NGU Report 2002.020 Postglacial mass movements in western Norway with special emphasis on the 2000-2200 BP and 2800-3200 BP periods - final report Geological Survey of Norway N-7441 Trondheim, Norway Tel.: 47 73 90 40 00 REPORT Telefax 47 73 92 16 20 Report no.: 2002.020 ISSN 0800-3416 Grading: Open Title: Postglacial mass movements in western Norway with special emphasis on the 2000-2200 BP and 2800- 3200 BP periods - final report Authors: Client: Reidulv Bøe, Aivo Lepland, Lars Harald Blikra, Norsk Hydro ASA, NGU Oddvar Longva and Eivind Sønstegaard County: Commune: Sogn og Fjordane, Møre og Romsdal Map-sheet name (M=1:250.000) Map-sheet no. and -name (M=1:50.000) Florø, Årdal, Ulsteinvik, Ålesund, Kristiansund Deposit name and grid-reference: Number of pages: 115 Price (NOK): 470,- Map enclosures: 0 Fieldwork carried out: Date of report: Project no.: Person responsible: June-July 2001 1st September 2002 293100 Summary: The Ormen Lange Gas Field was discovered in the Norwegian Sea outside Møre og Romsdal in 1997. The development of this field, which is located in the area of the Storegga Slide, requires safety assessment. NGU and Norsk Hydro ASA, in cooperation with the University of Bergen and Sogn og Fjordane College, have carried out a project with the aim to collect and compile data on slides, avalanches and gravitational faults that may have resulted from large earthquakes or tsunamis in northwest Western Norway. A major task in the present project has been to investigate the spatial extent and interpret the origin of a postulated mass-movement event ca. 2000 year ago and to evaluate its causes; climatic variations, a tsunami (possibly caused by an earthquake affecting the offshore area), an earthquake only affecting parts of western Norway, or a combination of an earthquake and a tsunami. Several other mass movements, including Storegga Slide tsunami deposits and pre-Storegga Slide slide and debrisflow deposits have been studied, both in fjord and lake sediments. Five of the 16 investigated fjords (Dalsfjorden, Førdefjorden, Syvdsfjorden, Voldafjorden, Ørstafjorden) provide evidence for a 2000-2200 years BP (calendar years before present, i.e. 1950) event. Previous investigations show no indication of a large shelf- edge slide in the Storegga area, that may have created a tsunami at that time, nor are any mass-movement deposits found on land or in the investigated lakes. This suggests that the 2000-2200 BP debrisflows and turbidites were most likely related to one or more earthquakes on land or close to the coast, and not an offshore megaslide-generated tsunami. Storegga Slide (8200 BP) tsunami deposits are observed in cores over most of the investigated area, both in the deep fjords and in lakes. Striking similarity between major slide and debrisflow deposits at the 2000-2200 BP and ca. 11 000-11 700 BP stratigraphic levels suggest a common triggering mechanism, probably earthquakes with epicenters in the Sunnfjord-Sunnmøre region. A period of debrisflows, turbidites and snow avalanches around 2800-3200 BP was most probably related to climatic irregularities, as also observed from previous studies elsewhere. Most of these deposits are only observed in cores and not in seismic data. Clusters of rock avalanches from the second half of the Holocene, in Romsdalen and Tafjorden, may be related to neotectonic activity of the newly discovered Berill fault, possibly around 3000 BP. Several other periods of mass-movement activity (turbidity currents, floods, snow avalanches) have been recorded, e.g. 1700-1800 BP and 5300-5600 BP. These are only observed in a few basins, and are thus interpreted to be related to local climatic variations. Clusters of rock avalanche deposits in the coastal area probably reflect old events, possibly just after the deglaciation. Many small debrisflows and turbidites in the early Holocene were probably related to rapid land uplift and high rates of erosion, at that time. Keywords: Marine Geology Tsunami Earthquake Core Debrisflow Dating Geological hazard Turbidite Slide CONTENTS 1. INTRODUCTION .............................................................................................................. 7 2. METHODS ....................................................................................................................... 12 2.1 Phase A1 ................................................................................................................... 12 2.2 Phase A2 ................................................................................................................... 12 2.3 Phase B1 ................................................................................................................... 12 2.4 Phase B2 ................................................................................................................... 13 2.5 Phase C ..................................................................................................................... 14 2.6 Phase D ..................................................................................................................... 15 2.7 Phase E ..................................................................................................................... 15 3. CLIMATE, MASS-MOVEMENT ACTIVITY AND HUMAN INFLUENCE OVER THE PAST 11 500 YEARS (10 000 14C YEARS) .................................................................. 15 3.1 Palaeoclimatic data ................................................................................................... 15 3.2 Climatic influence on mass movements and flood activity ...................................... 16 3.3 Timing of high mass-movement activity ................................................................. 17 4. ROCK AVALANCHES, GRAVITATIONAL FRACTURES AND NEOTECTONIC FAULTS ON LAND ................................................................................................................ 18 4.1 Romsdalen and Innfjorden ....................................................................................... 19 4.2 Tafjord-Geirangerfjord-Sunnylvsfjord ..................................................................... 24 4.3 Øtrefjellet, Oterøya and Sørdalen ............................................................................. 27 4.4 Sogn og Fjordane and Hordaland ............................................................................. 29 4.4.1 Aurlandsfjorden ................................................................................................ 31 4.4.2 Fjærlandsfjorden ............................................................................................... 34 5. FJORD INVESTIGATIONS ............................................................................................ 34 5.1 Sogn .......................................................................................................................... 34 5.1.1 Aurlandsfjorden ................................................................................................ 34 5.1.2 Barsnesfjorden .................................................................................................. 37 5.1.3 Fjærlandsfjorden ............................................................................................... 39 5.2 Sunnfjord .................................................................................................................. 42 5.2.1 Dalsfjorden ....................................................................................................... 42 5.2.2 Førdefjorden ..................................................................................................... 45 5.3 Sunnmøre ................................................................................................................. 45 5.3.1 Vanylvsfjorden ................................................................................................. 45 5.3.2 Syvdsfjorden ..................................................................................................... 48 5.3.3 Voldafjorden ..................................................................................................... 51 5.3.4 Ørstafjorden ...................................................................................................... 52 5.3.5 Sulafjorden ....................................................................................................... 55 5.3.6 Breisunddypet ................................................................................................... 59 5.3.7 Tafjorden .......................................................................................................... 62 5.4 Romsdal .................................................................................................................... 63 5.4.1 Mifjorden .......................................................................................................... 63 5.4.2 Julsundet ........................................................................................................... 64 5.5 Nordmøre ................................................................................................................. 68 5.5.1 Tingvollfjorden ................................................................................................. 68 5.5.2 Halsafjorden ..................................................................................................... 68 6. LAKE INVESTIGATIONS IN SUNNMØRE AND NORDFJORD .............................. 72 6.1 Storsætervatnet ........................................................................................................