## 1 Introduction  
The amplification of ground motion generated by sedimentary basins has been investigated by many researchers using analytical and computational methods (e.g., [1; 2; 3; 4; 5; 6]). However, standard seismic hazard maps only consider shallow geology conditions (i.e., the local soil conditions for the first 30 m depth of the stratigraphic profile). This is because most empirical equations for scaling peak ground acceleration (hereinafter, PGA) values only include the effects of local soil on a tens-of-meters scale and disregard the impact of deeper geological site conditions [7]. This is still the case, although there are studies that show that site resonance has little to do with the average shear-wave velocity of the upper 20 or 30 m [8]. Moreover, a series of recent seismic microzonation studies in the north-western Balkans [9; 10; 11; 12; 13; 14] revealed that deep geology conditions have a significant impact on the severity of both short- and long-period waves. Here, the term \"deep geology\" is used to describe the geological conditions that characterize sites on a scale of hundreds of meters or kilometers [15]. The 2004 version of Eurocode 8 [16] recognizes the potential importance of deeper geological site surroundings, stating in Clause 3.1.2(1) that the National Annex may establish the classification scheme that will account for deep geology. However, to date, no EU country, including Croatia, has included deep geology into the ground classification scheme. This study relies on our prior research [17; 18] on the severity of surface ground motion in the case study area of Osijek, Croatia, and is accompanied by a study of vertical spectra [19]. Osijek is once again used as the case study area for low-to-medium seismicity regions and with deep soils atop deep geological sediments. In our study of the horizontal PGA values [17], we have shown that deep soil atop deep geological sediments leads to the de-amplification of seismic waves and -17% lower PGA values than the values obtained at the rock sites. The subject of this particular paper will be the horizontal pseudo-absolute acceleration spectra (hereinafter, _PSA_). Table 1 shows the ground types defined by the shear-wave velocity for the top 30 m of the soil profile, V\({}_{5,30}\), and corresponding maximum spectral accelerations, as defined by Eurocode 8 [16]. Official seismic hazard maps in Croatia are incorporated in the Croatian National Annex to Eurocode 8 [20]. The hazard is expressed by the PGA values for ground type A, \(a_{\text{g}}\) (see Table 1), i.e., for the rock sites [16]. As can be seen in Table 1, the maximum spectral amplitudes are equal to the product of \(a_{\text{g}}\), 2.5 (a constant value for horizontal spectra), and the so-called soil factor [16], which is larger than 1 for all ground types. The fact that the soil factor is always larger than 1 indicates that there is an amplification of seismic waves regardless of the soil type. However, the empirical attenuation equations developed in the mid-1980s based on 1482 acceleration components from earthquakes in the western United States indicate that deep soil sites will lead to de-amplification of spectral amplitudes for vibration periods shorter than 0.4 s [21]. [PERSON] et al. [22] have recently studied the effects of site amplification and basin resonance in Jakarta, Indonesia. For short vibration periods (less than 1 s), they found that the levels of response spectral acceleration were significantly below those of the 2012 Indonesian building Codes' design response spectra, but approach or even exceed Code levels for longer periods [22]. They also discovered that available ground-motion prediction equations (hereinafter, GMPE) are incapable of capturing the impacts of basin geometry on seismic waves in a very deep basin filled with soft sediment [22].