Tree-ring reconstruction of reactivation phases of the Schimbrig landslide (Swiss Alps)
Reconstitution des phases de réactivation du glissement de terrain de Schimbrig (Alpes suisses) au moyen d’une approche dendrogéomorphologique
Résumé
One of the most important issues of current landslide research is related to the dating of their reactivations, both in spatial and in temporal terms. Landslide chronologies thus play a key role because they provide very essential information on past activity and thereby contribute substantially to hazard assessment, in particular in areas with intense anthropogenic uses. In most cases, however, knowledge of past landslide activity remains incomplete and archive records of past activity typically over represent the largest and miss the smaller events. In this paper, a chronology of past reactivations of the Schimbrig landslide (Swiss Alps) has been derived from the dendrogeomorphic analysis of 184 disturbed P. abies trees growing on the landslide body. In total, 318 growth disturbances identified and dated in the tree-ring series enabled reconstruction of 26 reactivation phases of the landslide body between 1859 and 2010 (mean return period: 0.17 event yr(-1)). Given the spatio-temporal completeness of the reconstruction, probabilities of landslide reactivation were computed and mapped using a Poisson distribution model for an event to occur within 5, 20, 50, and 100 years. Probabilities of landslide reactivation increase from 0.33 for a 5-year period to 0.99 for a 100-year period. High-resolution maps also indicate a strong spatial variability with lower values computed on the margins and in the central portion of the landslide body whereas less frequent reactivations are predicted in its lower part. The field-based reconstruction proposed in this paper provides quantitative probability maps of reactivation derived directly from the frequency of past events that appear complementary to conventional susceptibility maps, used for landslide zoning that provides an estimate of where landslides are expected to occur. This approach is considered a valuable tool for land managers in charge of protecting and forecasting people and their assets from the negative effects of landslides as well as for those responsible for land use planning and management. It demonstrates the reliability of dendrogeomorphic mapping that should be used systematically in forested shallow landslides.