Monitoring ambient vibrations in an active landslide: Insights into seasonal material consolidation and resonance directivity

This work aims at understanding the potential of ambient vibration monitoring in deriving information about material properties change related to landslide seasonal behaviour and resonance directivity in landslides. The case study is the Pietrafitta landslide in southern Italy, known for its effects on the SS87 National Road and for being influenced by artificial vibrations. The field experiment consisted in four two-day-long measurement cy- cles, completed between April and November 2016, by using two broad-band three components velocimetric stations: one station at the landslide's toe and a reference station outside the landslide. Obtained data were analysed in terms of root mean square, spectral amplitude, horizontal-to-vertical spectral ratio and directionality effects. Results indicated the existence of amplitude daily variations due to vehicular traffic for both the stations, and long-time variations in the amplitudes and frequency content at the landslide site. Polarization and direc- tional resonance showed a daily variation, with the most coherent results in the range of 20◦ - 60◦ at the night hours, in the absence of traffic. This azimuthal trend was transverse to the sliding direction. The most marked resonance effect was found in the April recordings. The observed horizontal-to-vertical spectral ratio curves changed over time, due to variation in shear stiffness related to seasonal consolidation. Results from the horizontal-to-vertical spectral ratio inversion suggested a decrease from April to October 2016 in the Vp / Vs ratio, and in the velocity contrast at the base of the sediments consistent with common decrease in landslide activity due to summer drought.