GNGTS 2021 - Atti del 39° Convegno Nazionale

GNGTS 2021 S essione 2.2 248 DAY-NIGHT CYCLE IN THE HVSR OF SEISMIC NOISE G.D. Chiappetta 1 , A. Gervasi 1,2 , M. La Rocca 1 1 Università della Calabria, Rende, Italy 2 Istituto Nazionale di Geofisica e Vulcanologia, Rende, Italy Introduction HVSR method (Nakamura, 1989) is one of the most efficient and widespread techniques to get useful information about seismic site effects. In more than 30 years of application many authors reported the effectiveness of this method in identifying the resonance frequency of the shallow geological structure in various geological context, particularly those characterized by horizontal soft layers (Mucciarelli, 1998; Bard, 1999; Bonnefoy-Claudet et al ., 2006a). The method has been applied to seismic noise and to earthquake signals, giving similar results in the most of cases. During the last 20 years many questions arose about the stability of the results obtained from this analysis and theoretical considerations have been proposed to estimate the contribution of different seismic wave types to the HVSR result. While the interpretation of results is quite obvious in cases of flat topography and simple layered structures upon bedrock with high impedance contrast, it may become much more difficult for sites where raw topography and complex geological structure with lateral heterogeneity characterize the area. The effects of transient signals has been investigated by many authors and results indicate that their contribution to HVSR is minor in many cases (Mucciarelli et al ., 2003; Parolai & Galiana-Merino, 2006). Thus many authors consider very stable the result of HVSR analysis, but some evidences of HVSR variability with time in particular geological contexts have been published recently (e.g. La Rocca et al ., 2020). Since this method has found wide application in engineering-related fields (such as microzonation studies and structures monitoring), the questions about the consistency of the results and the limits of the method have gained more importance in recent years, and not only for purely scientific purposes. In this study we analyzed months of seismic noise recorded at many sites in Calabria in order to investigate the features of seismic noise, its day-night cycle, and the HVSR variations with time. We found that in some cases the HVSR curve can remarkably change its shape, especially in terms of the resonance peak amplitude, and such variations show a clear correlation with the day-night cycle that characterize the amplitude of seismic noise. Local and regional earthquakes were also analyzed at the same sites, and HVSR results were compared with those obtained from the analysis of seismic noise. Data analysis and results Data analyzed in this work come from the regional seismic network managed by Università della Calabria (Rete Sismica Unical, international code IY, doi:10.7914/SN/IY, http://doi.org/ doi:10.7914/SN/IY, www.sismocal.org) . We started our analysis computing HVSR on many weeks of continuous signals for more than 30 seismic stations. Among the stations that show the presence of site effects we selected 9 sites (CHC2, GIMI, LADO, MANE, MNGA, SCAI, SMIN, UC01, VBL2, www.sismocal.org ) characterized by important variability of the HVSR peak amplitude. For the nine selected sites we performed further detailed analysis as described below. We computed the HVSR of continuous signals in different periods of the year. In this analysis we included the months of March and April 2020, a period characterized by a lower contribution of antropic disturbances in background noise due to the major lockdown adopted by the government to contrast the COVID-19 pandemic. In fact, for some seismic stations usually affected by a high level antropic noise at frequency greater than 1 Hz, that period gave the