Submarine landslides are marine landslides that transport sediment across the continental shelf and into the deep ocean. A submarine landslide is initiated when the downwards driving stress (gravity and other factors) exceeds the resisting stress of the seafloor slope material, causing movements along one or more concave to planar 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 sonar and other seafloor mapping technology.
Causes 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) groundwater seepage and high pore water pressure, vii) glacial loading, viii) volcanic island growth, and ix) oversteepening.
Weak geological layers 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 mapping and 3D seismic reflection data. 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 geotechnical work has been conducted on them. An example of a slide which was caused by weak geological layers is the Storegga Slide, near Norway which had a total volume of 3,300 km3.
Overpressuring 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 River Delta after Hurricane Camile struck the region.
Earthquakes 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 Banks landslide of 1929, where a 20 km3 submarine landslide was initiated after an earthquake. In the Mediterranean Sea, most submarine landslides on record appear to occur due to earthquakes or slope erosion, as in the development of submarine canyons.
Stormwave loading 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.
Gas hydrates 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 1300 m off the east coast of the United States and the Storegga slide off the east coast of Norway.
Groundwater seepage 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.
Glacial loading 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 moraines and till above hemipelagic interstaidal sediments. An example where glacial loading leads to submarine landsliding is the Nyk slide of northern Norway.
Volcanic island growth
Slope failures due to volcanic island growth are among the 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 2500 m but rare on edifices which are less than 2500 m. 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 200 km. Volcanic island submarine landslides occur in places such as the Hawaiian Islands and the Cape Verde Islands.
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