Slope instabilities

Instabilities on lateral slopes and delta slopes in Swiss lakes

The perialpine lakes of Switzerland are active sedimentation systems and sediment traps. Continuously accumulating deposits on prograding delta slopes and on moderately dipping lateral slopes are susceptible to instabilities, expressed as sublacustrine slides. Such mass movements may cause tsunami-type waves on the lake through the sudden mobilisation of large volumes of sediment. The collapsing masses may also directly affect shore or delta platforms. Such events are rare but nevertheless a potential hazard for infrastructure and communities around lakes.

Several historical examples of sublacustrine slope failures exist for Lake Lucerne for the last centuries. In AD 1601, a strong earthquake in Unterwalden with a magnitude around 6.2 triggered several synchronous slides within the different basins of the lake. The resulting waves reached heights of more than 4 m and led to extensive damage along the shores. A presumably spontaneous collapse of the Muota river delta near Brunnen occurred in AD 1687 when parts of the delta platform disappeared in the lake. The subsequent wave caused widespread flooding around the lake up to 5 m above normal level, as reported in historical accounts by Johann Lorenz Bünti and Jakob Billeter. Even in Lucerne, more than 20 km away and separated from the delta by narrow passages, lake level rose by more than 2 feet.

Lake sediments are a more or less continuous archive in which traces of these events are stored. The spatial and temporal distribution of such instabilities can be reconstructed also in prehistoric times by mapping and dating related mass movement deposits using geophysical methods (reflection seismic profiling), high-resolution bathymetric imaging and direct sampling (sediment cores). Extents and recurrence times can thus be estimated based on consistent event catalogues.

A geotechnical approach is taken in order to understand the underlying mechanisms (long-term causes and short-term triggers) controlling the occurrence of instabilities. This may include geotechnical testing in situ and on recovered samples, monitoring of parameters influencing slope stability such as pore pressure, as well as quantitative models for a back analysis of subrecent to recent failures.

The results may eventually be used for an assessment of the potential for future subaquatic mass movements in lakes, but also for other related issues such as paleoseismic reconstructions [link auf Projektseite...]. Ongoing work for this project focuses on the stability of prograding delta slopes in Lake Lucerne and other Swiss lakes.

Publications

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• Hilbe, M., Anselmetti F.S., Eilertsen R.S., Hansen L. (2008) Spuren von Massenbewegungen auf dem Grund des Vierwaldstättersees bei Weggis: Die Ereignisse von 1601 und 1795. Bulletin für Angewandte Geologie 13, 83-85.
  
  
• Stegmann S., Strasser M., Anselmetti F.S., and Kopf A. (2007) Geotechnical in situ  characterization of landslide deposits. The role  of pore pressure transients versus frictional  strength: Geophys. Res. Lett., 34, doi: 10.1029/2006GL029122.
  
  
• Strasser M. and Anselmetti F.S. (2008) Mass-movement event stratigraphy in Lake Zurich; A record of varying seismic and environmental impacts. Beiträge zur Geologie der Schweiz, Beiträge zur Geologie der Schweiz, Geotechnische Serie 95, 23-41, Schweizerische Geotechnische Kommission, Zurich, Switzerland.
  
  
• Strasser M., Stegmann S., Bussmann F., Anselmetti F.S., Rick B. and Kopf A. (2007) Quantifying subaqueous slope stability during seismic shaking: Lake Lucerne as model for ocean margins. Marine Geology 240, 77-97.
  
  
• Schnellmann M., Anselmetti F.S., Giardini D., and McKenzie J.A. (2006) 15,000 years of mass-movement history in Lake Lucerne. Implications for seismic and tsunami hazard, Eclogae Geologicae Helvetiae, 409-428.
  
  
• Strasser M., Anselmetti F.S., Fäh D., Giardini D. and Schnellmann M.Geology, 34, 1005-1008 (incl. suppl. material).
  
  (2006) Magnitudes and source areas of large prehistoric northern Alpine earthquakes revealed by slope failures in lakes.
• Schnellmann M., Anselmetti F.S., Giardini D. and McKenzie J.A. (2005)  Mass movement-induced fold-and-trust belt structures in unconsolidated sediments in Lake Lucerne. Sedimentology 52, 271-289.
  
  
• Schnellmann M., Anselmetti F.S., Giardini D., McKenzie J.A., and Ward S. (2002) Prehistoric earthquake history revealed by lacustrine slump deposits. Geology, 30, 1131-1134.