- Submarine landslide
Submarine landslides are
marinelandslides that transport sedimentacross the continental shelfand into the deep ocean. A submarine landslide is initiated when the downwards driving stress(gravity and other factors) exceeds the resisting stressof the seafloor slope material causing movements along one or more concave to planer rupture surfaces. Submarine landslides take place in a variety of different settings including planes as low as 1° and can cause significant damage to both life and property. Recent advances have been made in understanding the nature and processes of submarine landslides through the use of sidescan sonarand other seafloor mapping technology (Hampton et al. 1996; Locat & Lee 2002; Mason et al. 2006).
Submarine Landslides have different causes which relate to both the geological attributes of the landslide material and transient environmental factors affecting the submarine environment. Common causes of landslides include: i) presence of weak geological layers, ii) overpressure due to rapid accumulation of sedimentary deposits, iii)
earthquakes, iv) storm wave loading and hurricanes, v) gas hydrate dissociation, vi) groundwaterseepage and high pore water pressure, vii) glacial loading, viii) volcanic islandgrowth, and ix) oversteepening (Hampton et al. 1996; Locat & Lee 2002; Mason et al. 2006).
Examples of causes
The presence of weak geological layers is a factor which contributes to submarine landslides at all scales. This has been confirmed by seafloor imaging such as
swath bathymetric mappingand 3D seismicsystems. Despite their ubiquity very little is known about the nature and characteristics of the weak geological layers, as they have rarely been sampled and very little geotechnicalwork has been conducted on them. An example of a slide which was caused by weak geological layers is the Storegga slide, near Norwaywhich had a total volume of 3300km3 (Mason et al. 2006; Locat et al. 2003).
Overpressure due to rapid deposition of sediment is closely related to weak geological layers. An example of landslides caused by overpressure due to rapid deposition occurred in 1969 on the
Mississippi deltaafter Hurricane Camilestruck the region (Locat & Lee 2002).
Earthquakes are a key factor which trigger most major submarine landslides. Earthquakes provide significant environmental stresses and can promote elevated pore water pressure which leads to failure. Earthquakes triggered the
Grand Bankslandslide of 1929, where a 20km3 submarine landslide was initiated after an earthquake (Mason et al. 2006; Nisbet & Piper, 1998).
Stormwave loading and hurricanes can lead to submarine landslides in shallow regions and were recognised as one of the factors which contributed to the slides which occurred on the Mississippi delta in 1969 following Hurricane Camille (Locat & Lee 2002).
A number of studies have indicated that gas hydrates lie beneath many submarine slopes and can contribute to the triggering of a landslide. Gas hydrates are ice like substances consisting of water and natural gas, which are stable at the temperature and pressure conditions normally found on the seabed. When the temperature rises or the pressure drops the gas hydrate becomes unstable allowing some of the hydrate to dissociate and discharge bubble phase natural gas. If pore water flow is impeded then this gas charging leads to excess pore water pressure and decreased slope stability. Gas hydrate dissociation is thought to have contributed to slides at water depths of 1000 to 1300m off the east coast of the United States and the Storegga slide off the east coast of
Norway(Huhnerbach et al. 2004; Locat & Lee 2002).
Groundwater seepage and elevated pore water pressure can cause submarine landslides. Elevated pore water pressure causes reduced frictional resistance to sliding and can result from normal depositional processes, or can be coupled with other causes such as earthquakes, gas hydrate dissociation and glacial loading (Mason et al. 2006).
Sediment failure on glacial margins as a result of glacial loading is common and operates on a wide spectrum of dimensions, ranging from relatively small scale mass wasting processes in fjords to large scale slides covering several thousand square kilometres. Factors which are significant in glacial loading induced landslides are the flexing of crust due to the loading and unloading of a fluctuating ice front, variation in drainage and groundwater seepage, quick deposition of low
plasticity silts, rapid formation of morainesand tillsabove hemipelagic interstaidal sediments. An example where glacial loading lead to submarine landsliding is the Nyk slideof northern Norway(Lindberg et al. 2004; Locat & Lee 2002; Vanneste et al. 2006).
Slope failures due to volcanic island growth are among largest on earth involving volumes of several cubic kilometres. The failure occurs as large bodies of lava form above weak marine sediments which are prone to failure. Failure is particularly common on edifices which are over 2500m but rare on edifices which are less than 2500m. Variation in the behaviour of the slides is significant with some slides barely keeping up with the growth on the upper part of the volcano while others may surge forward great distances attaining landslide lengths greater than 200km. Volcanic island submarine landslides occur in places such as the
Hawaiian islandsas shown in figure 1 (Hampton et al. 1996; Mitchell, 2003; Moore et al., 1994).Oversteepening is caused by scouring due to oceanic currents and can result in the triggering of submarine landslides (Locat & Lee 2002).
In some cases the relationship between the cause and the resulting landslide can be quite clear (eg: the failure of an oversteepened slope) while in other cases the relationships may not be so obvious. In most cases more than one factor may contribute towards the initiation of a landslide event. This is clearly seen on the Norwegian continental slope where the location of landslides such as
Storeggaand Traenadjupetis related to weak geological layers. However the position of these weak layers is determined by regional variation in sedimentation style, which itself is controlled by large scale environmental factors such as climate change between glacialand interglacialconditions. Even when considering all the above listed factors, in the end it was calculated that the landslide needed an earthquake for it to ultimately be initiated. (Hampton et al., 1996; Mason et al. 2006).
The environments in which submarine landslides are commonly found in are
fjords, active river deltason the continental margin, submarine canyon fan systems, open continental slopes, and oceanic volcanic islands and ridges (Hampton et al., 1996).
ubmarine landslide processes
There are a variety of different types of submarine mass movements as demonstrated in figure 2. All of the movements shown above are mutually exclusive, for example a slide cannot be a fall. Some types of mass movements, such as slides, can be distinguished by the disrupted step like morphology which shows that there was only minor movement of the failed mass. The displaced material on a slide moves on a thin region of high strain. In flows the slide zone will be left bare and the displaced mass may be deposited hundreds of kilometres away from the origin of the slide. The displaced sediment of fall will predominantly travel through the water, falling, bouncing and rolling. Despite the variety of different landslides present in submarine environment, only slides, debris flow and turbidity currents provide a substantial contribution to gravity driven sediment transport (Locat & Lee 2002; Mason et al. 2006).
Recent advances in 3-D seismic mapping have revealed spectacular images of submarine landslides off Angola and Brunei, showing in detail the size of blocks transported and how they moved along the sea floor(Gee et al. 2005, 2007)
It was initially thought that submarine landslides in cohesive sediments systematically and sequentially developed downslope from slide to debris flow to turbidity current through slowly increasing disintegration and entrainment of water. However it is now thought that this model is likely to be an oversimplification, as some landslides travel many hundreds of kilometres without any noticeable change into turbidity currents, as shown in figure 3 while others completely change into turbidity currents near to the source. This variation in the development of different submarine landslides is associated with the development of velocity vectors in the displaced mass. The in-place stress, sediment properties (particularly density), and morphology of the failed mass will determine whether the slide stops a short distance along the rupture surface or will transform into a flow which travels great distances (Hampton et al. 1996; Locat and Lee, 2002).
The initial density of the sediment plays a key role in the mobilization into flows and the distances that the slide will travel. If the sediment is a soft, fluid material then the slide is likely to travel great distances and a flow is more likely to occur. However if the sediment is stiffer then the slide will only travel a short distance and a flow is less likely to occur. Furthermore the ability to flow may also be dependent upon the amount of energy transferred to the falling sediment throughout the failure event. Often large landslides on the continental margin are complicated and components of slide, debris flow and turbidity current may all be apparent when examining the remains of a submarine landslide (Hampton et al. 1996; Huhnerbach et al. 2004; Locat & Lee 2002; Gee et al. 2007).
The primary hazards associated with submarine landslides are the direct destruction of infrastructure and tsunami.
Landslides can have significant economic impacts on infrastructure such as the rupture of fibre optic submarine cables and pipelines and damage to offshore drilling platforms and can continue onwards on slope angles as low as 1°. An example of submarine cable damage was discovered in the Grand Banks slide of 1929 where the landslide and resulting turbidity current broke a series of submarine cables up to nearly 600 km away from the beginning of the slide (Hampton et al., 1996; Mason et al., 2006; Nisbet & Piper 1998). Further destruction of infrastructure occurred when Hurricane Camille hit the Mississippi delta in 1969 causing a landslide which damaged several offshore drilling platforms (Locat & Lee, 2002).
Submarine landslides can pose a significant hazard when they cause a tsunami. Although a variety of different types of landslides can cause tsunami, all the resulting tsunami have similar features such as large run-ups close to the tsunami, but quicker attenuation compared to tsunami caused by earthquakes. An example of this was the July 17, 1998 Papua New Guinean landslide tsunami where waves up to 15m high impacted a 20 km section of the coast killing 2,200 people, yet at greater distances the tsunami was not a major hazard. This is due to the comparatively small source area of most landslide tsunami (relative to the area affected by large earthquakes) which causes the generation of shorter wavelength waves. These waves are greatly affected by coastal amplification (which amplifies the local effect) and radial damping (which reduces the distal effect) (Mason et al., 2006; Mc Adoo & Watts, 2003).
Recent findings show that the nature of a tsunami is dependent upon volume, velocity, initial acceleration, length and thickness of the contributing landslide. Volume and initial acceleration are the key factors which determine whether a landslide will form a tsunami. A sudden deceleration of the landslide may also result in larger waves. The length of the slide influences both the wavelength and the maximum wave height. Travel time or run out distance of slide will also influence the resulting tsunami wavelength. In most cases the submarine landslides are noticeably subcritical, that is the Frounde number (the ratio of slide speed to wave propagation) is significantly less than one. This suggests that the tsunami will move away from the wave generating slide preventing the build up of the wave. Failures in shallow waters tend to produce larger tsunamis because the wave is more critical as the speed of propagation is less here. Furthermore shallower waters are generally closer to the coast meaning that there is less radial damping by the time the tsunami reaches the shore. Conversely tsunamis triggered by earthquakes are more critical when the seabed displacement occurs in the deep ocean as the first wave (which is less affected by depth) has a shorter wavelength and is enlarged when travelling from deeper to shallower waters (Mason et al., 2006; Mc Adoo & Watts, 2003).
The effects of a submarine landslide on infrastructure can be costly and landslide generated tsunami can be both destructive and deadly.
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