GNGTS2010 _______________________________________________________________________________________________ SESSIONE 2.2 SITE RESPONSE IN ORTIGIA PENINSULA (SIRACUSA, ITALY) F. Panzera, G. Lombardo, R. Rigano Dipartimento Scienze Geologiche - Universita di Catania, Italy The effects of topography have been widely studied by several analytical and numerical methods (e.g., Paolucci, 2002), instrumental evidences of topographic effects are however relatively few. Experimental techniques for investigating the topographic effects are quite expensive since they require the setting down and operation of the instruments for an undefined period of time to acquire earthquakes. For this reason, explosions and noise measurements can be very useful to estimate these site effects. Although such techniques have been rarely used to investigate on topographic effects, rather satisfactory results have been obtained (e.g., LeBrun et al., 1999; Pagliaroli et al., 2007). The surface topography has a great influence on strong-motion earthquake data, but there is no theoretical explanation why the HVNR method could give good results for purely topographic effects. The aim of present experiment is to estimate the seismic response of the study area and to test the reliability of using ambient noise recordings to define possible topographic effects. The study area represents, because of its geological and morphological setting, an useful test site to perform passive experimental techniques aiming to identify the site response directivity and the fundamental resonant frequency connected to the topographic effects. It is formed by a carbonate sequence whose dynamic properties were investigated through non-invasive techniques (MASW and ReMi). The results obtained through both techniques (Fig. 1) show a V (S 30) of about 1000 m/s, that is in good agreement with an A soil category. Twelve ambient noise recording sites were selected in the study area. The signals were acquired through Tromino, a compact 3-component velocimeter with a reasonable instrumental response in the frequency range 0.5-10 Hz. Time series of ambient noise, 30 minutes length, were recorded using a sampling rate of 128 Hz and processed through spectral ratio techniques (HVNR). The most stationary parts of the signals were selected, not including transients associated to very close sources, according to the guidelines suggested by the SESAME project. Fourier spectra were cal- culated and smoothed using a triangular average on frequency intervals of ± 5% of the central fre- quency. Moreover, HVNR were calculated after rotating the NS and EW components of motion by steps of 10 degrees from 0° (north) to 180° (south). This approach is powerful in enhancing, if any, the occurrence of site specific directional effects. A direct estimate of the polarization angle was 278