Microseismic signature of the internal deformation in the Åknes rockslide (Norway): Four years of downhole observations

Norway’s rugged western coastline is dominated by steep mountain slopes rooted within fjords. Mass movements on these slopes pose major hazards due to the potential for triggering massive tsunamis in the narrow fjords. However, our understanding of the internal triggering processes and potential precursory signals is still limited. We use the Åknes rockslide in western Norway as a natural laboratory to study the seismic footprint of the internal deformation in a slow-moving unstable rockslide (1-3 cm/yr). The Åknes rockslide is one of the most thoroughly instrumented rockslides in the world. We analyze four years (2021-2025) of microseismic data from an 8-level three-component borehole geophone string (15-50 m below ground level) intersecting at least one of the alleged sliding planes of the rockslide. The detected microseismicity shows bursts of highly similar events located close to the well (meters to a few tens of meters) with activity varying with depth. By connecting our long-term in-situ observations with comprehensive datasets of groundwater levels and deformation measurements from other boreholes within the rockslide, we shed light on the observed microseismic processes and their driving forces in the vicinity of the monitoring well. In addition to improved process understanding, our work aims to contribute to the development of robust, physics-informed strategies for early warning of sudden rock mass mobilization.