Characterization of Recent Mass Movements and Implications for Geohazard Evaluation on the Western Black Sea Margin

The integration of a large set of geophysical, geological, geotechnical and geochemical data through a multi-disciplinary team approach provided new insights into the geological processes occurring along the Western Black Sea Bulgarian Margin. These results provided a solid basis and key input parameters for slope stability and debris flows runout analysis. High resolution mapping of the continental slope revealed the presence of three main types of recent shallow slope instabilities. Each type of failure characterizes a geomorphic domain and, assuming a common seismic trigger, is inferred to be linked to peculiar preconditioning factors. Small scale retrogressive slumps occurred below the shelf edge, and generated debris flows with limited (few km) runouts and velocities up to 9 m/s. Such failures were favored by the presence of shallow gassy sediments. A regional composite headscarp crossing the slope is continuously cut back by retrogressive failures, separated in time by several hundred years. A similar slope failure return period is expected for future slides based on seismic-based slope stability analysis. Low-deformation superficial slumps occurred almost exclusively in finely laminated sapropels deposited in the last 8,000 years and formed widespread parabola-shaped sediment ridges.