Submarine Landslide

Primary reference(s)

Lee, H.J., J. Locat, P. Desgagnés, J,.D. Parsons, B.G. McAdoo, D.L. Orange, P. Puig, F. Wong, P. Dartnell and E. Boulanger, 2007. Submarine mass movements on continental margins. In: Continental Margin Sedimentation. pp. 213-274. Wiley.

Additional scientific description

Submarine landslides occur preferentially in particular environments, including fjords, active river deltas, submarine canyons, volcanic islands and the open continental slope. Evaluating the relative stability of different types of seabed sediment requires an understanding of driving stresses and sediment strength. Stresses can be caused by gravity, earthquakes and storm waves. Resisting strength can be reduced by pore water and gas pressures, groundwater seepage, rapid sediment deposition, cyclic loading and human activity. Once slopes have become unstable or have failed, sediment strength may continue to decrease so, following slope failure, the failed mass moves downslope under the influence of gravity and possibly other forces. If the moving sediment is a viscous fluid, this is termed a mass flow (gravity flow). If the movements are essentially rigid, internally undeformed masses along discrete slip planes, they are termed slides. If the movement is formed of ‘blocks’ of failed material which rotate along curved slip, they are termed slumps. Another kind of landslide involves movement on a planar surface and is termed a translational slide. In each type, movement can be fast or slow. Extremely slow movement is called creep. Submarine slides can become mass flows (gravity flows) as the failed mass progressively disintegrates and continuous downslope movement occurs. End members of disintegrating slides have different terms. Debris flows are where the sediment is heterogeneous and may include larger clasts supported by a matrix of fine sediment. Mud flows are predominantly muddy sediment. Turbidity currents involve the downslope transport of a relatively dilute suspension of sediment grains that are supported by an upward component of fluid turbulence. Recent submarine landslide research has: (i) shown that landslides and sediment waves may generate similar deposits, which require careful interpretation; (ii) expanded knowledge of how strength develops in marine sediment; (iii) improved techniques for predicting sediment rheology; and (iv) developed methodologies for mapping and predicting the medium- to large-scale regional occurrence of submarine landslides. Based on the identification of the different submarine sediment failures identified above and the classification of subaerial landslides (Varnes, 1958; Hungr, 2014), submarine landslides may be classified as mass sediment movements termed slides (translational and rotational slumps) and mass flows (mudflow, debris flow, liquefaction and turbidity current).

Almost all submarine landslides have multiple causes, which differ significantly to their subaerial counterparts, for example, seabed slope is not that important as shown by the largest volume submarine landslides being located on the shallowest slopes. Submarine landslides are triggered either by an increase in the driving stresses, a decrease in sediment strength, or a combination of the two. The following triggers show the interplay of these factors, but their relative importance is not well understood. For example, in some environments one of these triggers will dominate, whereas in others a different trigger will be most significant. The main triggers identified for submarine landslides are erosion (undercutting the landslide foot), a rapid rate of sedimentation and earthquakes. Erosion is common in deep-sea channels, submarine canyons and other active sediment-transport systems. When seabed surfaces are undercut, this can decrease the stability by increasing shear stress and/or decreasing the shear strength. With underwater earthquakes, the earthquake-induced shear stresses are large relative to sediment shear strength because the earthquake must accelerate all the sediment column including the interstitial water. The sediment shear strength is relatively low because it builds up in proportion to the submerged unit weight of the sediment and may be even lower if there are excess pore pressures. The ratio of driving stress to resisting strength is high relative to that on land. Rapid sediment accumulation contributes to failure in several ways. Because most of the weight of newly added sediment is carried by pore-water pressures. The shear stress acting downslope increases more rapidly. The shear stress may also increase because more sediment may be deposited at the head of the sloping surface than at the toe. In addition, the following may result in failure: retarded sediment shear strength development, increased development of shear stress because of thickness of the sediment body, and increased development of shear stress because of increases in the slope steepness. Metrics and numeric limits Landslide sediment movement has been measured in two events from breakage of submarine telephone cables. These indicate velocities of up to 28 m/s or 101 km/h (Grand Banks, 1929) and 5 to 16 m/s (18–57 km/h) in the Strait of Luzon between Taiwan and the Philippines between 2006 and 2015.

Metrics and numeric limits

Landslide sediment movement has been measured in two events from breakage of submarine telephone cables. These indicate velocities of up to 28 m/s or 101 km/h (Grand Banks, 1929) and 5 to 16 m/s (18–57 km/h) in the Strait of Luzon between Taiwan and the Philippines between 2006 and 2015.

Examples of drivers, outcomes and risk management

In coastal and offshore regions, submarine landslide impacts threaten submarine installations such as oil platforms, pipelines, cables, and wind installation. Mass submarine sediment failures are also one of the most important sources of sedimentation from shallow to deep-water environments and of shaping continental margins (McAdoo et al., 2000). Hence, a better understanding of submarine landslides is of great importance in the development of offshore resources exploration and protection, sustainable flood risk management, hazard assessments for engineering and environmental projects, and also in hydrocarbon reservoir managements (McAdoo and Watts, 2004; Masson et al., 2006). Development of ideas and understanding of submarine landslides has been based mainly on their role in generating tsunamis, with one example in 1969 where an oil platform in the Gulf of Mexico collapsed when the soft seabed was destabilised during a hurricane. The most important historical event with significant loss of life was in 1998 in Papua New Guinea when a slump generated tsunami killed over 2200 people on the nearby coast. Other important events include the 1929 Grand Banks landslide tsunami in which 27 people died, and in 1964 during the Great Alaska earthquake, when submarine landslides in Resurrection Bay and Port Valdez caused tsunamis that killed 45 people. An additional risk from submarine landslides are submarine telegraph and fibre optic cables. As noted, in 1929 trans-Atlantic telegraph cables off Newfoundland were broken by the Grand Banks landslide, and between 2006 and 2015 submarine telecommunication cables in the Strait of Luzon were broken by turbidite currents.