## 4 Uniform Hazard Spectra Calculations for Vertical Strong Ground Motion
Maximum radii for capturing 99 percent or 50 percent accuracy of the Tr = 475 years \(PSA\) estimations for vibration periods of 0.05 s, 0.3 s, and 0.5 s are shown as circles around the investigated area. Figure 10 shows the seismic microzonation maps for the study area for the return periods _Tr_ of 95, 475, 975, and 2475 years. Finally, uniform hazard spectra (hereinafter, UHS) for four distinct probability levels were created for the coordinates 45\({}^{\circ}\)32\({}^{\prime}\) N, 18\({}^{\circ}\)23\({}^{\prime}\) E, and compared to the Eurocode 8 [1] Type 2 spectra for Ground Type C, as shown in Figure 11. The UHS can be easily constructed for any point within the examined area by reading and linking the values for the provided coordinates in the hazard maps of Figure 11 with the same probability of exceedance. Figure 8: Disaggregation examples for different hazard probabilities and _PSA_ amplitudes for the location in Osjek with coordinates 45\({}^{\circ}\)32\({}^{\prime}\) N, 18\({}^{\circ}\)23\({}^{\prime}\) E.  
Figure 9: The magnitude recurrence curve for the most contributing seismic source zone surrounding Osjek (see Figure 8) and cumulative disaggregation for distances and magnitudes for the coordinates 45\({}^{\circ}\)32\({}^{\prime}\) N, 18\({}^{\circ}\)23\({}^{\prime}\) E.  
The resulting UHS spectra for various deep geology and local soil types were compared to the Eurocode 8 spectra for Ground Type C, scaled by the PGA values at deep soil locations (\(S_{L}=2\)) and for three different deep geology types (Figure 11). As can be observed, the ratios of maximum spectral amplitudes to PGA values, designated in Figure 11 as \(S_{PGA}\), diverge from the 3.0 factor recommended by Eurocode 8 [1] for vertical spectra. The difference between the \(S_{PGA}\) and the 3.0 factor is the smallest (up to 7%) for deep soil atop deep geological sediments (\(S=0\), \(S_{L}=2\)), and exists only for the 95-year and 475-year return periods. For the so-called stiff soils (those with a 15-75 m thick soil layer above the layer with V\({}_{5,30}>800\) m/s [12; 13]) atop deep geological sediments (\(S=0\), \(S_{L}=1\)), the \(S_{PGA}\) is up to 23% smaller the 3.0 factor. On the other hand, for the deep soils atop geological rocks (\(S=2\), \(S_{L}=1\)), \(S_{PGA}\) is up to 33% larger. Figure 10: Seismic hazard maps for the vertical spectral accelerations in the analyzed area, for four different probability levels and six vibration periods. Full circles denote the site chosen for UHS and disaggregation calculations. Based on the calculated UHS amplitudes, vertical to horizontal spectral ratios were calculated for four distinct probability levels, as shown in Figure 12. These ratios were also compared to the corresponding ratios of Eurocode 8 [1] for Type 2 spectra and Ground Type C and two sets of empirical GMPEs that consider the simultaneous impacts of deep geology and local soil [15,16]. The first set, labeled USC-CA in Figure 12, was created for California [15], whereas the second set, labeled USC-ExYU, was created for the former Yugoslavia [16]. While both empirical attenuation equations ignore the effect of local soil on the vertical ground motion, the California equation, which was based on 1482 acceleration components, indicated that deep geology affects seismic waves in the vertical direction [15]. The ratio shown in Figure 12 for the Californian GMPE corresponds to the vertical ground motion predictions at deep geological sediments. In USC-CA [15] and USC-ExYU [16] GMPEs, the product of the categorical variable and the frequency-dependent scaling coefficients is added to be able to predict either vertical or horizontal spectra. As a result, the exact vertical to horizontal _PSA_ ratios can be determined in the same way as was completed previously for the amplification factors for various soil and deep geology types.