Seismically induced slope instability maps validated at an urban scale by site numerical simulations

Maps of seismically induced instability at the urban scale can be drawn by means of geographic information system (GIS) tools that integrate different information layers such as (1) a landslide inventory; (2) a digital elevation model (DEM); (3) geo-hydro-mechanical site characterization, and (4) measured peaks or integral parameters at seismic stations. These maps are used to guide planning activities and emergency actions, but their main limitation is typically the lack of reliable analyses or calibrations. In this study, a possible method is proposed to control and increase the overall reliability of an hazard scenario map of earthquake-induced slope instability. The procedure can be summarized in the following steps: (1) GIS tools are used to describe the spatial distribution of the hydro-mechanical properties of the surface lithologies; (2) seismically induced instability maps of permanent displacements are drawn from the preceding information layers combined with seismic parameters spatially propagated by means of spatial interpolation tools; (3) point dynamic and stability numerical analyses are carried out by means of a commercial finite element method (FEM) code (e.g., Geostudio2004) to calculate permanent displacement by the Newmark's method along representative cross-sections. The numerical analyses are used to calculate a "depth factor", which can be considered as the contribution of the seismic local amplification to the surface calculations addressed by GIS tools. The ratio between the results drawn from the two approaches (GIS-based and FEM-based implementing Newmark's method) can be assumed as a scale factor related to the in-depth site-specific geo-lithotechnical characters to be addeded to GIS maps.