Initiation of earthquake-induced slope failure: influence of topographical and other site specific amplification effects

Increased structural damage caused by earthquakes on hilltops and along ridges has often been related to amplification of ground motion due to the presence of topography. However, comparison between observations and numerical modeling has shown that amplification is only partly dependent on the prominent surface morphology. Strong effects are also induced by soft layers, such as weathered rock material or colluvium, covering the topographies. Numerous seismically triggered landslides are reported to occur in the same materials that are likely to amplify ground motions. Therefore, it can be suspected that ground motion dynamics significantly contribute to the observed slopes failures. This potential relationship is the subject of the present case study, the Ananevo rockslide in the northeastern Tien Shan mountains. The survey included geophysical prospecting, earthquake recordings and structural analyses of the rock fabric. On the basis of the field data, observed amplification effects could be related to the local geological conditions and particularly to the surface morphology and to the presence of low-velocity layers-deeply weathered rocks-on the top of the bedrock. Surface layer- and topography-dependent amplification has also been studied numerically by 2D and 3D finite element modeling of ground motion dynamics. The present paper focuses on additional effects that may be induced by the presence of a fault zone and of the landslide scarp. Further, observed and computed ground motion dynamics are connected with slope failure susceptibility: 2D numerical simulations reveal that strain localization is closely related to wave amplification in surficial low-velocity layers, particularly below convex surface morphologies.