pockmarks are negative seabed structures that, like mounds, represent the surface expression of fluid migration pathways. Their physiographic variability is highly heterogeneous, as pockmarks are found in a wide range of geological settings. In the specific case of the Gulf of Palermo, they are located on the continental slope at depths ranging between 230 and 390 m bsl. The distinctiveness of the pockmark field in this study, which spans approximately 2.3 km², lies in its proximity to the coastline (≈ 5.8 km), its association with neotectonic lineaments, and its connection to the heads of canyons. Pockmarks exhibit diameters ranging from 90 to 320 m, with depths between 10 and 30 m. In continuity with the studies proposed so far, the study focuses on the analysis of acoustic and seismic geophysical data acquired in the Gulf of Palermo by the marine geology group of DISTEM. The analysis of seismic profiles allowed the identification of the acoustic basement and the fluid migration pathways, which are associated (though not exclusively) with normal faults located in Palermo offshore. Based on the literature data, the type of fluids rising corresponds to continental fresh water. So, the activity of the pockmarks is not continuous but intermittent, linked to the recharge and subsequent pressurization of fluids connected to the recharge of the continental aquifer through meteorological events. The integration of seismic profile with morphobathymetric data has revealed a weak layer, interpreted as such by its acoustic facies, at the base of landslide scars. This layer intersects the underlying fluid migration pathways associated with the pockmarks. Consequently, the identification of the areas and volumes of sediments weakened by fluid emission qualitative models regarding future retrogressive evolution of these areas. The analysis of the seismic profiles allowed the identification of a chaotic sediment deposit of about 5.75 million m³ along a landslide scar, near to the pockmark field. In order to validate the evolutionary model proposed for the study area the approach proposed by Nian et al. (2022), which employs a geotechnical model to analyze the equilibrium of a submarine slope, has been applied. The critical equilibrium is assessed by correlating the weight of the unstable block, fluid pressure, and shear force acting along the rupture surface. The extensive bibliographic research of geomechanical parameters and the measure of geometric parameters on seismic profile allowed the application of the Nian formula. Evaluating the instability of a submarine slope is very important for coastal risk studies. The case study of the pockmarks in the offshore of the Gulf of Palermo, where a tight connection exists between continental and marine processes (rainfall events and the upward movement of pressurized fluids), is particularly significant in the context of climate change impact studies.