Geophysics for investigation and analyses of large landslides

Rock avalanches and related tsunamis represent one of the most serious natural hazards in Norway, and during the last 100 years more than 170 people have lost their lives in western Norway. The Tafjord disaster of 1934 is one major example, where 3 millio ns m3 rock mass dropped into the fjord. The tsunami generated by the avalanche reached a maximum of 62 m above sea level, and several inhabited villages along the fjord were destroyed. A similar and potential field of high risk has been identified at Åker neset (Stranda, Møre og Romsdal) and extensive studies of the area have been planned for the next summer. The estimated volume of unstable rocks may be 10 times larger than in Tafjord in 1934. In order to constrain this information, systematic geophysical mapping is necessary, beside geological and geotechnical assessment. Through NORSAR participation in the Norwegian Centre of Excellency, the International Centre for Geohazards (ICG), preliminary contacts were taken in 2004 with two French research insti tutes, LGIT and LIRIGM, in Grenoble. They both have long experience in geophysical measurements in the Alpes. In order to plan more extensive geophysical investigation at Åkerneset during summer 2005, colleagues from Grenoble will be invited to share expe rience in geophysics for land sites, and to help investigate newer approaches, especially around the use of background seismic noise, to infer thicknesses and extent of fractured layers, micro-seismic monitoring and seismic tomography. In all cases, a bet ter mapping of the zone which may rupture will be the result, which in turn will give valuable information for geotechnical assessment, and input to numeric modeling of the potential slide. The size of the tsunami waves will then be deduced, which will in dicate what zones are at risk in case of a rupture. The french colleagues are invited to participate in geophysical experiments at Åkerneset during summer 2005, which will help establishing fruitful contacts.