Generic-To-Reference Rock Scaling Factors for Seismic Ground Motion in Italy

In this article, we apply the reference-rock identification method (RRIM; Lanzano et al., 2020) to the ITalian ACcelerometric Archive, which includes more than 1600 recording stations in Italy and in the neighboring countries, with different levels of site characterization. The RRIM is based on the identification and the evaluation of site parameters representing the reference site conditions (Steidl et al., 1996) and the construction of the scoring scheme to classify the candidate stations. Given the large number of sites, the preselection of can- didates is performed via residual analysis, selecting those characterized by flat site response and amplitude similar or lower than the one for the generic rock (average shear-wave veloc- ity in the uppermost 30 m, V S30 800 m= s). The main results of this study are: (1) a list of reference rock sites in Italy, with an associated score; (2) a scenario-independent generic-to- reference rock corrective factor for the ground-motion model for shallow active crustal events in Italy (ITA18; Lanzano, Luzi, et al., 2019); (3) a model for the generic-to-reference rockcorrectivefactor,parametrizedintermsofVS30 andκ0,thatis,thehigh-frequencydecay parameter (Anderson and Hough, 1984). A collateral product is a set of coefficients for the prediction of 81 ordinates of the Fourier amplitude spectra (FAS) in the frequency interval 0.1–30 Hz, calibrated with the same dataset and functional form of ITA18 for acceleration response spectra (SA). The application of RRIM allowed us to identify 116 stations with aver- age measured V S30 ∼ 900 m= s. The corrective factor allows to scale both SA and FAS spectra, and has a significant effect at high frequencies, reducing the ground motion by up to a factor 1.7 at f = 10 Hz. The introduction of κ0 in the corrective term modeling is effective from 2 Hz onward and results in a reduction of variability at high frequencies (f > 10 Hz).