The Storegga Slide has affected an area of approximately 95 000 km(2), and a sediment volume in the range of approximately 2400-3200 km(3) has been displaced. Around 250 km(3) of the volume has been deposited as turbidite sediments in the Norway Basin. This volume places the Storegga Slide event as one of the world's largest exposed submarine slides. Based on a comprehensive database, detailed morphological investigations and a dating programme have been performed to reveal the slide process and the timing of the slide. To date the slide event, detailed analyses of 89 cores within the Storegga Slide region have been undertaken. The investigations conclude: (a) The main Storegga Slide event represents one retrogressive event dated to be 7250 +/- 250 C-14 yrs BP or approximately 8100 +/- 250 cal. yrs BP. This age also corresponds with the age of a tsunami found along the western coast of Norway. (b) A few minor slide/slump events have been identified along the northern Storegga Slide escarpment, dated to be c. 5000 C-14 yrs BP and 2500-3000 C-14 yrs BP or approximately 5700 and 2200-2800 cal. yrs BP. The total volume of these events is interpreted to be less than 1 km(3) or c. 0.1% of the total volume calculated for the main Storegga Slide event. (c) The statistical analyses carried out on the lobes morphometrical parameters show a fairly good correlation (R-2=0.8-0.9) between the smallest and the medium/large size debris lobes within the Ormen Lange Field area. This means that the theological properties for this area can be described as fairly uniform for the slide masses and scaling of the morphometrical parameters should be possible with a great confidence. (c) 2005 Elsevier Ltd. All rights reserved.

We perform a statistical study of submarine debris flows based on recent data from the Storegga area off the western coast of Norway. Due to the high quality of the data and the uniformity of materials, the data from the Storegga database are far less dispersed than world-wide compilations available so far, allowing for a fit with good statistical significance. We find that both the run-out and the ratio between fall height and run-out are best described as a power-law function of the volume. For small to moderate volumes, visco-plastic models are able to reproduce the power law, in contrast to granular-frictional models. However, either progressive mixing of the shear layer with ambient water or hydro-planing must be invoked in order to explain the extreme run-out distances of larger debris flows.

The Storegga slide, released about 8200 years ago off the western coast of Norway, is a complex system of debris flows which occurred on the remains of previous Pleistocenic slides materials. Following geological mapping and interpretation of data gathered with geological and geophysical methods, the Storegga slide has been interpreted as consisting of a giant slide (volume ≈3100 km3 and runout of about 400 km) followed by a swarm of progressively smaller events. The flow of the giant slide, object of the present, study, represents a dynamical enigma. Indeed, if one simulates the flow of the giant slide with simple non-Newtonian soil properties constant along the path, extremely small yield strength must be attributed to the material. This finding is incompatible with observations of the smaller slides, where the simplest non-Newtonian soil properties models work well with a yield strength more than one order of magnitude larger. A possible conclusion is that the material has charged its properties during the flow. Numerical simulations are conducted with a newly introduced model where in analogy with soil mechanical models, the material progressively loses strength during the flow. The model gives results comparable to field data if the shear strength decreases by at least a factor of 30 during flow.

Two large submarine slides, The Storegga and the Traenadjupet Slides, occurred on the Mid-Norwegian margin during the Holocene. The Ormen Lange gas field is located within the scar of the Storegga Slide. This gigantic submarine slide occurred about 8200 years ago, and caused large waves (tsunamis) that reached the coasts of Norway, Scotland, Shetland and the Faroe Islands. The objectives of this chapter are to present the challenges and the slide risk assessment related to the development the Ormen Lange gas field. The risk evaluation is based on a qualitative approach for large natural slides, and a quantitative approach for new small slides in the vicinity of the development area.The work programme includes extensive, regional multi-disciplinary studies, carried out jointly by academia, industry and research institutions. The database includes an extensive grid of seismic data, detailed sea-floor morphology and sediment properties from a number of 'geoborings' (combined geological and geotechnical borings to sub-bottom depths of 200-400 m). Stability of the steepest slopes in the vicinity of the development area is calculated. Effects of excess pore pressures, earthquakes, reservoir compaction during depletion and underground gas blowouts into possible permeable layers have all been included in the stability calculations. To understand the recent slide history in the area and to find the frequency of the sliding, extensive sea-floor mapping and coring to date slide events are also included.A geological model for the Plio-Pleistocene of the area explains the large-scale sliding as a response to climatic variability. Over long periods, marine deposition prevailed with focused deposition due to current effects in the locations of the Storegga and the Traenadjupet Slides. During short intervals of peak glacial conditions, till and glacial debris flow sediments were deposited at high rates directly on the continental slope. This created excess pore pressures in the thick marine deposits. The most likely triggering mechanism of the slides is a strong earthquake following the onshore uplift after the glaciation. This explains why the slides take place after a glacial period. Since all the soft unstable clays were removed from the Storegga Margin during the last slide, it is concluded that a new cycle with sedimentation of soft clays and deposition of glacial sediments in the upper slopes are needed, to create a new unstable situation in the Storegga area. At present, the slopes in the Ormen Lange area have high safety factors, and the likelihood of new slides, both local and regional, is considered very low.

In this paper, slides and debris flows in overconsolidated clay materials are simulated numerically. As a case study, the models are applied to the Storegga slide in the Norwegian Sea and in particular to the sub-region called Ormen Lange, where the information available is the most precise for a subaqueous debris flow. Three different models for the rheology of clay are used: a viscoplastic (Bingham) fluid model, a viscoplastic fluid with interspersed solid blocks, and a viscoplastic model with yield strength increasing with depth. The small-scale debris flows in the Ormen Lange area can be reasonably well understood in terms of a pure Bingham model without granular effects and blocks. The presence of intact blocks in the region, however, indicates that at least the top layer of the sliding sediments was not destroyed by the flow. It suggests that the flow occurred mainly at high shear rate in a lubricating layer of mud deriving partly from the disintegration of the block's own material, and possibly from the entrainment of hemipelagic sediments along the flow path while the top part was left unsheared. The failure of the model with blocks probably stems from the use of the Coulomb friction law to represent the interaction between the block and the seabed. The Bingham model works better because during the flow of such fluids an unsheared plug region is formed naturally, even in unconsolidated materials. Combining the simulations with these three models, a possible scenario for the Ormen Lange debris flows is deduced according to which the lubricating layer supporting the blocks has a yield strength of about 10–15 kPa.

Seismic profiles and sediment cores from sixteen fiords and five lakes in western Norway have been investigated in a search for Holocene mass-movement deposits. Tsunami deposits caused by the Storegga Slide (8200 cal. BP) are observed over most of the investigated area, both in fords and in lakes. Five fords provide evidence for a 2000-2200 cal. BP mass-movement event. Debris flow deposits and turbidites related to that event occur in Sunnmore and Sunnfjord, suggesting triggering by one or more earthquakes close to the coast or on land. Similar mass-movement deposits occur in the same geographical area at 11 000-11 700 cal. BP. A period of debris flows, turbidity currents and snow avalanches, interpreted to be related to climatic irregularities, occurred around 2800-3200 cal. BP. Such events are recorded also from other periods, e.g. 1700-1800 cal. BP and 5300-5600 cal. BP, but they only occur in a few basins, and were thus probably related to local weather irregularities rather than regional climatic changes.

The detailed mapping of the Storegga Slide morphological elements and the analyses of the slide development are based on high-quality acoustic and sampling data sets acquired through a cooperation between academia and the petroleum industry. The Storegga Slide has affected an area of c. 95000 km2 and a sediment volume of minimum 2400 km3 and maximum 3200 km3 has been displaced with c. 250 km3 deposited as turbidite sediments in the Norway Basin. This volume places the Storegga Slide event as one of the world largest exposed submarine slides. The Storegga Slide can be divided into six distinctive morphological provinces. Associated, and superimposed, on these provinces a total of 63 slide lobe phases have been identified and mapped. The morphological investigations have furthermore made it possible to generate a set of numerical values for statistical analysis of slide sediment rheology. This knowledge also makes it feasible to model the Storegga Slide. The analyses of the slide have revealed that the slide has developed through a retrogressive process starting most probably on the lower slope. The most likely location for initializing is in an area close to the Faroe–Shetland Escarpment.

Throughout the Pleistocene the sedimentary environment on the SE Nordic Seas continental slope/outer shelf, off western Norway, has been strongly controlled by variability in the Norwegian Atlantic Current (NwAC), glaciations of the shelf areas and sea level changes. Acoustic and core data from the southern Vøring Plateau show a Pleistocene sequence characterised by hemipelagic sediments interfingered by diamictons on the upper slope. The area of the 7.25 14C ka BP Storegga Slide shows evidence of a long history of Pleistocene mega-slides. During the last interglacial, and most likely also during previous interglacials, the slide region has been the locus of rapid deposition between water depths of 800 and 1200 m, as a result of NwAC winnowing along the upper slope. The North Sea Fan region is strongly influenced by glacigenic debris flows (GDFs) deposited during glacial advances reaching the shelf edge, when the Norwegian Channel was occupied by the Norwegian Channel Ice Stream. It appears that GDF activity was initiated at ca. Marine Isotope Stage 12. Interbedded between the debris flow sequences, mega-slide events such as the Møre and Tampen slides have been identified. During glaciations, when the entire SE Nordic Seas continental shelf was covered by extensive grounded ice sheets, basal till were transported to the shelf edge from where subsequent mass movement occurred. During late glaciation/early deglaciation meltwater plumes were released at the time of disintegration of ice streams in the Norwegian Channel, as is evidenced from the last deglaciation of the margin at ca. 15 14C ka BP. The plume material was transported northwards by currents, before rapidly deposited as a thick package within the Storegga Slide area and on the south Vøring Plateau. Based on identification and dating of iceberg scourings, glacial erosion surfaces and delta deposits on the shelf, subsidence rates between 0.7 and 1.2 m/ky have been calculated for the last ca. 250 ka.

A huge slide (volume of 2400 km3 and run-out 450 km) was released in the Storegga area off the western coast of Norway during early Holocene, followed by numerous smaller debris flows. We perform numerical simulations of the giant slide using a Bingham model for the clay material. Agreement with present deposit distribution and run-out is found by assuming that the shear resistance between the debris flow and the seabed decreases during the flow, and we suggest sediment remolding or hydroplaning as possible explanations. Debris velocities are predicted and possible applications to the associated tsunami event are investigated.

The Holocene Storegga Slide is the last of a series of slides occurring in the same area during the last 500ky. The objectives of the present paper are to present the current understanding of the trigger mechanisms and development of the Storegga Slide, and to show the link between the sliding and Pleistocene climatic fluctuations in the area. Instability is created by the rapid loading of fine-grained hemipelagic deposits and oozes by rapid glacial deposition during peak glaciations. Postglacial earthquake activity was the most likely trigger. Although slide development is complicated and involves a number of slide mechanisms and processes, the overall development is retrogressive, starting at the mid- to lower slope. Sliding stops when the headwall reaches the flat lying, overconsolidated glacial deposits of the shelf.

The Traenadjupet Slide (14,100 km2) remobilised an up to 180 m thick package comprising late Weichselian glacigenic sediments and an underlying late Saalian — late Weichselian contourite drift. Rapid burial of the contourites and the presence of gas, is inferred to have caused development of excess pore pressure of the contourites which probably were the "weak layer" that initially failed. During triaxial compressional tests the contourite sediments show contractive behaviour and shear band development. Shear band development due to porewater pressure increase and liquefaction of contractive sediments is therefore regarded a possible mechanism for initial failure and sediment mobilisation of the Trænadjupet Slide.

Statistical analysis of the lobes of the Storegga slide reveals a power-law dependence of the runout distance on the release volume. For small to moderate volumes, visco-plastic models with a (remoulded) yield strength of about 10 kPa reproduce this dependence quite well, in contrast to granular-friction models. However, either progressive wetting of the bottom shear layer or hydroplaning has to be invoked to explain the extreme runout distance and the sediment distribution of the largest slide phase. Preliminary es timates of the turbidite volume put severe constraints on the formation rate and density of the turbidity currents accompanying the slide.

Abstract A joint industry initiative known as the Seabed Project has been established to investigate seabed conditions on the Mid Norwegian continental slope. This is in connection with deep water exploration licences awarded in the 15th Licencing Round, and involves the creation of a regional shallow seismic and geotechnical database covering an area of some 100,000 square kilometres. Building on previous work this paper presents results from the interpretation of over 10,000 km of new and reprocessed 2D seismic. The study has highlighted a number of different geohazards which must be carefully considered when planing future site investigations for exploration purposes. Such assessments can now be completed within the regional seismostratigraphic framework established during the study. Water depths in the study area range between 200 and 2500 metres. The main structural elements are the Mare and Varing Basins together with associated flanking basement highs. Sediments of mid to late Tertiary and Quaternary age have been investigated, and consist predominantly of thick sequences of glacigenic debris flows, slide sediments andunconsolidated hemipelagic sediments. The area includes part of the North Sea Fan and a number of slides and slumps including the Storegga Slide, one of theworlds largest. Other special features include sediment creep, diaprism, shallow faults, shallow gas, fluid escape features, evidence of gas hydrates and igneous intrusions. The understanding of the geological processes, which have shaped the continental slope and given rise to geohazards relevant to future exploration work, has been greatly enhanced by this work. As a joint industry initiative it is expected that the interpretation database will be extended by the acquisition and interpretation of Phase III data, currently ongoing, and input from 3D surveys and site surveys carried out by the operators and data from the European North Atlantic Margin (ENAM) project. Introduction The Seabed Project programme is multifaceted and designed to maximise operational safety related to the work of the petroleum industry in Norway as it takes its first steps into the deep water areas off Mid Norway. The programme is managed by Norsk Hydro on behalf of the operators of deep water licences awarded in 15th Licencing Round, i.e.: BP Norge, Esso Norge, Norsk Hydro, Norske Shell, Saga Petroleum and Statoil. The part of the programme which focuses on the seabed conditions and shallow geology has been divided into three phases, namely Geological and Geophysical (G & G) Interpretation Phases I, II and III. Phase I was carried out by the IKU, part of the Sintef Group based in Trondheim. This first phase involved the interpretation of 4100 kms. of an open grid of largely reprocessed conventional 2D digital seismic data. This was in order to establish a seismostratigraphical framework for the shallow geology (approximately the upper 1500 metres belowseabed) and identify features of importance, so as to gain an understanding of the geological and engineering conditions and to recommend additional work to be performed to aid this understanding. During 1996 additional high resolution 2D multichannel seismic data, together with mini sleeve gun, deep tow boomer and swathe bathymetry data were acquired on a regional grid in water depths between 400 and 1800 metres to compliment the Phase I data.

A lower limit of blockfields is inferred to indicate the maximum heights and thus thickness of the Late Weichselian ice sheet in the inner Nordfjord region. Ice movements in this area have been topographically controlled during the entire Weichselian glaciation. Prominent lateral moraines delimit the Younger Dryas valley glaciers in inner Nordfjord. Subsequent to the Younger Dryas Chronozone, the glaciers retreated rapidly due to calving in the fjord and climatic amelioration. In a later phase of deglaciation, in all probability around the early and middle part of the Preboreal Chronozone, an ice centre east of Strynefjellet dominated, while the Jostedalsbreen area is thought to have played a minor role as a centre of ice dispersal. The final deglaciation was dominated by vertically down-wasting ice remnants in the lake basins and tributary valleys. Terminal moraines in front of several outlet glaciers of Jostedalsbreen beyond the 'Little Ice Age' moraines indicate a climatic deterioration at the end of the Preboreal Chronozone.