Seismic Noise Interferometry to Disentangle Environmental Effects from Irreversible Subsurface Changes at the Åknes Rockslide in Western Norway

Abstract The Åknes rockslide in Western Norway represents one of the country’s most hazardous unstable slopes, with potential failure capable of generating destructive fjord tsunamis. Therefore, reliable monitoring of its internal deformation processes is crucial for hazard assessment and early warning. This study investigates the feasibility of using passive seismic interferometry (PSI) to monitor internal mechanical changes within the Åknes slope by analyzing relative seismic velocity variations (dv/v) derived from continuous recordings of a vertical borehole geophone array (15–50 m depth). Cross-correlation analyses in the 35–70 Hz frequency band reveal distinct seasonal patterns, with pronounced dv/v decreases during the spring snowmelt periods of 2023 and 2024. These velocity drops coincide with rapid groundwater level increases measured in a nearby borehole, suggesting that elevated pore pressure from meltwater infiltration temporarily reduces rock mass stiffness. Additional correlations with air temperature and snow depth indicate sensitivity to freeze–thaw and hydrological cycles, whereas wind-induced variations at shallow depth highlight the need for careful data processing. Beyond surface related forcings, a large dv/v decrease is observed in 2024 and correlates with an acceleration of displacement near the lower landslide shear zone, indicating sensitivity to localized changes in elastic properties. The results demonstrate that PSI is sensitive to relevant hydromechanical processes and allows distinguishing environmental effects from irreversible structural changes related to the slope creep.