Submarine Slumping and Mass Movements on the Continental Slope of Israel

Abstract

The continental margin of Israel has the shape of a lens with a foreset structure, which narrows from 40–50 km off northern Sinai to 5 km off southern Lebanon. The lens was formed by accumulation, since the Pliocene time, of mainly fine clastics derived from the Nile and transported by the counterclockwise currents of the south-eastern Mediterranean. After initial deposition, the detritus was redistributed over the continental slope and the adjacent deep sea by slumping. The slumped materials were probably largely transported downslope in the form of mudflows and debris flows, and occasionally by sliding of large blocks of sediments. The continental slope steepens from south (average 3–4°, maximum 5–7°) to north (average 6–8°, maximum 14–18°).

A geotechnical study was carried out on a large number of undisturbed core samples to provide a basis for the quantitative analysis of the slumping. Angles of internal friction of 24–25° measured by drained direct shear tests specify the maximum possible inclination of a stable infinite slope. The steepest slump-scar wall slopes are about 20° and indicate that a drained slumping mechanism is unlikely, and that these slopes have long-term stability. Consolidated-undrained triaxial compression tests and laboratory vane tests yielded angles of internal friction of 15–17° and ${c_u}/{\bar p_o}$ values of 0.22–0.91. An analysis of the force equilibrium within the slope sediments, based on these results, lead to the conclusion that horizontal earthquake-induced accelerations, as little as 5–6% of gravity, are sufficient to cause slope failures. Collapse resulting from liquefaction is unlikely, as the sediments are normally consolidated silty clays with intermediate sensitivity of 2–4.

Mass creep phenomena of apparently stable sediments (${c_u}/{\bar p_o}$=0.25) are ubiquitous over the sub-horizontal shelf edge and upper-most slope (0.5–2.5°), where they form undulating ground. Analysis of the static strength properties of the sediments, together with the stability of the slope, suggests that these phenomena result from long-term deterioration in shear strength of the sediments due to repeated loading effects. In view of the great water depth (above 80 m) and the mild oceanographic conditions in the region, this weakening could be caused by accumulating effects of earthquakes. This is presently being investigated by studies of the cyclic load properties of the sediments.