Seismic Triggering of Submarine Slides
Authors: Nadim Farrokh , Biscontin Giovanna , Kaynia Amir M. 
Published: October 2008

Earthquakes are known to have triggered major submarine mass movements all over the world. In offshore areas with minor sedimentation at present, earthquakes are the most important release factor for natural submarine slides. The paper describes the different earthquake-induced load effects that could make a submarine slope unstable and trigger a slide. In the assessment of seismic stability of a submarine slope, three scenarios should be evaluated and analyzed:Failure occurs during the earthquake. In this scenario, the excess pore pressures generated by the cyclic stresses degrade the shear strength so much that the slope is not able to carry the static shear stresses;Post-earthquake failure due to increase in excess pore pressure at critical locations caused by seepage from deeper layers; andPost-earthquake failure due to creep.

The paper discusses the mechanisms involved in the three scenarios and types of laboratory tests and numerical analyses that should be done to evaluate the seismic stability situation.

Stability of submarine slopes

Stability evaluation of submarine slopes under earthquake loading is one of the most challenging issues in offshore geohazard studies. To illustrate some of the important factors that need to be modeled in the assessment of seismic response of submarine clayey slopes, Nadim et al. (1997) considered a simple, one-dimensional (infinitely long) slope under seismic loading. When only gravity loads are acting, a generic soil element is subjected to a stress in the direction normal to the slope, represented by the effective normal stress (?n), and a stress in the plane of the slope, parallel to the dip, represented by the consolidation shear stress (?c) as shown at the bottom of Fig. 1. Given the simplicity of the formulation, the earthquake motion is assumed to consist only of shear waves propagating perpendicular to the slope, disregarding those propagating along the plane of the slope. This consideration is analogous to the assumption of vertically propagating "horizontal" shear waves for level ground conditions. The seismic motion then results in additional cyclic shear stress acting on the plane of the slope in a direction oriented at some angle with that of the consolidation shear stress. Although the seismic shear stress changes direction instantaneously, most analyses choose the critical direction to be parallel to the dip of the slope (i.e., the direction of shear shaking and initial shear stress coincide) as shown in Fig. 1.

Fig. 1 Infinite slope under one-dimensional seismic excitation. (available in full paper)

Earthquakes generate vibrations and mass inertia forces, which at times cause large shear stresses in the down-slope direction. However, the duration of load is short and in most situations, the main effects are accumulation of down-slope displacements accompanied by a moderate cyclic degradation of strength. This means that the focus of the seismic slope stability assessment must be on estimating the earthquakeinduced deformations, rather than computing a pseudo-static safety factor as commonly done by many geotechnical engineers. In a pseudo-static analysis, the inertial force caused by ground acceleration is applied as an effective static load equal to the mass of soil times the peak or the effective acceleration. Considering the equilibrium of the submerged