## 5 Discussion and Conclusions  
The motive for this research was the fact that most design codes, including Eurocode 8 [1], neglect the influence of the local soil on the vertical spectra. Moreover, while Eurocode 8 recognizes the potential importance of deeper geological site surroundings, stating in Clause 3.1.2(1) that the National Annex may establish a classification scheme that accounts for deep geology [1], Croatia has not included deep geology in its ground classification scheme to date. In this study, we analyzed vertical spectra for the deep soil sites atop deep geological sediments, in low to medium seismicity regions. The city of Osijek, located on the right bank of the Drava River and in the southern part of the Pannonian Basin, was chosen as the case study region, as it was in our accompanying paper on the horizontal spectra [18]. Underneath Osijek, geological sediments reach depths of up to 2.7 km, with a total thickness of 150-180 m above layers with an s-wave velocity greater than 800 m/s [18]. Recent research studies have suggested that the city of Osijek's building stock is particularly vulnerable [46, 47], necessitating the need for more reliable seismic action estimates for Osijek, especially after the devastating 29 December 2020, _Mw_ 6.4 earthquake in central Croatia [48]. This is also the case with many other cities with similar building typologies and similar seismic activity [49, 50, 51, 52, 53, 54]. New empirical regional _PSA_ attenuation equations were constructed using only regional strong motion data. A total of 218 vertical components of strong strong motion accelerograms recorded in the north-western Balkans from 112 earthquakes with magnitudes ranging from 3 to 6.8 were included in the strong motion database used to build the predictive equations. In these equations, both the effects of local soil and deep geological conditions were taken into account. For a 0.3 s vibration period, the maximum amplification of 1.48 times was found at deep soil atop deep geological sediments compared to the rock sites. Vertical _PSA_ amplitudes for vibration periods of 0.10 s to 0.15 s, as well as periods longer than 1.50 s, were shown to be de-amplified by up to 26%. As was also shown, using Eurocode 8 ratios of vertical to horizontal spectra results in a significant underestimation of vertical spectra. Vertical to horizontal _PSA_ ratios that are suggested by Eurocode 8 [1] for Type 2 spectra and Ground Type C were 2.0 to 2.3 times  
Figure 12: Vertical to horizontal spectral ratios for four distinct probability levels, calculated based on UHS and Eurocode 8 amplitudes from Figure 11. Empirical estimates of vertical to horizontal spectral ratios in California [15] and former Yugoslavia [16] are labeled USC-CA and USC-ExYU, respectively. lower than the empirically determined ratios for \(PSA_{vert}\) at deep soil atop deep geological sediments and with a vibration period of 0.3 s. The vertical to horizontal \(PSA\) ratios for a single soil type were shown to be influenced by the source-to-site distance, the probability of empirical estimates, as well as deep geology conditions. Hence, they cannot be captured by a constant that depends only on earthquake magnitude, as suggested by Eurocode 8 [1]. Seismic microzonation maps were constructed for the examined area using the proposed scaling equations. Vertical UHS amplitudes were also calculated and compared to the Eurocode 8 [1] spectra for Ground Type C, scaled by PGA values at deep soil sites, and for three different types of deep geology. The results show that the maximum spectral amplitudes to PGA ratios, referred to as \(S_{PGA}\) in Figure 11, do not deviate too much from the 3.0 factor indicated by Eurocode 8 [1] for vertical spectra at deep soils above deep geological deposits. However, for deep soils atop deep geological rocks, the \(S_{PGA}\) was equal to 3.9 for 95-year and 4.0 for 475-year, 975-year, and 2475-year return periods. In comparison to the 3.0 factor indicated by Eurocode 8, this is a 30-33% increase.