Landslides in vibrating sand box: What controls types of slope failure and frequency magnitude relations?

Although landslides are a worldwide significant natural hazard, their physics is not well understood. Here, landslides were induced in a vibrated box filled with wet (practically cohesive) sand, simulating natural slope failure. The questions addressed were (a) what controls the type of slope failure and (b) what controls frequency magnitude relations of landslides. In the experiments, two end-member slope failure types were obtained: during application of only horizontal acceleration, a failure plane rapidly developed, followed by a box-sized slump. In contrast, under application of only vertical acceleration, mode I fractures formed slowly, dissecting the slope into blocks. The fractures caused a strength heterogeneity and were followed by block-sliding. In the vertical shaking experiments, a power law size distribution of slide-blocks was measured, controlled by the fracture distribution. The experiments suggest that heterogeneity may be a major control on the size distribution of natural landslide inventories: In a homogeneous environment, the landslide will have a characteristic size of the whole system. In a heterogeneous slope, sizes of landslides will reflect the heterogeneity. Following the above experimental observations our hypothesis is that natural landslides may be divided into two groups small and large. The processes controlling their formation are different:I.The smaller natural landslides occur as slumps within the unconsolidated, rather homogeneous, sediments typical of the upper few meters close to the surface. The size of these slumps is determined by the dependence between failure depth (constrained to be the depth of the unconsolidated sediments) and area.II.In contrast to the homogeneous upper layer, rock mass below the unconsolidated sediment is always heterogeneous due to fractures, layers and bedding. This preexisting heterogeneity is the source of the power law decay observed for the large landslide portion in natural landslide distributions. Finally, in light of these insights, in hazard evaluation, it is advised to fit a regression line to the power law decay of the large landslide inventory only and consider the characteristic landslide separately.