GNGTS 2021 - Atti del 39° Convegno Nazionale

GNGTS 2021 S essione 2.2 252 hours compared with night hours. Higher rectilinearity of the particle motion corresponds to a greater amount of body waves, and perhaps Love waves, than Rayleigh waves. The HVSR of local earthquakes further support our assumption since in these signals the amount of body waves is far greater than surface waves, and HVSR results are very similar to the results obtained for day hour seismic noise. In many cases we found surprising results (CHC2, MNGA as described above) that need a deep investigation. For many years HVSR has been considered a stable feature of a given site. On the other hand, remarkable variations in HVSR shape or just in the peak amplitude have been observed and attributed to various factors, as topographic roughness and geological complexities (Burjanek et al ., 2014; Napolitano et al ., 2018; La Rocca et al ., 2020) or even changes in groundwater level (Rigo et al ., 2021). A large number of site effects studies are based on the assumption that the HVSR method is “fast and cheap” since half an hour of seismic noise recording is sufficient to get stable and reliable results. Our results demonstrate that in some cases the results of HVSR analysis computed on a short recording of ground motion can not be representative of the site features, and can lead to misleading interpretation of site effects. References Bard P.Y.; 1999. Microtremor measurements: A tool for site effect estimation? In K. Irikura, K. Kudo, H. Okada & T. Sasatani, The Effects of Surface Geology on Seismic Motion, 1251-1279, Rotterdam. Bonnefoy-Claudet S., C. Cornou, P.Y. Bard, F. Cotton, P. Moczo, J. Kristek, D., Fäh, D.; 2006a. H/V ratio: A tool for site effects evaluation. Results from 1-D noise simulations. Geophys. J. Int., 167 (2), 827-837, DOI:10.1111/j.1365-246X.2006.03154.x. Burjanek J., B. Edwards, D. Fah; 2014. Empirical evidence of local seismic effects at sites with pronounced topography: a systematic approach. Geophys. J. Int. 197, 608–619. doi: 10.1093/gji/ggu014. La Rocca M., G.D. Chiappetta, A. Gervasi, R.L. Festa; 2020. Non-stability of the noise HVSR at sites near or on topographic heights. Geophys. J. Int., 222, 2162-2171. doi: 10.1093/gji/ggaa297. Mucciarelli, M.; 1998. Reliability and applicability of Nakamura’s technique using microtremors: an experimental approach, J. Earthquake Eng., 2, 625–638. Mucciarelli M., M. R. Gallipoli, M. Arcieri; 2003. The Stability of the Horizontal-to-Vertical Spectral Ratio of Triggered Noise and Earthquake Recordings. Bull. Seism. Soc. Am., 93(3), 1407–1412. Nakamura, Y.; 1989. A method for dynamic characteristics estimations of subsurface using microtremors on the ground surface. Q. Rep. Railway Tech. Res. Inst. Japan 30, 25–33. Napolitano F., A. Gervasi, M. La Rocca, I. Guerra, R. Scarpa; 2018. Site effects in the Pollino region from the HVSR and polarization of seismic noise and earthquakes. Bull. Seism. Soc. Am., 108(1), 309-321. Parolai S., J.J. Galiana-Merino; 2006. Effect of transient seismic noise on estimates of H/V spectral ratio. Bull. Seism. Soc. Am., 96(1), 228-236. Rigo A., E. Sokos, V. Lefils, P. Briole; 2021. Season variations in amplitude and resonance frequencies of the HVSR amplification peaks linked to groundwater. Geophys. J. Int., 226, 1-13. Corresponding author: giuseppe.chiappetta@unical.it