GNGTS 2014 - Atti del 33° Convegno Nazionale

254 GNGTS 2014 S essione 2.2 horizontal directional amplification at rock sites and the relation with topography. Ambient noise and earthquake records of 226 three-component seismological stations installed on apparently stiff rock were investigated using H/V spectral ratios and horizontal polarization analysis. At first, H/V spectral ratios of ambient noise were calculated on rotated horizontal components from 0° to 180°, indicating that 127 (56% of the initial data set) of stations was affected by a significant amplification in site-dependent frequency bands (HVSR amplitude >2). This effect was also found to be strongly directional. A strict criterion, based on the covariance matrix diagonalization (Jurkevics, 1988) was then applied to select sites with a strong local tendency to polarize ground motion in the horizontal plane. Results indicated that 81 (36% of the initial data set) honored this condition. In order to check the stability of the effect, excluding the possibility that they could be artifacts of the noise signals or that they could be related to human activities, the analysis was repeated using earthquake records selected among those occurred in Italy in the period January 2008 – March 2011, with magnitude higher than 3. Pischiutta et al. (2011) concluded that an horizontal amplification larger than 2 was found at 66 stations (29% of the initial data set, which is a not negligible percentage) and that this amplification was strongly directional. Because of the Italian territory is affected by significant topographic variations, inevitably many stations of the Italian Seismic Network are installed on topographic irregularities. In order to characterize the local topographic conditions they have analyzed the DEM finding that, as expected, the most part of the selected stations showing directional amplification are installed on topographic irregularities (Rovelli et al., 2011). In Fig. 1 we provide a local-scale description for three stations chosen as representative of the most common cases. The top panel is relative to station CERT (Cerreto Laziale) located on Fig. 1 – Results of polarization analysis for three representative stations. The top panel shows the polarization result at station CERT (Cerreto Laziale), on a 1 km wide and 300 m high ridge with a nearly ellyptical shape. This station is an example of a systematic directional amplification occurring transversally to the topographic elongation. The middle and bottom panels represent results from stations SGTA (Sant’Agata Puglia) and ILLI (Lipari), respectively.