Statistical analysis of surface displacements – an example from the Åknes rockslide, western Norway

Abstract. Analyses of displacement measurements may provide valuable insight into the characteristics and behaviours of landslides. This paper demonstrates the application of statistical analysis to displacement data collected with Global Positioning System (GPS), total stations and extensometers at the Åknes rockslide site, western Norway. The Åknes rockslide has particular interest due to the potential for catastrophic consequences if the rockslide accelerates into a rock avalanche and hits the fjord below. This would generate a tsunami in the adjacent fjord system and pose a threat to local settlements and infrastructure as well as to the many tourists visiting nearby areas. The analyses reported in this paper pay special attention to the newly available time series obtained from seven permanent GPS stations. The results from these continuously monitored GPS stations are believed to be an important contribution to the understanding of the complex displacement pattern evident from previous investigations. Results from the statistical analyses show that the displacement rates can be modelled as linear trends superimposed with periodic (sinusoidal) components. This indicates constant average displacement rates with no persistent accelerations. The annual displacement rates estimated from GPS and extensometer measurements range from a few millimetres to about 8 cm, whereas the periodical fluctuations typically have maximum amplitudes of 1–2 mm. Some interpretations of the periodical fluctuations are presented. High correlations between displacements and the groundwater level, measured in a borehole at the upper part of the slope, are evident for extensometers located across the back scarp. For the GPS control points located further down the slope, this correlation is, however, not so clear.