International Geologiical Congress - Oslo 2008

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GHZ-11 Rock slope movements and early warning of catastrophic failure and related tsunamis

 

Anatomy of a catastrophe: The 1936 mass wasting and tsunami event in the Nordfjord region, western Norway

 

Nicolas Waldmann, The University of Geneva (Switzerland)
Guy Simpson, The University of Geneva (Switzerland)
Daniel Ariztegui, The University of Geneva (Switzerland)
Emmanuel Chapron, CNRS/University of Orleans (France)
Atle Nesje, University of Bergen (Norway)
Louise Hansen, Geological Survey of Norway (Norway)
Kristian Vasskog, University of Bergen (Norway)
Valentin Burki, Geological Survey of Norway (Norway)
 

 

Rock avalanches are one of the most devastating events in Norway. In August - September 1936, a number of smaller rock falls were observed at Ramnefjell (Loen Valley, Western Norway). These events culminated with the release of a 1 million m3 block from the upper part of the Ramnefjell Mountain, 800 m above Lake Lovatnet. The block fragmented and plunged into the lake causing a tsunami that achieved a maximum run-up of 74 m and killing 74 persons. This event was followed by a series of small rock falls that eventually developed a crack behind the 1936 original rupture surface. This rock failure opened a deep-seated fracture, which is still a potential sliding surface for a new series of failures.

Lake Lovatnet was surveyed combining simultaneously 1 in3 airgun multi-channel and 3.5 kHz (pinger) single-channel systems. The general seismic stratigraphy of the lake was reconstructed using the 1 in3 airgun multi-channel survey. Overall it shows a succession of glaciomarine sediments deposited during ice retreat and the transition to marine and lacustrine sedimentation as glacio-isostatic rebound turned the fjord into a land-locked lake. A succession of pre-historical and historical mass wasting events, including the megaturbidite and seiche deposits related to the 1936 landslide were further identified using the single-channel high-resolution device.

A specially tailored physically-based mathematical model allowed the numerical simulation of the 1936 landslide and related tsunami to better understand the effects of such events on a small lacustrine basin. The outcome of the model has been further validated against historical, marine and terrestrial information. The geophysical data allowed to characterize the anatomy of the event as registered in the sediments. It further permits to extend the record of mass wasting events beyond historical times providing a model that can be applied to comparable basins at various temporal and geographical scales.

 

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