Analysis of submarine slumping in the continental slope off the southern coast of Israel

Abstract

The continental slope off the coast of Israel is riddled with numerous large slump scars at depths greater than 400 m. Recent scar slumps are situated in the steepest central portions of the continental slope (400–450 m depth, α = 6°), frequently disfiguring older slump scars in its lower portions. The slumping materials were probably largely transported downslope in the form of density currents, and occasionally by sliding of large sediment chunks. Upslope retrogressive slumping phases progressively disfigure the shape of the slump scars until they totally disappear, causing net reduction of the thickness of the sedimentary column. To provide a basis for the quantitative analysis of slumping, laboratory vane tests, triaxial consolidated, undrained compression tests with pore‐pressure measurements, drained direct shear tests, and consolidation tests were performed oh undisturbed samples. Because the sediments consist of normally consolidated silty clays, the geotechnical properties measured on the core samples can be readily extrapolated for greater depths, assuming the sediments are homogeneous. Angles of internal friction measured by direct shearing under drained conditions are ?_d =24°‐25°, designating the maximum possible angle of a stable infinite slope. These angles are appreciably higher than the steepest slopes in the investigated area, and a drained slumping mechanism is therefore considered unlikely. The slopes of the slump scar walls are about 20°; therefore, in the absence of active erosional, sedimentological, or tectonic agents, these walls have long‐term stability (drained shear). Undrained shear failure resulting in slope instability may be attributable to rapid changes in slope geometry (undercutting or oversteepening of the slope), fluctuations in pore pressure, or accelerations associated with earthquakes. Undrained shear‐strength parameters were determined by both laboratory consolidated‐un‐ drained triaxial tests and by miniature vane shear tests. The angles of internal friction that were measured are ?_cu =15°‐17°, and the c_u/p _o values range between 0.22 and 0.75. An analysis of the force equilibrium within the sediments leads to the conclusion that horizontal earthquake‐induced accelerations, as little as 5–6% of gravity, are sufficient to cause slope failure in the steepest slope zone (400–450 m depth, α=6°, c_u/p _o=0.25). Collapse resulting from liquefaction is unlikely, as the sediments are normally consolidated silty clays with intermediate sensitivity, S_t =2–4.

The existence of slump scars in the lower portion of the continental slope, characterized by gentle slopes (α=1°‐3°) and sediments with high shear strength (c_u/p _o=0.30–0.50) is attributed to large horizontal accelerations (k= 12–16% of gravity). Owing to the wide range of geotechnical properties of the sediments (c_u/p _o= 0.20–0.75) and the inclination of the continental slope (α=1°‐6°), the same earthquake may generate a wide range of horizontal accelerations in different portions of the continental slope, and slumping may occur wherever the stability equilibrium is disrupted.