Rock avalanches, gravitational bedrock fractures and neontectonic faults onshore northern West Norway: Examples, regional distribution and triggering mechanisms

The present report aims on graviational slope failures in Møre & Romsdal andSogn & Fjordane counties. It presents selected examples and an overview of theregional occurrence of rock avalanches and other bedrock failures. The datadiscussed here have been collected through several NGU projects since 1995, thepresent one being: "Regional landslide occurrences and possible post-glacialearthquake activity in northwestern Norway" (phase D); a project funded byNorsk Hydro ASA and NGU. Geological studies show that several areas in westernNorway have been affected by a significant number of large rock avalanchesthroughout the postglacial period. The geographic distribution of rockavalanches and gravitational bedrock fractures clearly shows a clustering inspecific zones, with the highest frequency in the inner fjord areas of westernNorway. The highest number of such features are also found in two smaller areasof Møre & Romsdal, in a more coastal position around Oterøya and Syvdsfjorden.A group of gravitational failures also occur in the phyllite areas in Aurland,Sogn og Fjordane.The age of the features is still poorly constrained, but a review of existingdatashed som light on the time-frame. The bedrock fractures and rock avalanchesaround Oterøya probably occurred shortly after the deglaciation. The timeconstraint is weak for the coastal area further southwest, but the data pointto several events of different ages. Several dated events in the inner fjordareas of Møre & Romsdal suggests high rock-avalanche activity during the secondhalf of the Holocene. This can be seen in context with a neotectonic fault(Berill fault) in Innfjorden, west of Romsdalen. The Berill fault is at leastyounger than the Younger Dryas period (11500 cal. BP). Some dates of rockavalanches around 3000 cal. BP in Innfjorden and Tafjorden indicate arelationship with the faulting event. The data from Aurland in Sogn & Fjordanesuggest that the gravitational failures in the phyllite area are related to twostages, one shortly after the deglaciation and a smaller event at about 3000cal. BP.Review of historical earthquakes indicate that the triggering of large rockavalanches and deep-seated bedrock failures requires a minimum earthquakemagnitude of about 6.0-6.5 on the Richter scale. The distinct clustering offeatures in specific zones indicates that large earthquake played a role. Therecently detected neotectonic fault (Berill fault) further help to explain somerock avalanches in the fjords of Møre og Romsdal. The fault is situated in thearea of the highest land-uplift gradient in western Norway, which could be thecause for crustal instability. Earthquakes related to this fault were probablyof the order of M 6.5. The spatial distribution of the clustering in Møfre &Romsdal seems to fit a relationship between aerial extent of landslides andearthquake magnitudes of M 6.5 In conclusion, future work should especiallyaim on further investigations of possible neotectonic faults, in order to get abetter understanding of the palaeoseismic activity.

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