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preLiminary evidences of sLope instabiLities through seismic measurements at caLa rossa bay (itaLy) R. Iannucci1, S. Martino1, A. Paciello2, S. D’Amico3

1 Dip. Scienze della Terra & Centro di Ricerca per i Rischi Geologici (CERI), Università “La Sapienza”, Roma, Italy2 ENEA, C.R. Casaccia, Roma, Italy3 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.

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Data acquisition and processing. Between February and May 2015, geophysical campaigns of seismic noise measurements were carried out in Cala Rossa. Recently, seismic noise measurements were applied 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, e.g. HVSR, f-k analysis, site to reference spectral ratios, polarization analysis, base noise level variations.

Seismic noise was recorded in 47 single-station measurement sites over an area of approximately 0.15 km2, to cover both the unstable zones of the sea cliff and the stable carbonate plateau. Each station was equipped with a 3-component seismometer: 28 measurements were carried out using a LE-3D/5s seismometer by Lennartz Electronic GmbH and a REFTEK 130-01 datalogger set to a 250 Hz sampling frequency; 19 measurements were carried out using a 1.5 Hz SL06 acquisition unit by SARA Electronic Instruments, set to a 200 Hz sampling frequency.

The seismic noise records were processed by the Geopsy software (www.geopsy.org). The 1-hour time histories were divided into non-overlapping windows of 40 s and the Fast Fourier Transform (FFT) was computed for each component in the frequency range between 1.0 and 60.0 Hz. By averaging over the windows, the amplitude spectra and the H/V spectral ratio were finally achieved for each single record. The HVSR method (Nakamura, 1989) allows to obtain the resonance frequency of a site. The presence of a resonance peak in the HVSR curve has been interpreted both in terms of SH-wave resonance and in terms of the ellipticity of particle motion when the ambient noise wave train is made up predominantly of surface

Fig. 1 – Geological and geomorphological setting of the western part of Cala Rossa bay: a) excerpt of Favignana geological map in the Cala Rossa area (from Abate et al., 1997); b) picture of sea cliff showing the overlapping of the Lower Pleistocene calcarenites on the Pliocene clay deposits; c) picture showing one of the main joints on the calcarenites plateau.

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waves (Bonnefoy-Claudet et al., 2006). The wavefield is a combination of both types and the shape of the HVSR curve provides information about the shear wave velocity profile in shallow sediments (Galea et al., 2014).

Polarization analysis was introduced by Vidale (1986) and implemented by Burjánek et al. (2010) by adopting a continuous wavelet transform (CWT). The particle motion is characterized at given time and frequency by a 3D ellipse. The WAVEPOL package outputs the analysis of ambient noise time-series in visual representations of combined angular and frequency dependence. The ellipticity of the particle motion is defined as the ratio of the semi-minor axis to the semi-major axis of the ellipse (i.e. 1 represents circular particle motion and 0 purely linear motion), pointing out the polarization effects. It is represented as a 3D histogram of ellipticity versus frequency. Histograms of strike of the ellipse major axis are represented as circles on a polar plot, in which the frequency increases along the radius, and a colour ramp is used to denote amplitude in each histogram.

Results. HVSR analysis. The noise analyses pointed out a marked difference in the seismic response between the unstable areas and the stable plateau zone (Fig. 2). The HVSR curves show significant resonance peaks at frequency higher than 5.0 Hz in the measurements carried

Fig. 2 – Orthophoto of the Cala Rossa western sector showing: open and estimated fractures, the 47 noise measurement stations and HVSR curves grouped by different areas; for the unstable area (in the yellow frame) only the most significant HVSR curves are shown.

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out within and in proximity of the unstable zones, while these peaks are not present in the measurements carried out on the plateau zone.

The seismic energy in the frequency higher than 5.0 Hz can be related to the vibrational behavior of the dislodged rock blocks, according to Got et al. (2010) and Galea et al. (2014). In fact, Fig. 2 shows that resonance peaks in HVSR curves are much more evident in areas having higher density of fractures and blocks, as the zone where the three main joints are located.

Polarization analysis. Fig. 3 shows some examples of the polarization analysis results, giving information about the particle motion behavior in the different areas. For stations on the stable plateau area (e.g. FA01 showing in Fig. 3), the noise analysis points out that there are neither HVSR peaks nor polarization of the particle motion. Instead, the resonance peaks in the HVSR curves obtained on the unstable zone (e.g. FA07 showing in Fig. 3) show

Fig. 3 – Comparison of HVSR curve and polarization analysis results (ellipticity and strike) in the different areas.

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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.

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