A role of coastal erosion in a preparation stage of catastrophic landslide events revealed by multi-sensor observations

Coastal erosion makes coastal landslides unstable. However, it is not fully understood how the coastal erosion works for a preparation stage of a catastrophic landslide event because the change in safety of factor before and after coastal erosion events was not usually known. We carried out continuous, multi-sensor observations of the landslide displacement and toe erosion as well as the inclination and pore water pressure at a coastal landslide in Hokkaido, north-eastern Japan. Because this landslide exhibits a slip with a large displacement of 6-8 m once or twice a year, it is a suitable target to evaluate the effect of coastal erosion on landslide activity. We recognized small landslide slips in accompanied with high pore water pressure and toe erosion events. The stability analysis incorporating the effects of the ground water and coastal erosion well explained the time history of the observed landslide displacement, showing that the effect of coastal erosion on landslide activity is one of the major factors. Furthermore, the stability analysis even during the period without landslide displacement also showed that toe erosion events lowered the stability of the landslide. This fact was supported by the inclination data which suggested the occurrence of the deformation event of the landslide body. It was implied from the field observation data that coastal erosion played an important role in the preparation and on-going displacement of the coastal landslide.