GNGTS 2014 - Atti del 33° Convegno Nazionale

Description of results and concluding remarks. Site effects at a specific location are very important and can be used for engineering purposes to define the regional predictive law and the seismic hazard. A generalized site response concept is indeed useful to create a detailed shaking map for a region where the different outcropping lithologies are known. The results obtained set into evidence that major spectral ratio peaks are detected in the frequency range 2,0 - 4,5 Hz. Going into more details, we observe that these peaks often do not reach two amplitude units. Such behavior is more evident both in the south-eastern part of the island, as well as in its central and western portion. Comparison with the Lampedusa lithology point out that in these areas the most ancient and rigid terrains outcrop. This is in good agreement with the stiffness of the limestone formations extensively outcropping in the plateau located in the central part of the island. More pronounced spectral ratio peaks are detected in the measurement sites located close to the outcrops of more recent and soft deposits (eg. #55,#68 in Fig. 1b) and along the transects crossing the Cala Creta fault which marks out the eastern boundary of the island (e.g. #61, #64, #58, #59 in Fig. 1b). Similar well defined spectral ratio peaks are detected in the transects performed perpendicularly to the strike of the morphologic escarpments existing in the north eastern side of the island (e.g. #65, #66 in Fig. 1b). We can therefore affirm that since Lampedusa is almost entirely formed by calcareous deposits, as a consequence the amplification effects are mostly caused by either morphological or tectonic structures. In Fig. 2 examples of different site response features observed in either the plateau area or nearby the cliffs and the fault lines, are shown. Fig. 3 – Examples of fundamental frequencies as observed from SSNRs obtained through ambient noise recorded in two different buildings in Lampedusa. The polarization of the horizontal components of motion, evaluated in the measurement sites located nearby the fault lines, show that the largest amplifications occur at high angle from the fault strike (Lombardo and Rigano, 2006; Panzera et al. , 2012). On the other hand, measurements performed at increasing distance from the fault zone do not show a similar behavior and this suggests that the observed directional effects can be ascribed to the fault fabric (Panzera et al. , 2014). The measurement performed on buildings allowed us to infer the fundamental period of the investigated buildings, showing that there was no particular soil-to-structure effects. GNGTS 2014 S essione 2.2 207