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The severity of vertical seismic ground motions is often factored into design regulations as a component of their horizontal counterparts. Furthermore, most design codes, including Eurocode 8, ignore the impact of local soil on vertical spectra. This paper investigates vertical pseudo-absolute acceleration spectral estimates, as well as the ratios of spectral estimates for strong motion in vertical and horizontal directions, for low to medium seismicity regions with deep local soil and deep geological sediments beneath the local soil. The case study region encompasses the city of Osijek in Croatia. New regional frequency-dependent empirical scaling equations are derived for the vertical spectra. According to these equations, for a 0.3 s spectral amplitude at deep soils atop deep geological sediments compared to the rock sites, the maximum amplification is 1.48 times. The spectra of vertical components of various real strong motions recorded in the surrounding region are compared to the empirical vertical response spectra. The new empirical equations are used to construct a Uniform Hazard Spectra for Osijek. The ratios of vertical to horizontal Uniform Hazard Spectra are generated, examined, and compared to Eurocode 8 recommendations. All the results show that local soil and deep geology conditions have a significant impact on vertical ground motions. The results also show that for deep soils atop deep geological strata, Eurocode 8 can underestimate the vertical to horizontal spectral ratios by a factor of three for Type 2 spectra while overestimating them by a factor of two for Type 1 spectra. [ABSTRACT FROM AUTHOR]

This study presents a first developed site amplification model for Croatia using random vibration theory (RVT)-based site response analysis and measured shear wave velocity (Vs) profiles for different site parameters (V S30 – average shear wave velocity in the top 30 m, fundamental site frequency/period; H 800 – depth to the bedrock layer with reference to shear wave velocity V S = 800 m/s). The impedance between the topsoil layers and the engineering bedrock in the measured Vs profiles is more prominent than that in the soft transition modelled in the theoretical soil profiles. The results indicate that (i) the site amplification factors (AFs) are greater than that of the code-based AFs (current and revised Eurocode 8) for stiff soil/soft rock sites and (ii) significant nonlinearity in soft soils occur compared to that of recent ground motion models because of difference in variability between the measured and theoretical Vs profiles. These observations imply significant deviations from the site factors computed based on the site response models in this study and recent models, as well as from those provided in the present Eurocode 8 and its revised version. The presented site AF model based on different site parameters (V S30 , frequency/period or bedrock depth) can be integrated into existing or future developed regional ground motion models to characterize the influence of site effects on design response spectra following the variability in soil profile properties on weak and strong seismic responses. This is especially important when used in seismic hazard studies in areas with sparse empirical ground motion database. • Site amplification model for Croatia is derived based on RVT site response analysis. • Site amplification model is based on different site parameters V S30 , T 0 and H 800. • Results are comparable with the recent site amplification models. • Site amplifications are greater than the current and revised Eurocode 8 AFs. • Model can be used for seismic hazard studies in areas with sparse strong motions. [ABSTRACT FROM AUTHOR]

One of the challenging tasks for Croatian accession to the European Union is the completion of the National Annex to Eurocode 8 (EC8) that calls for the development of national seismic hazard maps. These maps need to be developed for several return periods (probabilities of exceedance) and should consider both bedrock and soil conditions. The latter is especially challenging as it calls for site characterization at a microzonation level, i.e., site characterization based upon Vs,30, SPT blow counts, and undrained shear strength, information available only in the departments of Public Works of major cities. In this paper, the authors discuss several shortcomings associated with the EC8 ground type definitions and propose their revision. The authors also present the ongoing work for preparation of the Croatian National Annex to EC8 and explain the rationale for the improvement of EC8 ground type definitions. [ABSTRACT FROM AUTHOR]