EARTHQUAKE-TRIGGERED LANDSLIDE HAZARD IN SENSITIVE CLAY DEPOSITS

In sensitive clay deposits of eastern Canada, bank erosion along streams is the most common natural trigger for large regressive landslides. Bank erosion leads to an oversteepening of the slope and induces a shear stress concentration at its toe, which can initiate, in strain-softening sensitive clays, a progressive failure. Depending on the mechanical properties of the sensitive clays, the stratigraphy, and the initial slope geometry, the failure can then evolve either towards a flowslide or a spread, or towards a mixture of these two distinct landslide types, or stops without significant retrogression. The size of these retrogressive landslides covers a wide spectrum ranging from about one hectare to a few hundred hectares. Earthquakes, on the other hand, are known to have triggered historical and prehistorical retrogressive landslides of much larger sizes, reaching several square kilometers. If the shaking is strong and long enough, field evidences indicate that these landslides tend to occur in clusters, sometimes leading to a valley-wide collapse, such as it happened during a M7+ earthquake in 1663 along the Gouffre valley in the Charlevoix region (Quebec). As for bank erosion, the retrogression process can be initiated by an earthquake-triggered first failure involving a slope along a river or a marine terrace, the retrogression being a consequence of this first failure. However, the retrogression process can also be directly triggered by the horizontal unloading of the slope generated by the seismic ground motions. Due to the rapidity with which the retrogressive failure propagates and due to the size of the potentially affected area, earthquake-triggered landslides may pose a serious threat to the population and infrastructure. The Geological Survey of Canada, in collaboration with the Quebec Ministry of Transportation, began a study to better identify within the context of intraplate seismicity the factors controlling the development of such large retrogressive landslides during or just after earthquakes. This communication presents some results of this study, in particular (1) the role of site effects (basin-edge and topographical effects) in modifying the seismic ground motion characteristics, and (2) the link between the energy released during earthquakes and the size of landslides. We then discuss how this knowledge can be incorporated into landslide hazard mapping and risk management strategies.