## 5 Discussion and Conclusions  
This research was motivated by recent seismic microzonation studies in the northwestern Balkans region [9; 10; 11; 12; 13; 14], which revealed that variations in deep geological site conditions can significantly increase seismic hazard estimates when compared to the hazard estimates based only on the effects of the local soil. At the same time, other recent studies indicated a high vulnerability of the city of Osijek's building stock [43; 44]. In this study, we analyzed horizontal UHS estimates for deep soil sites atop deep geological sediments in low-to-medium seismicity areas. For the case study, we chose the city of Osijek, Croatia, located in the southern part of the Pannonian Basin and on the right bank of the Drava River. Geological sediments underneath Osijek have depths ranging up to 2.7 km, while the total thickness above layers with an s-wave velocity greater than 800 m/s is 150-180 m.  
New empirical regional formulae for _PSA_ attenuation were first developed based solely on the regional strong-motion data. The strong-motion database used to create predictive equations includes 436 horizontal components of strong strong-motion accelerograms recorded in the north-western Balkans from 112 earthquakes with magnitudes ranging from 3 to 6.8. The provided equations take into account the effects of local soil and deep geological conditions at the same time. According to empirical estimations, the combination of deep soil and deep geological deposits results in short-period spectral amplitudes up to 37% lower than those obtained at rock soil locations. This suggests that energy dissipation in deep soil sites overcomes local soil amplification, which is consistent with other recent studies of soft sediment non-linear behavior (e.g., [45; 46]). Vice versa, spectral amplitudes for vibration periods \(T>\) -0.3 s are shown to be amplified. This is consistent with the findings of [PERSON] et al. [47], who investigated the impact of deep basin effects on structure collapse and discovered that vibration periods longer than 1.2 s increased the chance of structural collapse by up to 20%. In this study, the maximum amplification of 2.37 times was obtained for \(T=0.5\) s.  
The only GMPEs to which we can compare our findings (since they consider effects of deep soils and deep geological sediments at the same time) are those created for California  
Figure 10: The uniform hazard spectra calculated for the coordinates 45\({}^{\circ}\)32\({}^{\prime}\) N, 18\({}^{\circ}\)23\({}^{\prime}\) E for four different probability levels, and the Eurocode 8 [16] Type 2 spectra for ground type C.  
based on 1482 strong-motion records from earthquakes in the western United States [21]. Californian GMPEs, for example, show 30% lower _PSA_ amplitudes in the deep soil than in the rock sites for \(T=0.04\) s. According to Californian GMPEs [21], the amplification exists for vibration intervals longer than 0.3 s, and the highest amplification is 1.7, which is a little lower than what we obtained. Based on the presented scaling equations, UHS amplitudes were calculated for the analyzed area and the microzonation maps created. The resulting uniform hazard spectra were compared to the Eurocode 8 [16] spectra for ground type C, scaled by the PGA values at deep soil sites, and for three different types of deep geology. As can be observed in Figure 10, the ratios of the maximum UHS amplitudes to PGA values, \(S_{PGA}\), diverge from the 2.5 factor recommended by Eurocode 8 for horizontal spectra. In the case of UHS amplitudes for deep soils atop deep geological sediments, \(S_{PGA}\) is equal to 3.3 for 95-year, 3.4 for 475-year, 3.5 for 975-year, and 3.6 for 2475-year return periods. This is a 31-46% increase compared to the 2.5 factor suggested by Eurocode 8. Although the data used for the definition of the GMPEs used for hazard calculations are limited, we believe that the presented hazard maps and uniform hazard spectra can be seen as the first step towards more reliable hazard estimates for the analyzed region.