Conclusions

Given the our present state of knowledge attention is drawn to the fact that large areas of ocean floor not only in the NE Atlantic but also in the Norwegian and Greenland Seas are characterised by sediments that owe their origins to underwater slides and slumps. Most information is available for the Storegga Slides west of Norway. The ages of the slides are reasonably well known, whereas the ages of most of the other slides are not known. Most information is available for the Second Storegga Slide that took place approximately 7,100 years ago and which is believed to have generated a very large tsunami not only across the North Atlantic but also in the North Sea, Norwegian Sea and Greenland Sea. Tsunami flood levels in western Norway were locally as high as +20m, while in Scotland the generally accepted view that the highest flood levels reached between +4 and +6m OD. Geological analyses of tsunami sediments in the Shetland Islands also indicate the former occurrence of a large tsunami circa 5,700 years ago. The origin of this tsunami is not known. We consider here that most underwater slides and associated tsunamis were triggered either by the release of methane gas from seabed sediments or as a result of moderately high-magnitude offshore earthquakes. It is possible also that gas release and earthquake activity may have acted together to induce sea floor instability.

It should be noted that it is extremely unwise to locate major oil rig installations with foundations built upon fossil landslide deposits. Attention is also drawn in this report to the continental slope edge as the zone most likely to experience slope failure. In ocean areas located above such slope failures, strong vertical water motions may present an important risk to oil installations. We also draw attention to the potential dangers to oil installations and FPSOs in respect of the likely locations of future seafloor slope failures along the continental slope edge west of Norway.