GNGTS 2015 - Atti del 34° Convegno Nazionale

This paper reports the preliminary results of a geophysical campaign of seismic noise measurements carried out in May 2015 at Cala Rossa Bay. In recent years, several studies applied ambient noise techniques to investigate landslide-involved slopes (Del Gaudio et al. , 2008; Burjánek et al. , 2010, 2012) or to characterize blocks of unstable cliffs (Got et al. , 2010; Panzera et al. , 2012; Galea et al. , 2014), through different approaches among which H/V spectral ratios, f-k analysis, site to reference spectral ratios, polarization analysis, base noise level variations. The noise measurement stations were distributed in correspondence of the unstable sea cliff as well as on the stable plateau area (i.e. located about 100 m far from the cliff). The seismic noise was recorded for 1 hour and was analyzed by computing the Fast Fourier Transform (FFT) of the three ground-motion components as well as the HVSR. Because of the similar geological and geomorphological setting, the results obtained for the Cala Rossa Bay test site were compared to the ones obtained for Anchor Bay (Malta), reported in Galea et al. (2014) and partially reprocessed for the here discussed comparison. The goal of this research is to provide new evidences about local seismic response in unstable sea cliffs through the analysis of single-station noise records. The relevance of such a geophysical application consists on the possibility to manage the hazard related to sea-cliff landslides in order to mitigate the associated coastal risk. Geological and geomorphological setting. The Favignana Island belongs to the Egadi Archipelago and is part of the NW sector of Sicily (Italy), which represents the emerged western edge of the Sicilian-Maghrebian Chain, originated from the deformation of the Meso-Cenozoic Northern African continental margin (Scandone et al. , 1974; Antonioli et al. , 2006). Tectonic units, overthrusted in the Middle Miocene and Lower Pliocene, compose the Egadi Islands as well as the whole Trapani area (Nigro et al. , 2000). The Favignana Island is mainly built-up of Mesozoic-LowerTertiary carbonate deposits, 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, probably generated by transtensional faults activity (Slaczka et al. , 2011). In Cala Rossa Bay, located in the eastern portion of the Favignana Island, porous carbonate grainstones of the Lower Pleistocene widely outcrop, about 20 m thick and with almost- horizontal strata; these carbonate rocks overlie high-plasticity clays of Pliocene age with a thickness varying from 5 up to 10 m (Fig. 1a). The clays overlie massive carbonate grainstones of the Lower-Middle Miocene, with a thickness varies between 10 and 30 m (Abate et al. , 1995, 1997; Tondi et al. , 2012), which does not outcrop at Cala Rossa Bay. This geological setting characterized by the over-position of stiff rocks on a more plastic substratum (Goudie, 2004) leads to a lateral spreading phenomenon which widely involve the sea cliff. More in particular, the horizontal deformations affecting the clayey materials, with a visco-plastic behavior, induce cracks and failures in the overlying stiff rock causing the detachment of single rock blocks and the generation of falls and topples. Lateral spreading processes act selectively, shaping plateau of stiff rocks bordered by unstable cliffs. Lateral spreading phenomena represent a gravity-induced instability highly dangerous respect to the coastal cultural heritages and cause several touristic places in the Mediterranean area (Canuti et al. , 1990; Gigli et al. , 2012; Falconi et al. , 2015) to be inaccessible or avoided for tourists. The study area. In the western sector of Cala Rossa Bay in Favignana Island the ongoing lateral spreading produced a complex ground crack pattern which favors gravitational instabilities also influenced by the presence of an extensive network of underground tunnels related to historical quarry activities in the porous carbonate grainstones and terminated in the latter half of the last century (Falconi et al. , 2015). Based on field surveys, amain gravitational joint systemwas devised, consisting of three open fractures whose direction is nearly parallel to the coastline (Fig. 1b). Three different unstable zones, separated by the three opened ground cracks, and a fourth stable zone, consisting in the GNGTS 2015 S essione 3.2 57