Mega-pockmarks and fluid-escape systems in the Northern Tyrrhenian Sea: Morphometry, classification, and tectono-oceanographic controls

We document, for the first time, the occurrence of twelve mega-pockmarks in the Etruschi Seamount and in the surrounding sectors of the northern Tyrrhenian Basin (central Mediterranean Sea). They stand out from 230 mapped depressions, in having the dimensional criteria representative of mega-pockmarks: major axes exceeding 1000 m, internal depths greater than 100 m, and volumes in the order of ∼108 m3. Morphometric analysis indicates that size classes are primarily differentiated by dimensional parameters in the vertical aspects, rather than by planform geometry. While summit water depths (SWD) largely overlap among normal, giant, and mega-pockmarks, internal depth (H) and volume show a clear hierarchical increase. A log–log regression between maximum axis length and internal depth defines a robust positive power-law relationship (R2 = 0.75), consistent with previously documented global scaling trends. Spatial organization and morphology reveal a coupled morphogenetic system governed by tectonic structures, slope instability, and bottom-current reworking. Strings of pockmark oriented NNE–SSW suggest structural discontinuities acting as preferential pathways for focused seepage, whereas isolated depressions reflect more diffuse fluid-escape conditions. The progressive enlargement and local coalescence of pockmarks are closely associated with slope failure processes and bottom-current activity, which contribute to the maintenance and broadening of the pockmarks over time. The recognition of mega-pockmarks in the northern Tyrrhenian Sea has direct implications for Mediterranean geohazard assessment, as focused fluid migration may reduce sediment shear strength and precondition slopes to localized failures. High-resolution bathymetry and multitemporal surveys are therefore essential to monitor morphological evolution and assess potential risks to submarine infrastructure (e.g., telecommunication cables).