# Horizontal PGA Estimates for Varying Deep Geological Conditions--A Case Study of Banja Luka
## 2 Area Surrounding the Case-Study Site--Seismicity and Geology
### Official PGA Estimates for Banja Luka  
The ex-SFRY issued its first seismic-resistant design code in 1964 [43]. There was a seismic zoning map developed in 1950 based on the maximum (MCS--Mercalli-Cancani-Sieberg Scale) intensities observed at the time, which was utilized in conjunction with the 1964 code. In 1981, a new earthquake-resistant design code was approved [54], and a year later, a seismic zoning map was developed to be used temporarily with the new code [55]. The 1982 map was based on the maximum measured MCS intensities once more. In 1990, the 1981 code was modified with six new seismic zoning maps (for 50, 100, 200, 500, 1000, and 10,000 year return periods) [56]. Although the seismic hazard was again expressed in MCS intensity and for average soil conditions, these were the first seismic hazard maps in ex-Yugoslav created utilizing the PSHA (Probabilistic Seismic Hazard Assessment) approach. Please take note that we do not have return periods for the 1950 and 1982 maps, because they were developed using the largest observed intensities rather than a probabilistic hazard analysis. Table 2 shows MCS intensities for Banja Luka, as given in 1950, 1982, and 1990 official hazard maps. Table 2 also shows the corresponding empirical PGA estimates, calculated using the following equation [24]:  
\[\log(\mathrm{PGA})=-0.079+0.290\cdot I\pm P\cdot\sigma,\sigma=0.049, \tag{1}\]  
where PGA is the horizontal peak ground acceleration [cm/s\({}^{2}\)], \(I\) is the macroseismic intensity [\({}^{\circ}\)MCS], \(\sigma\) represents the standard deviation, and \(P\) = 0 for median estimates. For each intensity degree, Table 2 shows the PGA values calculated by Equation (1) for \(P\) = \(-1\) and \(P\) = 1. In 2018, two new hazard maps were included in Bosnia and Herzegovina's National Annex to Eurocode 8 [14]. The maps were created with the PSHA approach. The first map is for a 95-year return period, or a 10% chance of exceeding in 10 years, while the second is for a 475-year return period, or a 10% chance of exceeding within 50 years. The 95-year map correlates to Eurocode 8's \"damage limitation\" requirement, whilst the 475-year map corresponds to the \"no-collapse\" requirement. The 2018 maps were generated for ground type A (see Table 1), and on these two maps, it is the horizontal PGA that represents seismic hazard numerically. Furthermore, both maps disregard how deep geology affects surface ground motion. On these maps, the horizontal PGA values for the analyzed location in Banja Luka are 0.08 g for the 95-year and 0.17 g for the 475-year return period. We added these values in Table 2, together with the corresponding intens \begin{table}
\begin{tabular}{c c c} \hline
**Map** & **\(I\) [\({}^{\circ}\)MCS]** & **PGA** \\ \hline
1950 & VII & 0.081–0.102 g \\
1982 & VIII & 0.159–0.199 g \\
1990—50 years & VII & 0.081–0.102 g \\
1990—100 years & VIII & 0.159–0.199 g \\
1990—200 years & IX & 0.309–0.388 g \\
1990—500 years & IX & 0.309–0.388 g \\
1990—1000 years & IX & 0.309–0.388 g \\
1990—10,000 years & IX & 0.309–0.388 g \\
2018—95 years & VI–VII & 0.080 g \\
2018—475 years & VIII & 0.170 g \\ \hline \end{tabular}
\end{table} 
Table 2: Macroseismic (MCS) intensity degrees given in the official seismic zoning maps of former Yugoslavia [43; 54; 55; 56] for the selected location in Banja Luka, together with the associated median \(-\)\(\sigma\) and median + \(\sigma\) empirical PGA estimates computed using Equation (1). The PGA values of the 2018 official maps, which were used in conjunction with Eurocode 8 [14], are also shown. Equation (1). Table 2 shows that official hazard estimates from 1950 and 1982 are similar to those from 2018.