Sliding and regional slope stability in active margins: North Aegean Trough (Mediterranean)

Continuous seismic (air-gun) sub-bottom profiling in the North Aegean has revealed a large submarine slide. The failure zone extends from a depth of about 300 m down to 850 m and covers an area of 85 km2. The mean thickness of the slide reaches about 55 m and an estimated total volume of 3.8 km3 of late Pleistocene sediments has slid a distance of 6–7 km. Chronostratigraphic analysis of the acoustic reflectors imply that the major and secondary glide planes are the transitional zones between a lower sequence of strongly stratified reflectors (stiff, relatively coarser sediment layers) and the overlying acoustically transparent and/or poorly stratified sequence (weaker muddy layers with higher water content and lower shear strength). These weak transparent layers have been deposited during interglacial high sea level stands (isotopic stages 7, 5, 3). The major glide plane is assumed to have been developed within a muddy layer that was deposited about 170–245 ka BP. Stratigraphic, sedimentological data and AMS dates indicate an early Holocene age of the major failure event (not earlier than about 5.5 ka BP and not later than 7–8 ka BP). The regional seismic hazard and slope stability analysis shows that the North Aegean marginal upper–middle slopes are highly unstable. The calculated values of horizontal ground acceleration able to initiate failures (kc) range from 80 to 300 Gal downslope. This is lower than what is expected within 50 yr (especially in the upper–middle slopes).