GNGTS 2015 - Atti del 34° Convegno Nazionale

m thick. The blue clays lie on top of a not-outcropping stiff limestone of the Lower Miocene (Upper Globigerina Limestone, UGL). Also in this case, a lateral spreading is generated by the juxtaposition of the stiff limestones over the plastic blue clays. The phenomenon is responsible for the generation of four unstable rock blocks, behind which there is a stable plateau area. Seismic noise measurements were carried out on both the unstable blocks and the plateau area. Time-series of 20 minutes recorded using a three-component seismometer Tromino at a sampling frequency of 128 Hz were reprocessed for this study. In Anchor Bay, the HVSRs show a ubiquitous resonance peak in a narrow frequency range between 1 and 2 Hz and a following sharp dip of the spectral ratio. As this HVSR feature is widespread present in the Maltese archipelago, the dip of the HVSR curve has been interpreted in terms of a shallow shear-wave velocity inversion, which corresponds to the interface between the competent UCL and the plastic BC (Galea et al. , 2014). HVSR analysis comparison and interpretation. Given the similar outcropping lithologies, a similar response in the HVSR results of Cala Rossa Bay and Anchor Bay would be expected (Fig. 3). The HVSR peak at 5-6 Hz observed in Favignana and the peak at 1-2 Hz observed in Malta, as well as the following dips, could be related to a stratigraphic response. According to Galea et al. (2014), the HVSR peak can be interpreted in terms of Rayleigh-wave ellipticity and/ Fig. 2 – Amplitude spectrum and HVSR analysis results at A3 station. Left: results referred to the measurement on May 27 with stronger wind (on average 20 knots) and sea waves pounding directly on the cliff. Right: results referred to the measurement on May 30 with weak wind (on average 15 knots) and low sea waves on the cliff. 60 GNGTS 2015 S essione 3.2