EVIDENCE FOR LONG-TERM INSTABILITY IN THE STOREGGA SLIDE REGION OFF WESTERN NORWAY

Shallow-seismic surveys around the Storegga Slide off western Norway h ave allowed greater understanding of the development of this part of t he European margin. The northern flank of the scarp is formed of seism ically well-layered, hemipelagic and distal-glaciomarine deposits in w hich a variety of fluid-escape structures, probably due to gas, are lo cally abundant. There is evidence of slides that substantially pre-dat e the earliest slide previously recognized. Surveying on the North Sea Fan to the southwest of the Storegga Slide shows the markedly differe nt nature of the autochthonous sediments on the southern flank of the Storegga Slide; there is a predominance of glacigenic debris flows in the upper part of the sequence, lesser maximum slopes, and an apparent absence of interstitial gas and/or hydrates. This contrast has had co nsiderable effect on slope stability and has influenced the position o f the southwestern Storegga Slide boundary. The North Sea Fan successi on includes at least three major buried slides, termed the Vigra, More and Tampen slides, all of which substantially pre-date the Storegga e vent and probably pre-date predominantly glacigenic margin sedimentati on. Post-late Weichselian slope failure is locally significant. The re gion has a long, but as yet chronologically poorly defined history of instability in which seismic triggering is considered to have been imp ortant.