The role of material properties and landscape morphology on landslide size distributions

It has been observed that landslide size distribution shows power-law scaling for large landslide areas, with a roll-over for landslides below a certain threshold which frequently coincides with the modal peak of the distribution. The physical reasons leading to this distribution are not yet fully understood. The analysis of non-cumulative size distribution of two landslide inventories in the Italian Alps confirms the existence of a power-law scaling with exponent of 2.56 +/- 0.02, and demonstrates that the roll-over is not due to under-sampling since it occurs at landslide sizes significantly larger than mapping resolution. The control of topography on landslide size distribution is investigated by a topographic analysis using the virtual tiling method. We observe a power-law scaling of topography with a roll-over for finer scales, which is comparable with that of landslide size distribution. The control of material properties on landslide size and geometry is examined through deterministic and probabilistic 2D limit-equilibrium slope stability analyses. Incoherent materials favour shallow landslides with no limitation in size; cohesive materials favour deep landslides and show a limitation for small sizes. Multilayered materials with depth-dependent strength show a limitation for both small and large landslides. The joint probability of (1) the probability that a slope with a given size and slope gradient is unstable (i.e., failure probability from probabilistic stability analysis) and (2) the probability that a slope with a given size and slope gradient exists (topographic analysis) allows to build synthetic size distributions which show both power-law scaling and roll-over. For multilayered materials, we obtain a scaling exponent of the power law of 2.58 +/- 0.02. We suggest that the power-law scaling in landslide size distribution results, in the study area, from the constraint of topography and the presence of multilayered materials or depth-dependent strength. The roll-over is an intrinsic characteristic of the size distribution, due to the contribution of cohesion to slope stability. (C) 2012 Elsevier B.V. All rights reserved.