Submarine slide scars and mass movements in Karmsundet and Skudenesfjorden, southwestern Norway: morphology and evolution

Shallow seismic sections, multibeam bathymetric images and geotechnical inv estigations of the seafloor in Karmsundet and Skudenesfjorden, SW Norway, h ave revealed the occurrence of large slide scars and associated mass-moveme nt deposits. The slide escarpments are up to 30-40 m high and 8 km long, wi th up to 20 x 10(6) m(3) of sediment released during a single event of retr ogressive sliding. The slide scars show a range of associated seafloor topo graphy, reflecting various modes of sediment displacement. The variation in the escarpment morphology and sediment deformation style can be attributed to the varied intensity of liquefaction. Sliding involved mud deposits of Allerod-Younger Dryas to early Holocene age, but partly also deposits older than ca. 12,000 C-14 years BP. The sediment failures were probably trigger ed by seismic reactivation of a local fault by postglacial regional isostat ic rebound. Prior to failure, the seafloor deposits were undercompacted and metastable due to high pore pressure, the presence of shallow gas and the relatively steep local slope created by the formation of a major scour trou gh by sea currents. Minor gravitational sliding may still occur along some of the escarpments, although engineering slope-stability analysis indicates that the deposits can be regarded as stable in the present-day conditions of static load and the anticipated weak sporadic earthquakes with an occurr ence likelihood of 10(-4)/year and a peak acceleration of 0.25 g. (C) 2000 Elsevier Science B.V. All rights reserved.