Comprehensive review of pockmarks and first "Susceptibility Map" of the Italian Continental Margins

Fluids, encompassing gases and liquids, possess lesser density than solids, therefore exhibit an upward movement within sedimentary strata due to buoyancy. Seafloor "fluid flow" is a well-established phenomenon in diverse geodynamic settings, spanning active and rifted continental margins, compression zones (subductions), and depositional environments characterized by high-rate sedimentation such as deltas and contourite drifts. This phenomenon manifests in a range of positive (e.g., mud volcanoes) and/or negative seafloor morphologies (pockmarks).

Pockmarks, recognized since the 1970s, represent the dominant morphological features associated with fluid escaping from the seafloor. These seafloor morphologies can reach diameters and depths of several kilometers and over 100 meters, respectively, featuring circular to elongated planforms and flat-bottomed to conical cross-section profiles. Despite insights from geological and geophysical data, the comprehensive understanding of the mechanisms governing pockmark formation, growth and maintenance remains elusive. Various hypotheses and conceptual models, including those involving near-bottom currents, have been proposed to elucidate the genesis and development of pockmarks. These models encompass continuous processes (seeps) or sudden episodic events of fluid releases and blowouts (vents).

Pockmarks hold significance for various reasons. Pockmarks are often linked to fluid-driven sedimentary failures, highlighting their role as a significant geohazard associated with fluid migration, excess pore pressure, and potential landslide triggering. Therefore, studying pockmarks is crucial for geohazard assessment and the planning of submarine and seafloor infrastructures, where their inherent instability at the seafloor requires safety assessments. Studies on seismicity in marine areas suggest that pockmarks may serve as important earthquake precursors, with observed increase in seeping water temperature before seismic events and continued venting of gas bubbles immediately afterward. The plausible relationship between fluids and seismicity is due to the lubricating effect of fluids on faults. Furthermore, pockmarks release hydrocarbons, which are of great importance for the global carbon cycle, their implication in the climate change and in the sustaining of specialized biological communities. Pockmarks are also indicators of petroleum generation, making their study pertinent in oil and gas exploration.

This study, concerning the assessment of approximately 6,000 pockmarks mapped on the central Mediterranean Sea, utilizes morphological, sedimentological, and tectonics evidences, employing a GIS-based and data-driven approach to generate the pockmark susceptibility map for the Italian continental margins. The map is the outcome of a deep learning architecture tasked with the classification of the seafloor based on binary classification obtained by training a neural network with locations where pockmarks have been mapped and locations where the same are certain to be absent.