The Storegga Slides

The most extensive and complex area of submarine slides along the continental shelf and continental slope of the eastern North Atlantic region occurs in the Storegga area located off the west coast of Norway. In marked contrast, there are no fossil submarine slides in the North Sea region since the area is characterised by an extensive area of shallow and relatively flat continental shelf. Thus, whereas tsunamis can theoretically be generated in the North Sea region as a result of offshore earthquakes, it is impossible to generate tsunamis in this area as a result of submarine slides. The last major earthquake that took place in the central North Sea region took place in 1931 and did not generate a tsunami.

The Storegga submarine landslide complex in the Norwegian Sea is one of the largest areas of slope failure in the world (Figure 1). In this area, three very large underwater landslides are known to have taken place during the last 100,000 years (Bugge et al., 1987; Jansen et al., 1987). Seismic stratigraphic surveys in the area have also shown that several (presently undated) older slides also occur beneath the main Storegga Slide complex (Evans et al., 1996). The most detailed information for palaeotsunamis associated with these fossil slides is for the Second Storegga Slide that took place circa 7,000 years ago. The tsunami generated by this landslide is believed to have been the most widespread coastal flood known to have struck coastlines bordering the eastern North Atlantic and North Sea. Several sites showing possible former tsunami inundation associated with the Second Storegga Slide have been reported from western Norway and have been associated with flood runup values as high as 20m (Svendsen and Mangerud, 1990; Bondevik and Svendsen, 1993) (Figure 1).

The three principal submarine slides that occur in the Storegga area were first described by Bugge (1983) and detailed accounts of the features have been subsequently provided in several papers (Bugge et al., 1987; Jansen et al., 1987; Bugge et al., 1988). Later, Dawson et al. (1988) suggested that the Second Storegga submarine landslide generated a large tsunami that led to extensive flooding of coastlines bordering the Norwegian Sea and North Sea. These authors showed that evidence for such flooding was evident in eastern Scotland where many Holocene coastal sequences contain a layer of sediment thought to have been deposited by this tsunami. Detailed field evidence for the tsunami was provided in a subsequent paper (Long et al., 1989) while additional accounts describe the potential risk to the northwest Europe coastlines posed by such landslide-generated tsunamis (Long et al., 1990; Smith and Dawson, 1990). More recently, Svendsen and Mangerud (1990), Hansom and Briggs (1991) Bondevik and Svendsen (1993) and Bondevik (1996) have described evidence from sites in western Norway and northwestern Iceland for the same tsunami at this time. More recently, it has been suggested that this tsunami may have caused severe flooding as far south as the Netherlands (De Groot, pers. comm.).

These studies have been paralleled by the development of numerical models of the tsunami generated by the Second Storegga Slide (Harbitz, 1991, 1992) and Henry and Murty (1992). These research investigations have been particularly valuable since they have enabled comparisons to be made between the theoretical model predictions and the empirical field information on tsunami run-up. In addition, studies have been undertaken on the physical characteristics of the coastal sediments considered to have been deposited by this tsunami (Dawson et al., 1991; Shi et al., 1993) and this has helped determine former maximum flood levels at the coast.

Ages of the First, Second and Third Storegga Slides

The oldest of the three Storegga Slides (known as the First Storegga Slide) was one of the world's largest slides and involved a movement of approximately 3,880 km3 of sediment. The age of this slide is not known with any accuracy although estimates of its age based on oxygen isotope stratigraphy suggest values between circa 28,000 - 30,000 years (E. Jansen, pers. comm.).

The Second and Third Slides appear to have taken place on separate occasions between 8,000 and 6,000 years ago (Jansen et al., 1987). The older of these two slides, which involved the translation of more consolidated sediments than the First Slide, cut back 6-8 km headwards beyond the First Slide and removed approximately 450 km2 of the continental shelf edge. This slide involved the movement of some very large quantities of sediment. In particular, two blocks, 150-200 m thick and up to 10-30 km wide, were transported as far as 200 km downslope over seabed surfaces with average slopes of no greater than 1-2°. Radiometric dating of coastal sediments in Scotland that were deposited by the tsunami caused by this slide suggests that it took place at approximately 7,000 years ago (Smith et al., 1991).

The Third Storegga Slide was limited to the upper part of the Second Slide scar area and may represent the final stages of movement of the Second Slide. Estimates of the age of this slide are placed near 6,000 years by Jansen et al. (1987).