Thesis title: Spatial and Morphometric analysis of gravitational instabilities along the Tyrrhenian and Ionian continental margins of Calabria
Submarine landslides are among the most widespread and hazardous geomorphic processes affecting continental margins. Their occurrence can severely impact offshore infrastructure, submarine communication networks, and coastal areas. Despite their significance, a comprehensive understanding of their controlling factors remains limited due to the restricted accessibility of the seafloor and the scarce availability of in-situ lithological and geotechnical data.
The PhD research focuses on the Tyrrhenian and Ionian Calabrian continental margin, an area characterized by high tectonic activity, complex morphology and sedimentary dynamics, and widespread evidence of gravitational instability. The main scientific problem addressed by this work is the full exploitation of the exceptionally high quality morpho-bathymetric data available in the marine realm, to investigate submarine landslides and to understand how their morphometric attributes are reliable and can be related to geological control factors.
To address the challenge, a systematic workflow was developed that integrates high-resolution multibeam bathymetry (5–20 m grid size), geomorphological mapping, morphometric analysis, and statistical modeling. After identifying and quantifying potential biases due to data heterogeneity and interpretative subjectivity, nearly 2,000 submarine landslides were originally mapped across the Calabrian margin using rigorous and ad-hoc standard, forming a very reliable base for the geodatabase. Over thirty geometric and morphometric parameters were originally measured or computed to describe landslide morphology and setting. The relationships among variables were investigated using several statistical routines, such as Exploratory Data Analysis (EDA), Analysis of Variance (ANOVA), Spearman’s rank correlation and Size-frequency distribution.
The analysis revealed that, although the study area encompasses two distinct geological domains, the Ionian forearc and the Tyrrhenian back-arc, the observed differences in morphometric characteristics are mainly driven by local morphological factors rather than by broader geodynamic settings. Landslides were clustered into two principal physiographic domains: those occurring within canyons and confined channelized settings, and those developing in unconfined open slope environments.
From a morphological point of view, canyon-related landslides generally occur over steeper slopes and are characterized by smaller dimensions. The relationships between morphometric variables are strong, suggesting a more uniform set of controlling factors primarily associated with topographic steepness, confinement, and local sediment dynamics. Conversely, landslides developing on open slopes are typically larger and less morphologically constrained, and in a few cases, the landslide deposit is present. The correlation among parameters is weak and shows greater variability, reflecting the interplay of multiple and spatially heterogeneous controlling factors.
From the perspective of sediment transport toward deep basins through gravity-driven flows, the open slope and canyon landslides show comparable proportions of seafloor affected by landslides (~4% of the total domain area), suggesting similar likelihood to fail. However, considering the greater size of landslides and the wider area of open slope environments, these account for nearly 75% of the total volume of sediment transferred towards the basin.
The availability of high-resolution data and many gravitational instability features made it possible to characterize the size distribution, showing a behavior consistent with a log-normal distribution across both domains, while power law was able to describe only a truncated portion of data (heavy tail). In addition to the definition of the statistical frequency distribution, the analysis of the modal class revealed distinct characteristic behaviors across the different domains. Landslides occurring in canyon environments display a fractal-like behavior, with an exponential increase in the number of detected landslides as bathymetric resolution increases. Conversely, in open-slope settings, this trend is interrupted around values of approximately 30,000 m², indicating the presence of a characteristic scale limit, open up new research perspectives in the dimensional characterization of submarine landslides.
The analysis of regional controlling factors of gravitational instabilities, roughly extrapolated to the marine realm, revealed distinct behaviors across the different physiographic domains. In canyon environments, landslides show no correlation with peak ground acceleration (PGA) or fluid presence (derived by pockmark susceptibility) but appear to be more influenced by high uplift rates, suggesting a possible role of tectonically induced oversteepening. Conversely, in open slope environments, landslides display an inverse correlation with seismicity, indicating that low-magnitude earthquakes may instead promote sediment consolidation, while showing stronger correlations with tectonic uplift and fluid activity.
From a methodological standpoint, the definition of the area–volume scaling parameters allows the avoidance of time-consuming reconstructions of the failed mass; the development of empirical degradation functions enabled the quantification of resolution-dependent detectability loss and allows comparison among the growing number of inventories in the literature acquired at different resolutions; and the application of geometric parameters such as compactness as quality-control tools improve the reliability of the morphometric database and minimizes potential mapping errors.
Overall, considering the study of submarine landslides is extremely costly and challenging, and available in-situ measurements of lithological and geotechnical properties are limited, this research has demonstrated that, starting from only high-resolution bathymetric data, exploiting morphometric characteristics through statistical analyses, and applying appropriate procedures to minimize biases, it is possible to extract valuable even if only partial scientific information on gravitational instability processes.
Furthermore, the methodological framework developed in this research, from the standardization of mapping and morphometric characterization procedures to the definition of empirical detectability loss functions and the use of compactness as a proxy for mapping errors, establishes a reliable and replicable quantitative approach for the analysis of submarine instabilities.