The role of sedimentation rate on the stability of low gradient submarine continental slopes

Abstract

Submarine landslides at open continental slopes are the largest mass movements on Earth and can cause damaging tsunamis. To be able to predict where and when such large landslides may occur in the future requires fundamental understanding of the mechanisms that cause them. Due to the inaccessibility of these features this understanding is based on poorly tested hypotheses. Recent studies have proposed that more landslides occur during periods of sea level rise and lowstand, or during periods of rapid sedimentation. These hypotheses are tested by comparing a comprehensive global data set of ages for large submarine landslides to global mean sea level and local sedimentation rates. The data set does not show statistically significant patterns, trends or clusters in landslide abundance, which suggests that the link between sea level and landslide frequency is too weak to be detected using the available global data base. The analysis also shows no evidence for an immediate influence of rapid sedimentation on slope stability, as failures tend to occur several thousand years after periods of increased sedimentation rates. Large submarine landslides occur on remarkably low slope gradients (<2?), which makes them difficult to explain. A widely used explanation for failure of such low angle slopes is high excess pore pressure due to rapid sedimentation and/or focused pore fluid flow to the toe of the slope. If these hypotheses are universal, and therefore also hold for continental margins with comparatively low rates of sediment deposition (where numerous large landslides are observed), is tested in this thesis. Fully coupled 2D stress-fluid flow finite element models are created that simulate the excess pore pressure and drainage response of a continental slope to the deposition of new sediment. Homogeneous models with a wide range of physical-mechanical properties as well as models with an aquifer are loaded by low rates of sediment deposition. All models turn out stable and resulting excess pore pressures are too low to significantly decrease effective stress anywhere along the slope. Hence, factors other than sediment deposition must be fundamental for initiating slope failure, at least in locations with slow sedimentation rates. The results obtained in this thesis not only indicate that failure mechanisms that have previously been considered important may not be universal. They also emphasise the large uncertainties in our current understanding of the occurrence, timing and frequency of large submarine landslides at open continental slopes.

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