A field-based model of permafrost-controlled rockslide deformation in northern Norway
Introduction
Rockslides pose a significant geohazard in the cold, periglacial fjord landscapes of northern Norway. The Jettan rockslide at Nordnes at 69°30′N (Fig. 1) may cause serious consequences to the inhabited fjord areas due to its potential of generating destructive tsunamis (Kristensen and Blikra, 2011). A clear seasonal deformation pattern has been identified for the Jettan rockslide, which has been demonstrated not to be controlled by rainfall and snowmelt alone (Nordvik et al., 2010).
Large parts of the north Norwegian fjord landscapes are located in the arctic zone with altitudinal permafrost. The permafrost regional limit descends from ca. 990 m a.s.l. in the west, to 550 m in the interior in the eastern part of northern Norway (Christiansen et al., 2010). This means that the Jettan rockslide area, extending from 400 to 800 m a.s.l., is located at the regional modern permafrost limit. However, the irregular topography of the rockslide area, with deep open fractures, a complex fracture geometry and snow accumulation pattern, creates special local terrain and subsurface conditions. This complex topographic setting strongly suggests that permafrost can be locally present, which can affect the rockslide deformation. Studies of processes and driving mechanisms of rockslides in such environments are very limited.
The seasonal deformation of large landslides is mainly thought to be controlled by rainfall causing increased water pressure (e.g. Crosta and Agliardi, 2003, Sartori et al., 2003, Hong et al., 2005), a factor assumed also as fundamental for several rockslide events in Norway (Bjerrum and Jørstad, 1968). Snowmelt is also thought to be an important factor increasing the water level in fractures and thus controlling displacements, as assumed to be the case for the Rio Colorado Rockslide in Chile (Casassa and Marangunic, 1993), the rockslope at Affliction Creek in British Colombia (Bovis, 1990) and documented displacements based on the monitoring data from the Åknes rockslide in western Norway (Grøneng et al., 2011). Alternatively, Watson et al. (2006) interpreted the cyclic deformation of the Checkerboard Creek landslide in British Colombia as being controlled by a thermal response of the rock mass due to seasonal temperature variations. However, detailed process understanding is lacking, and the displacements in all other previously investigated sites can also be linked to the variations in water pressure.
Even though permafrost and its potential influence on landslides have been assumed to be important (e.g. Davies et al., 2001, Harris et al., 2001, Geertsema et al., 2006, Gruber and Haeberli, 2007, Sosio et al., 2008, Huggel et al., 2010), detailed field studies on the subject are still sparse. However, the slide scar of the Felik landslide in the Aosta valley in Italy has been observed to have fractures completely filled by ice down to more than 30 m depth (Bottino et al., 2002). The review of recent large slope failures in ice and rocks in Alaska, New Zealand and the European Alps (Huggel et al., 2010) indicates that periods of high air temperatures, melting of snow and ice and rapid thaw reduce the overall slope strength. Also, studies of periglacial landforms such as rock glacier dynamics provide some background for the state of knowledge concerning the seasonal dynamics in landforms characterized by open fractures and large pore spaces. A clear seasonal rhythm has been identified in the European Alps for rock glacier movement rates. The highest velocities were recorded mainly between summer and early winter; while the lowest velocities were generally observed in spring or early summer (Delaloye et al., 2010). To some degree seasonal changes in rock glacier kinematics appear to primarily reflect permafrost temperature variations, mainly considered to be thermally-induced (Delaloye et al., 2010). Davies et al. (2001) found minimum shear strength of frozen rock joints at temperatures between − 1.5 °C and 0 °C using centrifuge modeling. Also other authors have collected various indications that the deformation of frozen debris or rock glaciers increases with increasing ground temperatures up to 0 °C (e.g. Hoelzle et al., 1998, Frauenfelder et al., 2003, Ikeda et al., 2003, Kääb et al., 2007).
Since 2007, continuous rockslide deformation measurements have been carried out in different parts of the Jettan rockslide using various methods, enabling quantification and characterization of the deformation style (Nordvik et al., 2010). Contemporarily, continuous records of air, rock, snow and ice temperatures have been collected in and around the rockslide area. Meteorological data were recorded close to the unstable area. This, in combination with direct field observations of ice in the deeper part of fractures, has enabled the identification of the key seasonal frozen ground processes controlling the rockslide deformation.
The main scientific aim of this work is to obtain a better understanding of the seasonally active processes of the complex terrain and permafrost environment in the fractures of the Jettan rockslide. Here we focus on data collected from early autumn 2007 until the end of summer 2011, providing a four-year data record.
Section snippets
The Jettan rockslide study area
The Jettan rockslide is part of several large instabilities along a steep plateau of the Nordnesfjellet Mountain at the east side of the fjord Storfjorden, extending up to about 900 m a.s.l. (Fig. 1). Among the instabilities, the largest active movements have been documented on the Jettan rockslide (Fig. 2). Geological studies were initiated by the Geological Survey of Norway (NGU) in 1999 (Blikra and Longva, 2000, Braathen et al., 2004), and have been continued using various investigation
Ground deformation
Extensive surface displacement monitoring was established in 2007 (Blikra et al., 2009, Nordvik et al., 2010), consisting of 11 crackmeters, 15 tiltmeters and three lasers all installed in the Jettan rockslide area. In 2010 a DGPS network consisting of 11 antennas and three extensometers was added, providing valuable 3D movement vectors. The noise level can be relatively large for GPS data, and movement pattern can be difficult to see if the deformation is low. All GPS antennas are located in
Ground deformation
As displacement patterns at the Jettan rockslide was earlier analyzed including lasers and crackmeters in a 16-month period (2007–2009) (Nordvik et al., 2010), we focus here on a two year period from summer 2009 to summer 2011. The data presented include mainly sensors close to the main active fracture and the high-velocity northern part. The overall displacement data clearly document that only smaller parts of the unstable structure have active displacement (Fig. 2). The GPS data and the
Seasonal processes in fractures
The deformation at the Jettan rockslide has a significant seasonal pattern, and we propose a model divided into four seasonal stages (Fig. 16). The model is focused on the seasonal process changes in the fractures, and is discussed below.
Conclusions
The unique field data set collected for several years in the periglacial environment of the Jettan rockslide in northern Norway has enabled a detailed understanding of the interactions between permafrost and deformation processes. The permafrost controlled rockslide model we present clearly shows that the primary controlling factors are the spring snowmelt, the early to middle winter cold air accumulation into the fractures, and the middle to late winter snow isolation of the fractures. The
Acknowledgments
Our best thanks to ‘Nordnorsk fjellovervåking’, the monitoring center for the Jettan rockslide and its crew, for drilling the three 2.5 m boreholes, good assistance with logistics during field work, permitting us to use their equipment and facilities on Nordnes, and providing data included in this paper. Thanks to Håvard Juliussen for assisting with the setup of some of the temperature installations. Many thanks to Ulrich Neuman, Geokolibri, and to Markus Eckerstorfer for carrying out the BTS
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