International Geologiical Congress - Oslo 2008


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


Event tree analysis of the Åknes rock slope


Unni Eidsvig, NGI (Norway)
Suzanne Lacasse, NGI (Norway)
Farrokh Nadim, NGI (Norway)
Lars Harald Blikra, Geological Survey of Norway (Norway)


Rockfalls and rock slides can be disastrous, also because of their tsunamigenic potential. A massive rock slide at Åknes in the Stranda municipality in Norway would have dramatic consequences, as the tsunami triggered by the slide would endanger several communities around Storfjord. Site investigations, monitoring, and warning system for the potentially unstable rock slopes have been implemented to reduce the hazard and risk. The slopes displacements are continuously monitored. The project also includes a regional susceptibility and hazard analysis.

An integrated risk assessment has four basic stages: (1) data collection, (2) hazard assessment, (3) consequence assessment, including identification of elements at risk and vulnerability assessment, and (4) risk assessment, including risk analysis and risk evaluation. Risk management is the consideration of all the four steps and making decisions on the way to proceed. For a system where the conditions that could lead to an undesirable event are complex, an event tree analysis can represent the optimum way to quantify hazard and risk. Given a number of possible consequences resulting from an initiating event, the sequence of following events is identified and their probability of occurrence quantified.

As part of the site-specific hazard and risk assessment, event trees were constructed by pooling opinions with objective of reaching consensus on the hazard, vulnerability and elements at risk (consequences) associated with a rock slide at Åknes. Numbers were set by experts on the hazards (probability of the rock failure and tsunami occurring) and the potential losses (human life and material/environmental damage). The probability of occurrence and the risk were calculated through an analysis and a consolidation of all the branches of the event tree. The event tree analysis of the risk associated with the potentially unstable rock slope resulted in a map of the risk to the residents for the municipalities close to Åknes. The paper presents the evaluation process followed and the results of the analysis.


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