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Seismic monitoring of the unstable rock slope at Åknes, Møre & Romsdal

Summary

NORSAR has installed a permanent microseismic monitoring network in autumn 2005 at Åknes and has been operating the system continuously since then. The network is recording seismic events directly related with the mass movement as well as secondary triggered events as rock falls and small-scale rock avalanches. The seismic system is considered complementary to the direct measuring systems (e.g. extensometers, laser ranging, inclinometers, etc.). It is expected that an acceleration of the slope will be accompanied with an increasing seismic activity. Data from the seismic network are processed in real time and are made available immediately to the early warning centre located in Stranda, Møre og Romsdal.

Location and site-specific hazard

The Åknes site is situated in the county of Møre og Romsdal, Norway, where steep valleys within the extensive fjord systems are prone to gravitational failures. Many rock avalanche deposits with volumes up to more than 100 million m3 could be identified and have been mapped in the county. The county is sparsely populated and the majority of rock slides cause only local damage. However, if a rock failure occurs in the fjord system it can affect a larger area, because of its potential to generate flood waves. In the last century three major rock slides in Norway created flood waves claiming 175 victims.

   

The total area of the instability at Åknes covers about one square kilometer and it has clear limits at the top (fractures several meters wide and 20-30 m deep) and in the Western part (a distinct gorge). The lower limit is not clearly visible, because it is covered with boulders and dense vegetation, but several springs at about 150 m above sea level are considered to be indicative for the foot of the instable part. The volume is estimated to be between 40 and 70 million m3. The average slope steepness is between 35 and 40 degrees. In the upper part two distinct blocks have been identified. The Western block has a movement rate of about 8 cm/year, whereas the Eastern block moves with a rate of about 2-3 cm/year.

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