Multidisciplinary investigation of a shallow near–shore landslide, Finneidfjord, Norway

The 1996 landslide near Finneidfjord, Norway, involved the displacement of c. 1 x 106 m3 of sediment. Failure initiated offshore and developed in a retrogressive manner, back-stepping 100 – 150 m inland, and removing a 250 m long section of the main North-South highway. The landslide caused the loss of four human lives, and may have been triggered by human activity (e.g., blasting for road works and/or placement of fill along the shore). Acquisition of an extensive and multi-disciplinary data set, including high-resolution swath bathymetry, 2D/3D seismic data, multiple short (up to 6 m) and two long (12 m and 14 m, respectively) sediment cores, and in situ Free-Fall Piezocone Penetrometer (FF-CPTU) profiles complemented with geotechnical laboratory data, has afforded detailed analysis of both the landslide morphology and stratigraphic controls. Using regional 2D parametric sub-bottom profiler (TOPAS) profiles and a targeted decimetre-resolution 3D Chirp seismic volume (950 m x 140 m), we focus on post-failure material transport/deposition, correlating the failure plane against one of several regionally extensive packets of high–amplitude, composite reflections. In seismic reflection data, the slide plane lies within a distinct, thin (< 0.5 m) stratigraphic bed of lower acoustic impedance than the background sedimentation (indicated by high amplitude reverse-polarity top reflection), which is extensively deformed or completely scoured by motion of the overlying material. Within the body of the landslide, two different flow facies are identified. Inversion of these broadband (1.5 – 13.0 kHz) seismic data has allowed the calculation of remote physical properties (using acoustic quality factor, Q), affording a depth and spatial assessment of the relationship between morphology and grain size. These remote physical properties have been correlated against high-resolution geotechnical data from core logs and FF-CPTU profiles, identifying the slide plane as a weak, laminated, clay-rich bed. This combined geophysical/geotechnical assessment of the landslide morphology and internal architecture supports previous work indicating a complex, multi-stage failure. These combined data illustrate how seafloor stability is strongly influenced by shallow subsurface structure, with the geotechnical properties and lateral continuity of stratified beds acting as a primary control on slide plane depth and failure probability.