The role of sedimentation rate and permeability in the slope stability of the formerly glaciated Norwegian continental margin: the Storegga slide model

Abstract

Despite the gently dipping slopes (ca 1°), large-scale submarine slope failures have occurred on the mid-Norwegian continental margin (Storegga, Sklinnadjupet, Traenadjupet), suggesting the presence of special conditions predisposing to failure in this formerly glaciated margin. With a volume estimated between 2,400 and 3,200 km 3 and an affected area of approximately 95,000 km 2 , the Storegga slide represents one of the largest and best-studied submarine slides of Holocene age known worldwide. Finite element modeling of slope failure indicates that a large (6.5< Ms<7.0) seismic triggering mechanism would not be sufficient to cause failure at more than 110 m below the seabed as observed for the slip planes at Storegga (northern sidewall). This implies that other factors (e.g., liquefaction, strain softening, gas charging, rapid burial) are needed to explain the occurrence of the Storegga slide with a deep surface of failure. In this paper, we discuss the importance of the compaction effect of rapidly accumulated sediments in the slide area. During compaction, sediment grains reorganize themselves, thereby, expelling pore water. Consequently, depending on sedimentation rate and permeability, excess pore pressures might result beneath less permeable sediments. Our modeling and cross-checking illustrate how excess pore pressure generation due to high sedimentation rate could explain the development of layers of weakness, and thus, how such a large slide might have been initiated in deep sediments. Using the highest sedimentation rate estimated in the area (36 and 27 m/kyr between 16.2 and 15 kyr BP), 1D modeling shows excess pore pressure values of around 200 kPa at a depth of 100 m below the seafloor 15 kyr BP and 60 kPa at a depth of 100 m at the time of the slide (8 kyr BP). Excess pore pressure apparently drastically reduced the resistance of the sediment (incomplete consolidation). In addition, 2D modeling shows that permeability anisotropies can significantly affect the lateral extent of excess pore pressure dissipation, affecting, that way, normally consolidated sediments far from the excess pore pressure initiation area.Landslides 4 • (2007) 309

Submarine mass movements on glaciated and non-glaciated European continental margins: A review of triggering mechanisms and preconditions to failure

“…Thus, rapid changes in sedimentation rates could have been responsible for excess pore pressure build up in the Storrega area (e.g. Leynaud et al, 2007). Since several pre-Holocene slides have been observed in the surrounding area of the Storegga Slide (i.e.…”

“…due to gas hydrate dissociation) or migrating laterally (pressure gradient) in a fully consolidated sediment might trigger timedependent viscoplastic deformation leading to more favourable conditions to failure after a specific time period. This might be the case during lateral dissipation of excess pore pressure after a high sedimentation rate period (Leynaud et al, 2007). However, such conditions are not enough to trigger a slide on a 1 slope.…”

Timing and frequency of large submarine landslides: implications for understanding triggers and future geohazard

“…margin is the third failure in this otherwise quiet period. Laberg et al (2003) suggest that a contourite underlying the Traenadjupet slide acted as a mechani- Flemings, 2000;Bryn et al, 2005;Leynaud et al, 2007). A delay time is 1026 necessary for the fluid migration to take place which mainly depends on the 1027 permeability of the sediment as well as the distance the fluid has to travel, and 1028 may involve several thousand to a million years (Dugan and Flemings, 2000;1029Dugan, 2012.…”

Landslides: a review of achievements in the first 5 years (2004–2009)

“…Landslide risk to cultural heritage is socially eye-catching, but not always cited in technical papers. The 2007 best paper was the role of sedimentation rate and permeability in the slope stability of the formerly glaciated Norwegian continental margin: the Storegga slide model by Leynaud et al (2007), but it is not included in this table. Guzzetti et al (2008) had 11 citations; it is the largest number of citations in 2008.…”