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We present a combined 3-D geoelectric and seismic tomography study conducted on the large Åknes rockslide in western Norway. Movements on the slope are strongly inuenced by water inltration, such that the hydrogeological regime is considered as a critical factor aecting the slope stability. The aim of our combined geophysical study was to identify and visualize the main shallow tension fractures and to determine their eect on hydraulic processes by comparing the geophysical results with information from borehole logging and tracer tests.To resolve the complex subsurface conditions of the highly fractured rock mass, a threedimensional set-up was chosen for our seismic survey. To map the water distribution within the rock mass, a pattern of nine intersecting 2-D geoelectric proles covered the complete unstable slope. Six of them that crossed the seismic survey area were considered as a single data set in a 3-D inversion. For both methods, smoothing-constraint inversion algorithms were used, and the forward calculations and parameterizations were based on unstructured triangular meshes. A pair of parallel shallow low-velocity anomalies (<1400 m/s) observed in the nal seismic tomogram was immediately underlain by two anomalies with resistivities < 13 kΩm in the resistivity tomogram. In combination with borehole logging results, the low-velocity and resistivity anomalies could be associated with the drained and water-lled part of the tension fractures, respectively. There were indications from impeller owmeter measurements and tracer tests that such tension fractures intersected several other waterlled fractures and were responsible for distinct changes of the main groundwater ow paths. 1

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