Risk Assessment for Earthquake-Induced Submarine Slides

Abstract

Stability evaluation of submarine slopes under earthquake loading is an important and challenging issue in many offshore geohazards studies. Generally, three scenarios of earthquake-induced slope failure should be evaluated and analyzed: (1) 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, (2) Post-earthquake failure due to increase in excess pore pressure at critical locations caused by seepage from deeper layers; and (3) Post-earthquake failure due to creep. Soils that have strong strain-softening characteristics and high sensitivity are most susceptible to failure during earthquake shaking. The scenario of excess pore pressure migration from deeper layers into critical areas, leading to slope instability, is quite important and could occur over a time span of years or even decades in deep marine clay deposits. However, post-earthquake creep-type failure is believed to be the most common mechanism for clay slopes. In a risk assessment framework, the uncertainties in all the parameters and models used in the stability assessment must be addressed and the consequences of slope failure must be evaluated. It is often difficult to separate the uncertainties due to lack of knowledge (epistemic uncertainties) from the natural variability of the physical parameters such as soil shear strength and earthquake characteristics. The risk assessment procedure outlined in the paper integrates the results of geotechnical evaluations with other evidence, like dating of the previous slide events, to provide a more rational estimate of the annual probability of earthquake-induced submarine slope instability.