GNGTS 2019 - Atti del 38° Convegno Nazionale

348 GNGTS 2019 S essione 2.2 HVSR VARIABILITY AT SITES NEAR AND UPON TOPOGRAPHIC HEIGHTS G.D. Chiappetta 1 , A. Gervasi 2,1 , R.L. Festa 1 , M. La Rocca 1 1 Università della Calabria, Rende, Italy 2 Istituto Nazionale di Geofisica e Vulcanologia, Rende, Italy Introduction. The horizontal to vertical spectral ratio (HVSR, Nakamura, 1989) is one of the most efficient techniques available for the analysis of site effects, especially in the estimation of the resonance frequency in case of a soft surface layer upon a bedrock (Mucciarelli, 1998; Mucciarelli and Gallipoli, 2001; Bonnefoy-Claudet et al. , 2006a). The layer seismic resonance produces a well defined peak in the HVSR curve, and its frequency is strictly related to the layer thickness (Bonnefoy-Claudet et al. , 2006a). Interesting site effects are produced by the interaction of seismic waves with the topography. Topographic effects generally produce an amplification of the horizontal ground motion and a polarization in the same direction of the maximum slope, or normal to the ridge crest, thus mountain places are commonly affected by such effects in case of earthquake (Pedersen et al. , 1994; Spudich et al. , 1996). The analysis of seismic noise provides useful information about the topographic effects (Chavez-Garcia et al. , 1996; Panzera et al. , 2011). Results obtained from HVSR analysis are generally considered as a stable feature of a given site, infact a number of experiments have shown that the frequency of the resonance peak is almost the same among weak stationary noise, transient noise, traffic noise, calm or windy days, and even earthquakes (Mucciarelli, 1998; Mucciarelli et al. , 2003; Parolai et al. , 2004; Guillier et al. , 2007; Cara et al. , 2010). Actually, the most of these observations were carried out on sites characterized by quite simple geological structures where a well-defined resonance peak occurs, while evidences of non-stability of the HVSR curves are very few (Benkaci et al. , 2018). In this paper we show some examples of sites where the HVSR shape may change considerably with the appearence of one or more occasional peaks when the amplitude of background noise increases due to weather conditions. Data analysis and results. We recorded a large amount of data at many different sites in Calabria (Italy) through three component seismic stations (Fig. 1). Then we performed standard HVSR analysis on selected data using the software GEOPSY (www.geopsy.org ). Spectra were computed on 120 s sliding window, applying an anti- triggering algorithm if necessary to remove transients related to vehicles and other human activities, and they were used to compute the average HVSR and its standard deviation. Through the same software we computed also the HVSR versus azimuth to study the polarization of the HVSRpeaks, which generally is a consequence of the amplification Fig. 1 - Topography map showing the position of seismic stations used in this paper. MMN, ROI, CMPN and SMIN are permanent stations of the Università della Calabria Seismic Network, while T002, T017 and TP02 shown in the inset are temporary stations.