Landslides DOI 10.1007/s10346-019-01338-w Received: 1 August 2019 Accepted: 18 December 2019 © Springer-Verlag GmbH Germany part of Springer Nature 2020 G. Pappalardo I S. Mineo I S. Imposa I S. Grassi I A. Leotta I F. La Rosa I D. Salerno A quick combined approach for the characterization of a cliff during a post-rockfall emergency Abstract The research presented in this paper is aimed at testing an innovative surveying protocol based the integration of quick methodologies employed for the characterization of a carbonate cliff affected by a rockfall and showing signs of further instability. During a post-rockfall emergency, one of the most important activities is evaluating if reactivation of the movements is possible, and if it may represent a threat for a series of elements at risk. On 5 January 2019, intense rainfalls triggered the detachment of a sig- nificant volume of fractured limestone and dolostone below the main square of a historical village located in southern Italy in the municipality of Messina, giving rise to one of the greatest land- slides occurred at one of the most tourist areas of northeastern Sicily. Fallen blocks damaged the two main infrastructures of the area reaching also a bus terminal, which fortunately was vacant at the time of the rockfall. With the aim of proposing a quick surveying protocol during a post-rockfall emergency, terrestrial laser scanner, infrared thermography, and horizontal to vertical spectral ratio surveys were employed for the geostructural charac- terization of the cliff and for the investigation of the subsoil below the main square. In particular, the survey through terrestrial laser scanner returned an accurate 3D model of the cliff, where some key structures were highlighted; infrared thermography allowed recog- nizing caves and fractures along the cliff, with specific reference to a hollow area arising from past rockfalls. Such remote data, along with direct rock mass surveys performed by expert climbers allowed ascertaining that the instability of this cliff is driven by wedges formed by the intersection of 2 and 3 discontinuity systems, which are likely related to the main tectonic systems of the area. The horizontal to vertical spectral ratio survey allowed the identification of a peculiar vertical contrast of impedance, which may be related to a mechanical discontinuity located below the main square of the village, well matching with one of the systems responsible of the instability. The integration of these surveys methodologies resulted a useful quick protocol for the achievement of the key information on the stability of a rock cliff in the initial stage of its securing. Keywords Rock cliff . Rockfall . Infrared thermography . Laser scanner . HVSR Introduction In a post-rockfall emergency, the quick survey of affected areas is essential to ensure a rapid characterization of the problem, fore- cast its potential evolution and suggest the most suitable urgent remedial measures. To this purpose, the integration of quick sur- vey methodologies is welcomed for the management of the emer- gency, especially when no previous engineering-geological data are available and the most exposed elements at risk have to be located. When the setting of rock cliffs does not offer suitable spots for direct rock mass surveys or the unstable rock volumes are located at relevant heights, the remote and/or indirect analysis of the main geostructural features is a feasible solution to achieve reliable data. The recent technological development in the analysis of slope instability has highlighted the potential of some innovative, non- destructive, and stand-alone survey methodologies acting from both a remote position (e.g., terrestrial laser scanner, infrared thermography) and in situ (geophysical HVSR prospecting). More specifically, terrestrial laser scanner (TLS) provides detailed point cloud data to obtain a highly accurate 3D model of the slope, thus overcoming the limitations of traditional surveying methodologies applied to high cliffs or to hardly accessible rock faces, although a direct rock mass survey, when feasible, is welcomed for a more realistic interpretation of TLS results. The scientific literature offers numerous cases of TLS application for the study and mon- itoring of rock masses (e.g., Lim et al. 2005; Rosser et al. 2005; Armesto et al. 2009; Fanti et al. 2012; Abellán et al. 2013; Gigli et al. 2014a; Mineo et al. 2018). Its most significant advantage is the possibility of achieving the accuracy of the best topographic in- struments, thus ensuring a precise analysis of the rock face, with the possibility of focusing on the main instability features. Infrared thermography (IRT) is a particular imaging technique based on the infrared radiation emitted by all forms of matter with temperature above the absolute zero (Hillel 1998). Its application to the rock mechanics as a diagnostic tool for instability phenom- ena has still a limited case history. It allows building images highlighting the thermal variations occurring along a rock slopes, which can be related to specific features of the rock. The most recent experiences proved the usefulness of IRT for either the study of weathered unstable slopes, or the rock mass fracturing characterization, or the remote survey and mapping of large land- slides (Baroň et al. 2012; Casagli et al. 2017; Pappalardo et al. 2018a; Pappalardo and Mineo 2019). Some application cases in literature are focused on the integration between IRT and geomechanical surveys and/or TLS (e.g., Gigli et al. 2014b; Martino and Mazzanti 2014; Mineo et al. 2015a, b; Teza et al. 2015; Pappalardo et al. 2016a). The horizontal to vertical spectral ratio (HVSR) is an indirect geophysical surveying methodology employed for the calculation of the spectral ratio between the mean horizontal on the vertical components of the ground motion (Nakamura 1989). HVSR tech- nique allows identifying the amplification frequency of the ground motion due to resonance effects. This can be related to the pres- ence of tectonic and/or stratigraphic discontinuities, landslides, or peculiar local conditions. Several authors employed HVSR tech- nique to get information on the seismostratigraphy features of the subsoil (e.g., Lermo and Chavez-Garcia 1993; Lachet and Bard 1994; Ibs-von Seht and Wohlenberg 1999). Pappalardo et al. (2016b) coupled this survey method to geostructural analyses to reconstruct the degree of fracturing of a rock cliff in a cultural heritage site previously investigated from the geostructural point of view (Imposa et al. 2010, 2015), while Imposa et al. (2017a) proved its utility in the survey of a landslide body. A successful application was achieved also by Pappalardo et al. (2018b), who detected a relict landslide underneath an archeological site in Sicily. Landslides Original Paper