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Blikra L.H.,Aknes Tafjord Early Warning Center | Blikra L.H.,Sogn og Fjordane University College | Christiansen H.H.,University Center in Svalbard | Christiansen H.H.,Copenhagen University

Knowledge about the detailed processes linked to the existence of permafrost in rockslide fractures is sparse. Large parts of the Jettan rockslide are located right below the discontinuous permafrost limit in the arctic part of the alpine landscape of northern Norway. Combining four years of meteorological and rockslide deformation data with temperature measurements from different parts of open fractures, shallow bedrock boreholes and air, as well as daily snow cover observations, allows a detailed identification of the key processes involved. These field data are the basis for the development of a permafrost controlled rockslide model. Seasonally, the deformation has a very distinctive pattern with high deformation starting abruptly right after snowmelt in May, and lasting until snow isolation in winter. Then there is a gradual transition to medium deformation as the ground is cooled further for another 1-2. months. Finally, the winter period, when maximum snow occurs in the fractures, is characterized by limited or almost no deformation. The primary controlling deformation process is meltwater percolation into fractures in summer with significant refreezing, ice formation and temperature increase in the lower part of the fractures from - 1. °C to 0. °C. Sporadic permafrost exists below the discontinuous permafrost limit, and may extend into open fractures and sliding planes. Another primary process is the significant cold air accumulation in fractures in early winter, due to the Balch effect, which significantly cools the fracture and surrounding rock promoting permafrost development. Finally, the cold air effect is stopped by snow isolation once enough snow has accumulated in the fracture by late winter. The deformation itself is thought to be controlled by changing shear strength of the brecciated sliding planes due to either changing ice temperatures and/or variations in water infiltration to the unsaturated sliding zones. The overall system is very locally controlled driving itself, and the effect of a future climate change can thus be of minor importance. © 2013 Elsevier B.V. Source

Oppikofer T.,University of Lausanne | Oppikofer T.,Geological Survey of Norway | Jaboyedoff M.,University of Lausanne | Pedrazzini A.,University of Lausanne | And 4 more authors.
Journal of Geophysical Research: Earth Surface

The basal sliding surfaces in large rockslides are often composed of several surfaces and possess a complex geometry. The exact morphology and location in three dimensions of the sliding surface remains generally unknown, in spite of extensive field and subsurface investigations, such as those at the Åknes rockslide (western Norway). This knowledge is crucial for volume estimations, failure mechanisms, and numerical slope stability modeling. This paper focuses on the geomorphologic characterization of the basal sliding surface of a postglacial rockslide scar in the vicinity of knes. This scar displays a stepped basal sliding surface formed by dip slopes of the gneiss foliation linked together by steeply dipping fractures. A detailed characterization of the rockslide scar by means of high-resolution digital elevation models permits statistical parameters of dip angle, spacing, persistence, and roughness of foliation surfaces and step fractures to be obtained. The characteristics are used for stochastic simulations of stepped basal sliding surfaces at the Åknes rockslide. These findings are compared with previous models based on geophysical investigations. This study discusses the investigation of rockslide scars and rock outcrops for a better understanding of potential rockslides. This work identifies possible basal sliding surface locations, which is a valuable input for volume estimates, design and location of monitoring instrumentation, and numerical slope stability modeling. Copyright 2011 by the American Geophysical Union. Source

Fenton C.R.,Helmholtz Center Potsdam | Hermanns R.L.,Norges Geologiske Undersokelse | Blikra L.H.,Aknes Tafjord Early Warning Center | Kubik P.W.,ETH Zurich | And 6 more authors.
Quaternary Geochronology

Two rock avalanches in Troms County - the Grøtlandsura and Russenes - were selected as CRONUS-EU natural cosmogenic 10Be production-rate calibration sites because they (a) preserve large boulders that have been continuously exposed to cosmic irradiation since their emplacement; (b) contain boulders with abundant quartz phenocrysts and veins with low concentrations of naturally-occurring 9Be (typically<1.5ppb); and (c) have reliable minimum radiocarbon ages of 11,424±108calyr BP and 10,942±77calyr BP (1σ), respectively. Quartz samples (n=6) from these two sites contained between 4.28×104 and 5.06×104 at 10Be/g using the 1.387Myr 10Be half-life. Determination of these concentrations accounts for topographic and self-shielding, and effects on nuclide production due to isostatic rebound are shown to be negligible. Persistent, constant snow and moss cover cannot be proven, but if taken into consideration they may have reduced 10Be concentrations by 10%. Using the 10Be half-life of 1.387Myr and the Stone scaling scheme, and accounting for snow- and moss-cover, we calculate an error-weighted mean total 10Be production rate of 4.12±0.19 at/g/yr (1σ). A corresponding error-weighted mean spallogenic 10Be production rate is 3.96±0.16 at/g/yr (1σ), respectively. These are in agreement within uncertainty with other 10Be production rates in the literature, but are significantly, statistically lower than the global average 10Be production rate. This research indicates, like other recent studies, that the production of cosmogenic 10Be in quartz is lower than previously established by other production-rate calibration projects. Similarly, our findings indicate that regional cosmogenic production rates should be used for determining exposure ages of landforms in order to increase the accuracy of those ages. As such, using the total 10Be production rate from our study, we determine an error-weighted mean surface-exposure age of a third rock avalanche in Troms County (the Hølen avalanche) to be 7.5±0.3kyr (1σ). This age suggests that the rock avalanche occurred shortly after the 8.2kyr cooling event, just as the radiocarbon ages of the Grøtlandsura and Russenes avalanches confirm field evidence that those rock-slope failures occurred shortly after deglaciation. © 2011 Elsevier B.V. Source

Nordvik T.,Geological Survey of Norway | Nordvik T.,Norwegian University of Science and Technology | Blikra L.H.,Geological Survey of Norway | Blikra L.H.,Aknes Tafjord Early Warning Center | And 2 more authors.
Engineering Geology

The Nordnes rockslide in northern Norway poses a threat to local settlements along the nearby fjord due to its potential of generating tsunamis. Therefore, a monitoring program was initiated in 2007. Evaluation of the resulting monitoring data is expected to provide important contributions to the understanding of the sliding mechanisms.This paper focuses on statistical analyses of continuous laser and crackmeter measurements at the Nordnes rockslide during a period of 16. months. Annual linear displacements and seasonal fluctuations were estimated from time series of 3 lasers and 10 crackmeters.Results from the analyses show that the north-westernmost part of the area has the largest movement of more than 5. cm per year, which makes this part the most critical in terms of generation of a rapid rockslide. The amplitudes of the seasonal fluctuations estimated from crackmeter time series were approximately 0.5. mm. The largest displacements clearly occur in autumn and early winter with a stagnation or retreat phase in spring and summer. Thus, the movements are not increased during snowmelt which is a normal seasonal characteristic elsewhere. Although the temperature changes have a significant effect on the observed displacements, the seasonal variations could not fully be modelled with temperature terms in the regression models suggesting that there are other additional controlling factors. The rockslide is localized in arctic and periglacial conditions, and the documented seasonal variations are interpreted to be linked to effects of deformations caused by seasonal frost and permafrost.Prediction intervals for future displacements were also derived from the current time series. These prediction intervals are considered useful for the evaluation of future measurements and may serve as basis for defining alert thresholds for possible future early warning systems. © 2010 Elsevier B.V. Source

Kristensen L.,Aknes Tafjord Early Warning Center | Blikra L.H.,Aknes Tafjord Early Warning Center | Blikra L.H.,Sogn og Fjordane University College
Landslide Science and Practice: Early Warning, Instrumentation and Monitoring

In Norway, four large rock slides are considered high-risk objects and equipped with real time early warning system. The mountain Mannen is the most recently instrumented of those. Construction and instrumentation was started in 2009 and was almost completed in 2010. To ensure redundancy, several types of instruments are used, such as lasers, realtime DGPS, extensometers, ground based InSAR system, borehole instrumentation and a meteorological station. The surface displacement is about 3 cm/year, with the largest velocity in the upper part, where the annual probability of failure estimated to 1/100. Strong subsurface deformation is measured in a 120 m borehole, suggesting a complex movement in a graben structure at the back-crack. © Springer-Verlag Berlin Heidelberg 2013. Source

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