Directivity of slope dynamic response to seismic shaking

A four year-long accelerometric monitoring of a landslide-prone slope area in Italy provided evidence of amplifications with systematic directional differences in shaking energy by a factor of 2-3, apparently correlated with the directions of local topographic features. The examination of regional-scale data revealed similar site response directivity phenomena also on some steep rock slopes not affected by amplification. The spectral properties of the site responses were investigated using HVSR analysis and a new approach based on the multiplication of spectra from several events. The results suggest that such phenomena are caused by a directional redistribution of spectral energy controlled by a combination of interrelated topographic and geologic factors acting in similar directions.