Seismic response of landslides to natural and man-induced ground vibrations. Evidence from the Petacciato coastal slope (central Italy)

This study focuses on the vibrational response of a landslide mass to natural and man-induced vibrations. The landslide is located in the Molise region (Italy) and involves a coastal slope extending approximately 5 km2, from the Petacciato village to the coastline. The landslide activity is well documented since 1909 with historical reactivations and resulted in severe damage to infrastructure and to the ancient village. This activity is characterised by a roto-translational mechanism involving blue clays, sands, and conglomerates of Pleistocene age. A multitemporal geomorphological analysis of the landslide activity allowed us to detect a retrogressive style corresponding to the crown area as well as a prograding toe that reaches the submerged nearshore. Seismic ambient noise measurements revealed that the local seismic amplification is strictly concentrated in the backcrown area, i.e., the area affected by the most recent retrogressive effects of the landslide mass. Moreover, ground vibrations generated by train transits were analysed to identify possible amplification phenomena of dynamic external input, that can occur inside the landslide mass with respect to the surrounding area. Therefore, a Spectral Aggravation Index (SAI) was defined by the ratio between the Fourier spectrum during train transit and that obtained for an equal time window in the absence of train traffic sources. The SAI was applied to compare the response to man-induced vibrations inside the landslide mass with the one outside, by computing a ratio of the SAI at different measurement points inside/outside the landslide mass. The obtained results highlight the intense activity of the landslide both in space and time, revealing a joining between the ongoing retreat areas, surveyed by multitemporal geomorphological analysis, and local seismic response, by geophysical investigations. On the other hand, most of the landslide mass from the middle- to downslope position is not resonant and its higher damping causes a lower intensity of ground motion in response to man-induced vibrations with respect to the surrounding areas.