GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione 2.2 363 Preliminary evidences of slope instabilities through seismic measurements at Cala Rossa bay (Italy) R. Iannucci 1 , S. Martino 1 , A. Paciello 2 , S. D’Amico 3 1 Dip. Scienze della Terra & Centro di Ricerca per i Rischi Geologici (CERI), Università “La Sapienza”, Roma, Italy 2 ENEA, C.R. Casaccia, Roma, Italy 3 Department of Geosciences, University of Malta, Msida, Malta Introduction. In the Mediterranean area, sea cliffs are high-risk elements due to the diffused landslide processes that affect sites of touristic and heritage relevance. Since the Roman Age and until the last century, an intense quarry activity took place at Favignana Island (Sicily, Italy) that significantly changed the morphology of the eastern part of the Island. This mining activity produced an extensive network of open air quarries, underground quarries and tunnels, locally named “Pirrere”, hosted into Pleistocene calcarenites. These quarries represent one of the most important cultural heritage of Sicily and currently the safety of this important site is threaten by landslide processes. The geological setting of the western sector of Cala Rossa bay predisposes a large lateral spreading responsible for instabilities of the sea cliff (Falconi el al. , 2015), associated to falls, slides and/or topples of different-size rock blocks as evidenced by the wide block-size talus distributed all along the coast line. This paper reports preliminary results of geophysical campaigns of seismic noise measurements carried out in February-May 2015 at Cala Rossa bay. The noise measurements were performed in several stations distributed in correspondence of the unstable sea cliff as well as on the stable plateau. The three ground-motion components of the seismic noise were analyzed to obtain the horizontal-to-vertical spectral ratio (HVSR) as well as to perform a frequency-dependent polarization analysis. The goal of this research is to point out evidences of the ongoing slope instabilities through the local seismic response. Such a geophysical application can be useful to zone hazardous coastal areas in the frame of natural risk mitigation for designing protection strategies to manage the landslide risk in Cala Rossa bay and preserving the “Pirrere” quarries. Geological and geomorphological setting. Aegadian Islands represents a part of the Egadi Thrust Belt of the Sicilian-Maghrebian system, originated from the deformation of the Meso-Cenozoic Northern African continental margin (Antonioli et al. , 2006). Favignana Island is mainly composed by Mesozoic-Lower Tertiary carbonate deposits, overthrusted in the Middle Miocene and Lower Pliocene (Nigro et al. , 2000) and covered by transgressive Plio-Pleistocene shallow-water marine deposits (Abate et al. , 1995, 1997; Catalano et al. , 1996). The Lower Pleistocene deposits widespread outcrop along the eastern slope of the emerged paleo-Favignana Island in a pull-apart basin generated by transtensional faults activity (Slaczka et al. , 2011). Cala Rossa bay is located in the eastern part of Favignana Island. In this area, porous calcarenites of the Lower Pleistocene widely outcrop (Fig. 1a), about 20 m thick and with almost-horizontal strata. The calcarenites overly high-plasticity clays of Pliocene age having thickness of 5-10 m (Fig. 1b). Under the clays there are massive calcarenites of the Lower- Middle Miocene (Abate et al. , 1995, 1997), not outcropping at Cala Rossa bay. The juxtaposition of stiff rocks (calcarenites) on a plastic substratum (clays) leads to a lateral spreading phenomenon (Goudie, 2004) at the Cala Rossa bay western side: the horizontal deformations affecting the clayey materials, with a visco-plastic behavior, induce cracks in the overlying stiff rock (Fig. 1c). Lateral spreading shapes a plateau of stiff rock bordered by jointed unstable cliffs, favoring fall, slide and/or topple of single rock blocks. A detailed characterization of the joint net was carried out on May 2016, to obtain distribution and geomechanical properties of the joints (Iannucci et al. , 2015). The instability processes take place in a rock mass extremely re-shaped by the extensive network of the “Pirrere” quarries, threating the safety of this important cultural heritage.