The difficult prediction of earthquake-induced landslide displacements: the case of Gaggio Montano (northern Apennines, Italy)

One of the most damaging side effects associated with seismic shaking within a certain distance from the seismogenic source are possible landslide movements. Seismic-induced landslide movements can cause serious problems to local communities even if the seismic shaking is of low intensity. To face these problems, local administrators are often forced to take unpopular planning decisions that affect the population. Therefore, detailed knowledge of landslide behavior to seismic shaking appears crucial.

In this study, we analyze the seismic response of a slow-moving landslide, which mainly affects clay soils. This landslide is located along a built-up area of about one thousand inhabitants (Gaggio Montano, northern Apennines, Italy), in a zone classified as medium seismic risk and its hazard potential has been assessed by considering geomorphological, geological, hydrological and seismic issues.

Firstly, we compiled a database of the existent geognostic investigations (cores, inclinometers, piezometers and CPT) and of the new geophysical surveys (HVSR and MASW) specifically acquired for this study. This allowed us to realize a detailed geological model of the slope and of the landslide body. Secondly, we run a 2D-numerical model of the seismic response of the slope. This allowed to evaluate the topographic and stratigraphic effects in response to hypothetical earthquakes with return period of 475 years. Finally, we applied the Newmark method to estimate the seismic-induced landslide movements.

A detailed sensitivity analysis was performed to account for the strong uncertainty in the data, especially regarding the geotechnical input parameters. The results show a dramatic influence of the shear strength parameters on the seismic-induced displacements. For example, by changing the effective friction angle by just 1°, the displacements vary more than 30 cm. This highlights the limitations of the conventional approach and the difficulty to predict the response of existing landslides to seismic shaking.