A Future 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:-

1. 3 major slides have taken place during the last 30,000 years. True.
   
2. 3 known major slides have taken place during the last 1 million years. True.
   
3. 3 major slides have taken place within an interval of 25,000 years (30,000-5,000 years BP). True.
   
4. 2 major slides (Storegga 2 and 3) have taken place within a time interval of circa 1,300 years. True.

There is the obvious hazard that future slumps and slides on the continental slope may be triggered by offshore earthquakes. There is a possibility that a future offshore earthquake of sufficient magnitude could lead to a slope failure. Another mechanism of slope failure (less well understood) involves the release of methane gas compounds as a mechanism by which seabed sediments may be subject collapse and failure. The process of clathrate production in underwater sediments has been widely discussed in the geological literature for many years, and although detailed discussion of these processes is beyond the remit of this report, it should be noted that gas release should be considered seriously as a possible mechanism for seabed sediment failure.