Deciphering the dynamics of a heterogeneous sea cliff using ambient vibrations: case study of the Sutta-Rocca overhang (southern Corsica, France)

We report here an interesting case study of dynamic characterization of a limestone sea cliff area, named the Sutta-Rocca overhang, located near the medieval town of Bonifacio (southern Corsica, France). The site belongs to an UNESCO protected area with an important number of visitors per year. Therefore, it is of particular interest to evaluate the potential rockfall hazard along the cliff. With the objective of evaluating the feasibility of an operational technique to monitor the cliff using ambient vibrations, two seismic stations were installed during six months (from 2016 November to 2017 April) in order to identify and monitor the resonant frequencies of the a priori unstable rock mass. Several techniques of classical seismic monitoring are tested on the site: single-station spectral analysis for identification of frequency peaks related to the unstable mass, relative spectral amplifications between stations and the operational modal analysis of the cliff. In particular, data analysis from a temporary campaign of two days of continuous recordings by a linear array of seismometers perpendicular to the sea cliff highlights the main characteristics of the ground motion: the overall motion of the promontory mainly in the north–south direction (perpendicular to the coast) and bending vertical and parallel to the cliff (east–west direction) at higher frequencies. The strong vertical and east–west seismic amplifications are also confirmed by the analysis of five local seismic events recorded at both semi-permanent stations. Even if ambient vibrations and seismic data analysis allows to decipher the dynamics of the Sutta-Rocca overhang, we conclude that classical seismic monitoring of just one single frequency peak with a limited number of sensors is not recommended for this particular site.