## 4 Uniform Hazard Spectra Calculations for Vertical Strong Ground Motion  
We now conduct a probabilistic seismic hazard analysis (hereinafter, FSHA) for the seismic microzonation of the area within the blue rectangle shown in Figure 7. Several recent regional microzonation studies [26; 27; 28; 29; 30; 31] have shown that empirical predictions based on scaling equations that consider both deep geology and local soil are in excellent agreement with recorded ground motions in the region. In this study, we define the GMPEs for  
Figure 5: Empirical predictions of vertical to horizontal \(PSA\) amplitudes for four vibration periods, \(T\) = 0.05 s, 0.10 s, 0.30 s, and 0.50 s, calculated using Equation (1) and coefficients from Table 2, for deep soil sites (\(S_{L}\) = 2) and the three different deep geology parameters. Solid lines show empirical predictions developed for all epicentral distances, while dashed lines show the predictions developed for distances smaller than 30 km. Figure 6: Empirical predictions of vertical to horizontal \(PSA\) amplitudes for four vibration periods, \(T\) = 0.05 s, 0.10 s, 0.30 s, and 0.50 s, for deep geological sediments (\(S\) = 2) and the three different shallow geology parameters, including the median \(\pm\) 1 \(\sigma_{log}\) predictions for deep soil sites (\(S_{L}\) = 2). the probabilistic hazard analyses for Osijek using Equation (1) and the coefficients from Table 2, as was completed for the horizontal spectra [18]. We assumed deep soil sites and deep geological deposits for the entire case study area. The REASSES V2.0 program was used to create the PSHA estimates [38]. Although the seismic source zone can be defined based on available seismological data [39; 40; 41], to compare the results to our other studies for the same region [17; 18] we utilized the pan-European seismic source zone model, which was one of the outcomes of the \"Seismic Hazard Harmonization in Europe\" (SHARE) Project [42; 43; 44]. Figure 7 shows the boundaries of the seismic source zones used in this study's hazard calculations. The circles in Figure 7 depict the source-to-site distances of 111 km, 185 km, and 229 km, estimated to contribute to 99 percent of the overall PSHA estimations for the 475-year return period for spectral amplitudes at 0.05 s, 0.3 s, and 0.5 s, respectively. As expected, distant severe earthquakes will have a larger effect on long-period vertical spectral amplitudes, while the short-period amplitudes will be dominated by local seismic activity. As also shown in Figure 7, the distances that will contribute to the 50% accuracy of the PSHA calculations for the 0.5 s spectral amplitude are designated by a circle with a radius of 58 km. Figure 8 shows examples of hazard disaggregation [45] for the coordinates \(45^{\circ}32^{\prime}\) N, \(18^{\circ}23^{\prime}\) E. Figure 9 shows cumulative disaggregation examples for distance and magnitude. It also shows (in the bottom-right plot) the magnitude recurrence curve for the most contributing seismic source zone around Osijek. As can be seen, the real return periods of the most contributing earthquakes differ for different vibration periods and are much smaller than the so-called return period \(Tr\), which is equal to the reciprocal value of the mean yearly rate of occurrence, \(N(PSA)\), of seismic events that will cause a \(PSA\) amplitude that will exceed the expected value-see the related discussions in [17; 18]. This should be noted because Eurocode 8 [1] does not define Types 1 and 2 spectra in terms of conservative upper limits for magnitudes, but rather in terms of the magnitudes that \"contribute most to the seismic hazard defined for the site for the purpose of probabilistic hazard assessment\" (see Eurocode 8 [1]: 3.2.2.1, (4), Note 2 and 3.2.2.2, (2)P, Note 1). Figure 7: Map of areal seismic source zones used for PSHA calculations in this study.