## 1 Introduction
These maps show that the horizontal PGA values for the site in Banja Luka are 0.08 g for the 95-year return period and 0.17 g for the 475-year return period. Multiplying the horizontal values by the Eurocode 8 values of 0.90 and 0.45, the vertical PGA values for the Type 1 Spectrum (with \(M_{S}>5.5\) for the \"most contributing\" earthquakes) become 0.072 and 0.153, and 0.036 and 0.077 for the Type 2 Spectrum (\(M_{S}\leq 5.5\)), for the 95- and 475-year return periods, respectively. Table 1 shows these values and the corresponding  
All former Yugoslav mays expressed the hazard in terms of seismic intensity and for average soil conditions [36; 37; 38; 39]. The map that was issued in 1950 was used in conjunction with the first design code for earthquake-resistant structures in the former Yugoslavia, enacted in 1964 [36]. This map was compiled based on the data on the greatest MCS intensities ever recorded. The 1982 interim seismic zoning map was based on the greatest observed intensities at the time and utilized in conjunction with the 1981 code [37; 39]. In 1990, six new maps, for the 50-, 100-, 200-, 500-, 1000-, and 10,000-year return periods, were added to the 1981 code [38]. Although the seismic hazard was still displayed in terms of MCS intensity and for average ground conditions, the 1990 maps were the first in the former Yugoslavia to be created using a PSHA method. For each MCS intensity, Table 1 also displays the associated empirical vertical PGA estimates, obtained using the equation [20]:  
\[\log(PGAvert)=-0.593+0.331\cdot I\pm P\cdot\sigma,\sigma=0.016, \tag{1}\]  
where \(PGAvert\) is the vertical peak ground acceleration [cm/s\({}^{2}\)] and \(I\) is the MCS (Mercalli-Cancani-Sieberg Macroseismic Scale) intensity degree. Furthermore, \(P=0\) for the median empirical estimate and \(\pm 1\) for the median \(\pm 1\) standard deviation, \(\sigma\). Table 1 displays the vertical PGA values computed by Equation (1) for \(P\) equal to \(-1\) and \(1\), for each intensity degree. The vertical PGA predictions for Banja Luka based on seismic intensities from the 1950 and 1982 maps are fairly consistent with the vertical PGA values obtained from the current official hazard maps. However, the 2018 vertical PGAs for the 475-year return period are significantly lower than the 1990 vertical PGAs for the 500-year return period. Even the vertical PGA values for the Type 1 Spectrum, which imply higher seismic activity, are significantly smaller than the 1990 values. Unfortunately, we do not have any information regarding the seismic source models and attenuation functions that were used to compile the 1990 maps. Hence, we were unable to provide a systematic interpretation of a significant discrepancy between the vertical PGA values from the 1990 maps and the more recent 2018 Eurocode-based estimates. Table 2 shows the vertical PGA values (divided by the gravitational constant, \(g\)) estimated by Equation (1) for various intensities, as well as the empirical VHPga [20]. Table 3 includes the VHpga values used by the earthquake-resistant design codes in the former Yugoslavia [36; 39], as well as those defined by two empirical GMPEs that incorporate the simultaneous effects of deep geology and local soil, one for California [19] and the other for the former Yugoslavia [22]. While both GMPEs neglect the local soil effect on vertical ground motion, the equation constructed for California, USA, which was based on 1482 acceleration components, takes into account the effects of deep geology on the amplification of vertical ground motion [22].