GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione 2.2 367 marked directivity properties as evidenced by polarization analysis results. The ellipticity diagrams show a corresponding sharp drop to zero at the same frequencies of the HVSR peaks, indicating a high degree of linearity in the particle motion, and the polar plots show directivity of particle motion approximately orthogonal to the fractures. In general, the results obtained by the polarization analysis show a polarization perpendicular to the main fractures that can be interpreted as vibrational motion of individual blocks (Galea et al. , 2014), where vibration frequency is related to geometrical and mechanical properties of the dislodged rock blocks (Got et al. , 2010). Conclusions. In the western part of Cala Rossa bay (Favignana, Italy), an ongoing lateral spreading process threats the “Pirrere” quarries, an important heritage site. On February-May 2015, seismic noise measurements were carried out both on the stable plateau area and on the unstable zone that pointed out a different seismic response in the two areas. In the unstable area, several HVRS peaks were observed at frequencies higher than 5.0 Hz and these peaks are characterized by marked directivity properties. The results by HVSR and polarization analyses allowed to associate the obtained seismic response to the vibrational motion of dislodged rock blocks. Considering the obtained preliminary results, more-detailed studies will be carry out to characterize the vibrational modes of single rock blocks, in the frame of designing protection strategies to manage the landslide process in Cala Rossa bay. Acknowledgements. The Authors are grateful to J. Burjánek for the use of the polarization codes. The Authors wish to thank the local authorities for supporting the geological and geophysical surveys. References Abate B., Ferruzza G., Incandela A. and Renda P.; 1995: Tettonica trascorrente nelle Isole Egadi (Sicilia occidentale). Studi Geologici Camerti, vol. spec. 1995/2 , 9-14. Abate B., Incandela A. and Renda P.; 1997: Carta geologica delle isole di Favignana e Levanzo (Arcipelago delle Egadi), Scala 1:12.500. Dip. di Geologia e Geodesia, Università di Palermo. Antonioli F., Kershaw S., Renda P., Rust D., Belluomini G., Cerasoli M., Radtke U. and Silenzi S.; 2006: Elevation of the last interglacial highstand in Sicily (Italy): A benchmark of coastal tectonics. Quaternary International, 145-146 , 3-18. Bonnefoy-Claudet S., Cotton F. and Bard P.Y.; 2006: The nature of noise wavefield and its applications for site effects studies: a literature review. Earth-Sci. Rev., 79 (3), 205-227. Burjánek J., Gassner-Stamm G., Poggi V., Moore J.R. and Fäh D.; 2010: Ambient vibration analysis of an unstable mountain slope. Geophys. J. Int., 180 (2), 820-828. Burjánek J., Moore J.R., Yugsi Molina F.X. and Fäh D.; 2012: Instrumental evidence of normal mode rock slope vibration. Geophys. J. Int., 188 (2), 559-569. Catalano R., Di Stefano P., Sulli A. and Vitale F.P.; 1996: Paleogeography and structure of the central Mediterranean: Sicily and its offshore area. Tectonophysics, 260 , 291-323. Del Gaudio V., Coccia S., Wasowski J., Gallipoli M.R. and Mucciarelli M.; 2008: Detection of directivity in seismic site response from microtremor spectral analysis. Nat. Hazards Earth Syst. Sci., 8 , 751-762. Falconi L., Peloso A., Puglisi C., Screpanti A., Tati A. and Verrubbi V.; 2015: Rockfalls monitoring along eastern coastal cliffs of the Favignana island (Egadi, Sicily): preliminary remarks. In Lollino G., Giordan D., Marunteanu C., Christaras B., Yoshinori I. and Margottini C., eds: Engineering Geology for Society and Territory, 8 , 287-291. DOI: 10.1007/978-3-319-09408-3_50 Galea P., D’Amico S. and Farrugia D.; 2014: Dynamic characteristics of an active coastal spreading area using ambient noise measurements-Anchor Bay, Malta. Geophys. J. Int., 199 , 1166-1175. Gigli G., Frodella W., Mugnai F., Tapete D., Cigna F., Fanti R., Intrieri E. and Lombardi L.; 2012: Instability mechanisms affecting cultural heritage sites in the Maltese Archipelago. Nat. Hazards Earth Syst. Sci., 12 , 1883- 1903. Got J.-L., Mourot P. and Grangeon J.; 2010: Pre-failure behaviour of an unstable limestone cliff from displacement and seismic data. Nat. Hazards Earth Syst. Sci., 10 , 819-829. Goudie A.; 2004: Encyclopedia of Geomorphology. Routledge, London (UK), 1156 pp. Iannucci R., D’Amico S., Farrugia D., Galea P., Martino S. and Paciello A.; 2015: Seismic measurements in unstable sea cliff areas. Atti del 34° Convegno Nazionale Gruppo Nazionale di Geofisica della Terra Solida. Tema 3: Geofisica Applicata. Trieste (TS), 17-19 November 2015, vol. 3 , 56-63.