Major Offshore Earthquakes and Tsunamis
Introduction
Any consideration of offshore earthquake activity in the northeast Atlantic and Norwegian Sea is limited by the relatively short period of time for which accurate historical information is available. In the following section an account is given of offshore earthquakes that could possibly have generated tsunamis over the area of the Storegga Slides. The discussion is confined only to the highest magnitude earthquakes and is based largely on the NORSAR report of Ringdal et al. (1982) as well as those of Nielsen et al. (1984), Bungum et al. (1986), Engell-Sorensen and Havskov (1987) and Havskov et al. (1989) (Figures 3-5).
Submarine Slides, Tsunami Risk and Offshore Seismicity
Submarine slides are common features of the eastern Atlantic continental slope and outer continental shelf edge. Any submarine slide that moves comparatively rapidly is likely to generate a tsunami. Investigations of fossil slides in the Storegga area shows that three major slides have taken place during the last 30,000 years and that two of these slides are two of the largest features in the world. Any realistic assessment of tsunami risk to the UK coastline has to consider this area as the key area in which large tsunamis can be generated in the future. Since major submarine slide activity has taken place in the Storegga area on at least three occasions in the geologically recent past and since additional older slides have recently been discovered beneath the main slide complex, there is a risk that a major landslide and tsunami may take place in the future in this area. This is a subject for particular concern since the NORSAR earthquake analysis of Ringdal et al. (1982) shows that a number of historical earthquakes in the Norwegian Sea area have taken place over the scarp slopes of the Storegga Slides with some of the largest offshore earthquakes in the last 30 years having taken place in the Storegga area (cf. also Havskov et al. 1989) (Figures 3-5).
The trigger mechanisms that initiate submarine landslides in this area are not well understood. Long et al. (1990) suggested that in the case of the Storegga Slides, slope failure may possibly have taken place as a result of the release of shallow gas, of biogenic or petrographic origin, that occurs in either free phase or in solution but often as gas hydrates beneath areas of deeper water on the continental slope. Any phase change from solid (as a gas hydrate or clathrate) to gaseous, may have led to a rapid increase in pore pressure and a reduction in the shear strength of the sediments, thereby causing slope failure (Long et al., 1990). The latter authors also considered that the slides (and hence the tsunamis) may have been triggered by offshore earthquakes. (Long et al., 1990). Recent research has suggested that high magnitude seismicity (M>6.5-7.0) along 'stable' continental areas is restricted to passive margins or within failed rifts. Long et al. (1990) have pointed out that the Storegga area is located at the junction of the North Sea/Viking Graben rift and the continental margin and hence may be susceptible to high-magnitude seismicity. If it was the case that the Second Storegga Slide was generated by a high-magnitude earthquake, there is a considerable likelihood that a future high magnitude earthquake in this area would trigger a new slide and a major tsunami.
The greatest risk of a future submarine slide-generated tsunami affecting the UK coast therefore may resolve into an estimate of the probability of a magnitude 6.0-7.0 earthquake taking place in the Storegga area. This, however, is not an easy task although some estimate could be made. Caution should be exercised, however, since gas release rather than an earthquake may trigger such a submarine slide.
Another approach to the problem of estimating risk would be to make a statistical assessment of the frequency of observed submarine slides in the Storegga area in order to make a probabilistic assessment of the likelihood that another slide might take place during the next thirty years. However, from a statistical point of view this is almost impossible since, in order to accomplish this, one would have to choose whether the probability estimate of another slide and tsunami originating in the Storegga area within the next thirty years should be based on one of the following observations:-