GNGTS 2015 - Atti del 34° Convegno Nazionale

According to Varnes (1978) and Hutchinson (1988), the resulting landslide process should be defined as a complex-type one since the failure mechanism is a combination of lateral spreading, rock fall and rock topples. Moreover, the two main isolated blocks show a counter-slope top, indicating that a roto-translational component is also present in the spreading process. Data acquisition and processing. In May of 2015, a field-campaign was carried out in the western part of Cala Rossa Bay to record seismic ambient noise. Over an area of approximately 0.01 km 2 , 25 single-station measurements were deployed to cover the aforementioned landslide zones (Fig. 1b). More in particular, 5 stations were placed in each of three unstable zones while 10 stations were deployed on the stable plateau area, at various distances from the f3 ground crack. Each station was equipped with a 3-component seismometer: 20 measurements were carried out using a LE-3D/5s seismometer by Lennartz Electronic GmbH coupled with a REFTEK 130-01 datalogger, set to a 250 Hz sampling frequency; the other 5 measurements were carried out using a 1.5 Hz SL06 acquisition unit by SARA Electronic Instruments, set to a 200 Hz sampling frequency. Ambient noise records of 1-hour duration were acquired in each station in two days characterized by different weather conditions: on May 27 with a strong wind (on average 20 knots according to the Trapani forecast station) and sea waves directly pounding against the cliff and on May 30 with a weaker wind (on average 15 knots) blowing on the opposite direction and so avoiding the generation of sea waves against the cliff. The seismic noise records were processed by the use of 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) in the frequency range between 0.4 and 60.0 Hz was computed for each component. By averaging over the windows, the amplitude spectra and the H/V spectral ratio, as well as the distribution of their values in horizontal plane (i.e. spectrum rotate and H/V rotate), were finally achieved for each single record. The HVSR (Nakamura, 1989) analysis is worldwide used to predict the resonance frequency of a site, particularly when layers having low shear-wave velocity cause a sharp impedance contrast with the bedrock. The presence of a resonance peak in the HVSR curve has been interpreted both in terms of SH-wave resonance in soft surface layers, or in terms of the ellipticity of particle motion when the ambient noise wave train is made up predominantly of surface waves (Bonnefoy-Claudet et al. , 2006). In practice, the wavefield is expected to be a combination of both types and the HVSR curve contains information about the shear wave velocity profile in shallow sediments (Galea et al. , 2014). Results. By analyzing the FFT of the Cala Rossa Bay records (Fig. 2), a significant energy contribution clearly appears at frequencies lower than 1 Hz in May 27, when a strong wind and sea waves acted against the cliff; on contrary, in May 30, with more favorable weather conditions, such a spectral contribution disappears. The HVSR curves show a peculiar shape both in the unstable and in the plateau zones, a cave can be observed in the frequency range between 1 and 2 Hz, corresponding to a marked amplification of the vertical ground motion component. The HVSRs also show an asymmetrical peak between 5 and 6 Hz and several peaks at higher frequencies (10-60 Hz). Even though it is not always significant according to SESAME guidelines (Bard, 2005), the HVSR peak at 5-6 Hz frequency is present in the measurements carried out within and in proximity of the unstable zones, while it is not visible in the plateau area. Discussion. The Anchor Bay test site. A comparison between the results obtained at Cala Rossa Bay in Favignana Island and the ones of Anchor Bay in Malta was possible taking into account the data already published by Galea et al. (2014). The Anchor Bay test site was chosen for such a comparison because of the lithotechnical setting very similar to the one of Cala Rossa Bay. According to Pedley et al. (2002), in Anchor Bay stiff limestones of Late Miocene age (Upper Coralline Limestone, UCL), with a thickness of about 20 m, overlie blue clays of the Middle Miocene (Blue Clay formation, BC) about 30 GNGTS 2015 S essione 3.2 59