Back-analysis and failure mechanisms of a large submarine slide on the ebro slope, NW Mediterranean

The causes that brought a portion of a submarine slope to fail, and whether this slope may fail again or not are issues not always easy to answer because of the inaccessibility of the deep marine environment. This study tackles those questions for a relatively large (2200 km2, 26 km3), 11 kyr old landslide off the Ebro margin, NW Mediterranean by means of geophysical (multibeam bathymetry, seismic reflection profiles and side-scan sonar data), sedimentological and geotechnical data. Back-analysis of the slide and interpretation of the geophysical data sets determine that undrained failure occurred in the form of small rotational retrogressive slides. The measured strength profile is compared to that obtained from back-analysis and found to be in substantial agreement if relatively small earthquake ground motions are considered (∼0.05 g). Pre-conditioning factors promoting instability of the slope included oversteepening, reduction of shear strength along the failure plane and overloading due to differential compaction and faulting associated to a deep-rooted inactive volcanic dome. Seismic activity associated with these faults is considered as the most likely triggering mechanism since regional earthquakes do not appear to induce accelerations large enough. The slope resulting from the slide appears to be presently safe both under static drained and undrained conditions. An earthquake that is able to induce accelerations of ∼0.1 g is needed to produce permanent deformations of the slope, which could finally induce failure.