Rock avalanches - distribution and frequencies in the inner part of Storfjorden, Møre og Romsdal county, Norway

Abstract

Past landslide deposits store valuable and important information that can be used for geohazard analysis. This includes data on frequency of events, run-out distances and dynamics of rock avalanches. Investigations of rock-avalanche events also give possibilities for back-calculation (modelling of avalanches), such as conditions for sliding in potential unstable slopes.\rA complete swath bathymetry dataset from the inner parts of Storfjorden and its tributary fjords has been studied. Based on this dataset and seismic data, rock-avalanche deposits on the fjords bottom have been mapped. An inventory of avalanche debris distribution and volumes has been compiled. A relative chronostratigraphy is elaborated, and slides are tentatively put into this frame based on seismostratigraphic position and freshness of surface morphology. In many cases it is difficult to distinguish between large rock-avalanches with volumes more than 100000m3, and rock fall and alluvial cones. Hence the number of avalanches may be higher than listed in the inventory.\rA total of 108 rock-avalanches have been mapped. Twentyfive events occurred during the deglaciation (12500-10000 14C years BP) and the deposits from these avalanches constitute 522mill. m3, or 89%, of a total avalanche volume of 587mill. m3 registered in the fjords. 5 out of these 25 avalanches contribute 79% of the total debris volume. Twenty-one slides are equal to or larger than the 1934 Tafjord Slide, but only six of these occurred during the Holocene. The frequency of rock-avalanches was very high during the Younger Dryas and Preboreal periods, i.e. from 11 000 to 9000 14C years BP with approximately 45 recognised slides. After that, we see a more or less even distribution of rock-avalanches throughout the Holocene, with5-8 slides per 1000 years. The largest avalanche deposit is found in Sunnylvsfjorden, below Blåhornet, slightly north of the Åknes Slide, which is presently active.\rThe rock-avalanche activity is highest in Geirangerfjorden with a frequency of one slide every 350 years. The slides are, however, smaller than in other fjords. In Tafjorden the frequency is one slide every 650 years, in Synnylvsfjorden one slide every 1300 years and in Storfjorden from Stordal to the mouth of Sunnylvsfjorden, the observed slide frequency is one every 3000 years. The present data clearly shows that the Storfjorden area has a high frequency of large rock-avalanche events, and that there is a need for geological investigations to map unstable and active rock slopes in the fjord areas.