Submarine landslide hazard in the Sines Contourite Drift, SW Iberia: slope instability analysis under static and transient conditions

Abstract

The Sines Contourite Drift (SCD) is a plastered drift with terraced-like morphology, formed by the Mediterranean Outflow Water (MOW) that emplaced in the Alentejo Margin, SW Iberia. The high (~ 27 cm/kyr) and variable sedimentation rates experienced since the Pleistocene resulted in low consolidation, and the development of excess pore pressure, which, associated with the seismicity in SW Iberia, offer significant conditions for slope instability in the SCD’s Late Pleistocene-Holocene muddy-contourite and hemipelagic sediments. Therefore, it is crucial to assess the susceptibility of the area to submarine landslide hazards. Slope instability is assessed both under static and transient conditions, based on the sediment mechanical properties, obtained through drained and undrained triaxial tests performed on sediment samples of three gravity cores (CO14-GC-2B, CO14-GC-3B, and CO14-GC-7B), respectively, retrieved at 1208-, 1280-, and 1425-m water depth (mwd). Those properties consist of internal friction angle (ϕ), with average values ranging between 28.5° and 35.1°; sediment unit weight (γ) that ranges from 16.9 to 18.1 kN/m3; and undrained shear strength (Su), ranging between 5.81 and 6.50 kPa. Cohesion (c) values are incipient and thus considered as 1 kPa in the modeling under static conditions. The analysis also accounts for the peak ground acceleration (PGA), determined according to the magnitude of seismicity recorded in the area. The SCD is prone to slope instability and collapse, especially in gradients > 5°, due to the sediment’s low consolidation, strength, and permeability. Seismicity greatly reduces the factor of safety (FS), promoting slope instability