Landslides and landslide deposits on the seafloor

Figure: Submarine slide at Finneidfjord, June 20, 1996. Figure produced using data collected using a multibeam echosounder ( J.S. L'Heuereux).

Mapping landslides underwater is complicated and expensive, which is not helped by that fact that these slides can be triggered on slopes with grades of less than one degree. Therefore, vast areas must be studied. In the same manner as on land, submarine landslides occur when sediment that becomes unstable moves to greater depths as the result of changes in the physical conditions on the seafloor. The sea floor is not always a flat and quiet place. Often, its topography is rough and changing, as it is shaped by currents, waves and tides. The Storegga Landslide occurred 8200 years ago on the continental shelf outside Møre and Romsdal and is the largest submarine landslide known to date. This slide was the size of Iceland and created a tsunami 15 to 20 meters high along the coast of Norway. Loose soil can move many times faster under water than in air. Even where the seafloor is nearly flat, the mass can attain a speed of over 100 kilometers per hour. 

Clay-slides that have started within lakes and reached inland have occurred several times in Norway. An example is Sokkelvika i Nordreisa, Troms, on May 7, 1959, where parts of the steep slope that formed the lake bottom failed, creating a submarine slide that reached far onto land (fig. 1). Nine people were killed. Another example is Finneidfjord, in June 1996, when a wave from a submarine landslide engulfed several houses and part of the E6 highway slid out. Four people lost their lives (Fig. 2). Human activity has repeatedly triggered shoreline events over the last ten years. Landfill dumping along shorelines can result in the sliding of both the landfill material and the existing underlying material. Examples of this are the landslide of 1988 in Balsfjord, Troms, where two people died, and the collapse of an area built up by landfill in the harbour of Kristiansand in 2008 (fig. 3).

At NGU, we have worked extensively to understand where and how submarine landslides occur, and map high risk areas. We study the sea floor by using sonar, multibeam echo-sounders, core samples, seabed samples, seismic surveys etc.