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ABSTRACT: Submarine Slope Instabilities in the Vicinity of the Hellenic Arc, Greece

Lykousis Vasilios

The continental shelf and slope of Greece is very complex morphologically, resulting from intense neotectonic activity, and is related with the post-Miocene evolution of the Hellenic arc system. It is active seismically with fairly frequent shallow earthquakes >5R. The expected peak horizontal ground accelerations (%g) within 100 yrs range from 10-20%g and locally exceed 30%, while values of 25-33%g were measured recently after strong earthquake shocks (5.5-6.1R).

A variety of slope failures have been identified owing to ground motion intensity (%g), slope angle, vertical variation of Cu/Po, and to subbottom sediment sequence stratigraphy.

Small-scale peripheral slumping and mud flows are the predominant shelf sediment instabilities in relatively low angle prodelta slopes (0.5°-1.5°). Rotational and transitional slides, as well as debris and turbidity flows, were observed on steeper prodelta slopes (2°-7°). The shear plane is believed to be the gas-charged sediments and/or the finer grained (and low shear strength) basal sedimentary layer of the prodelta sediment sequences.

Large-scale translational or rotational slides were detected particularly in the fore-arc continental slopes. Possible failure planes within continental slopes were regarded to be Pleistocene interglacial hemipelagic mud sheets or sapropelic layers.

Rotational slides were observed, as well, within the shelf-edge late Pleistocene delta sequences. These failures were activated soon after sequence deposition (18,000 yr B.P.) and are presently inactive.

Infinite slope stability analysis indicates that slopes >1° are unstable in the areas of expected peak ground accelerations >13-15%g.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990