Site effects observed in the Norcia intermountain basin (Central Italy) exploiting a 20-year monitoring

This work aims to analyse the response to ground shaking of the Norcia intermountain basin (central Italy), where a temporary seismic network (Pilz and Parolai in Norcia basin (Italy) temporary seismic network. GFZ Data Services. https://doi.org/10.14470/8u7554472182, 2009) was installed from January to May 2009, during the L’Aquila (Mw 6.1) seismic sequence. Here we present the results of the application of various empirical approaches for the evaluation of local site effects, considering hundreds of records relevant to earthquakes in the local magnitude range 3.0–5.4. The site amplification was estimated considering either the standard spectral-ratio (SSRs, Borcherdt in Bull Seismol Soc Am 60:29–61, 1970) or the horizontal to vertical spectral-ratio technique (HVSRs, Lermo and Chavez-Garcia in Bull Seismol Soc Am 83:1501–1506, 1993) techniques, applied to the S-phase and the S-wave coda of selected earthquakes. The results evidence the amplification of both horizontal and vertical components of ground motion at frequencies spanning from about 0.5 Hz, in the deepest part of the basin, to 4 Hz at basin edges. HVSRs show lower amplitudes than SSRs, due to the amplification of the vertical component, highlighting as the single station spectral technique, suitable to estimate the fundamental resonance frequency of the sites, is not able to reliably describe the actual seismic response. Considering the shape of the basin, possible predominant amplification with particular direction are investigated by rotated SSRs that allow to evidence in the central part of the studied area a prevailing amplification of the S-train waves along the 140°N direction. Finally, the generation of surface waves within the basin was investigated by the Multiple Filter Technique analysis (MFT, Dziewonski et al. in Bull Seismol Soc Am 59:427–444, 1969), estimating the backazimuth of the identified surface waves using the method proposed by Baker and Stevens (Geophys Res Lett 31:L09611, 2004. https://doi.org/10.1029/2004GL019510). In particular, the favourable conditions for the generation of surface waves inside the basin, have been evaluated by comparing records relevant to earthquakes with different sources and magnitudes recorded in the last twenty-year by the permanent NRC accelerometric station.