45. The 1978 Quick Clay Landslide at Rissa, Mid Norway: Subaqueous Morphology and Tsunami Simulations

The 1978 landslide at Rissa is the largest to have struck Norway during the last century and is world-famous because it was filmed. Swath bathymetry data and seismic reflection profiles reveal detailed information about the subaqueous morphology of the mass-transport deposits (MTD). Results show that the landslide affected nearly 20% of the lake floor and that it exhibits a complex morphology including distinct lobes, transverse ridges, longitudinal ridges, flow structures and rafted blocks. The rafted blocks found at the outer-rim of the MTD travelled a distance of over 1,000 m in the early stage of the landslide on an almost flat basin floor. Simulation of sediment dynamics and tsunami modelling show that the rafted blocks most likely triggered the flood wave with a recorded maximum surface elevation of 6.8 m.