Creep Bursts on the Åknes Landslide, Norway: Detection, Characterization and Possible Mechanisms

Creeping landslides may fail catastrophically, posing significant threats to infrastructure and lives. Landslides weaken over time through rock mass damage processes that may occur by steady-state creep or transient accelerations of slip, called creep bursts. Creep bursts may control landslide stability by inducing short-term damage and strain localization. This study focuses on the Åknes landslide in Norway, which moves up to 6 cm per year and could potentially trigger a large tsunami in the fjord below. An 11-year data set is compiled and analyzed, including kinematic, seismic, and hydrogeological data acquired at the landslide surface and in a series of boreholes. An annual average of two creep bursts with millimeter amplitude has been recorded within the shear zone in each borehole, accounting for approximately 11% of the total displacement. Creep bursts detected simultaneously in multiple boreholes are preceded by increased seismic activity and rising water pressure. However, most creep bursts are observed in only one or a few boreholes. These bursts often happen during seasonal high and low groundwater levels in autumn and spring, respectively, correlating with local peaks in water pressure. No such correlation is observed during summer. We propose that creep bursts can have different causes and hypothesize that rock degradation leads to some creep bursts independent of water pressure variations. In contrast, the largest creep bursts are correlated with variations in absolute water pressure or gradients of water pressure within the shear zone. Our findings emphasize the complexity of a dense data set requiring multiple mechanisms to explain creep burst dynamics.