## 4 PSHA for the Case Study Site in Banja Luka  
In this section, we will conduct a PSHA analysis for the location in Banja Luka with coordinates \(44^{\circ}46.5^{\prime}\) N, \(17^{\circ}15^{\prime}\) E, using Equation (3) as the GMPE. PSHA estimations will be generated with the REASESS V2.1 software [52], which is based on Cornell-McGuire \"deductive\" methodology [53; 54]. The PSHA computations are used to evaluate the likelihood of encountering a certain value of _PGAvert_ while accounting for all earthquakes that may occur in the analyzed area over the expected life of a building. The seismic sources in the surrounding area of the analyzed site are first identified. In the present study, we will employ the SHARE Project's pan-European seismic source zone model [55; 56; 57]--see Figure 5, which also displays the epicenters of several large historical earthquakes, as well as a few recent catastrophic earthquakes in the northwestern Balkans [29]. This source zone model was based on the new homogeneous earthquake catalogue (SHEEC--the \"SHARE European Earthquake Catalogue\" [58]), which was compiled for the scope of the SHARE project. Thorough explanations and details regarding the completeness, homogeneity with respect to \(M_{\text{W}}\), and other important features of this catalog can be found in [56; 58; 59; 60] and will not be repeated here. Figure 5: The seismic source zones from the SHARE Project [55; 56; 57] used to evaluate the PSHA for the site with coordinates \(44^{\circ}46.5^{\prime}\) N, \(17^{\circ}15^{\prime}\) E in Banja Luka. In Figure 5, the two larger circles, whose radii are 103.5 and 73.0 km, respectively, show the maximum distances that we must include in the PSHA computations to achieve 99 percent of the estimated hazard at the analyzed location for the return periods of 95 and 475 years; the two smaller circles with radii of 52.5 and 32.5 km, respectively, show the maximum distances needed to obtain 90 percent of the total hazard for the same return periods. Figure 5 shows that the _PGAvert_ probabilistic estimates are dominated by local seismicity, with only the M6.4 1969 Banja Luka and M6.4 2020 Petrinja earthquakes occurring within the depicted radii, while all other events occur in areas that are not significantly contributing to the seismic hazard. After the designation of seismic sources, the next step in the PSHA analysis will be to estimate the severity of ground motion at different distances. In the present study, we shall apply Equation (3) to this aim. Finally, the contributions from all analyzed source zones are aggregated and tallied across all magnitudes and distances. Recent regional microzonation investigations [23; 24; 25; 26; 27] found that horizontal PGA is dominated by local seismicity and is not particularly responsive to distant and very strong earthquakes, such as those that occur in the Vrancea area in Romania. It is reasonable to suppose that the _PGAvert_ in Banja Luka will be dominated by local seismicity as well. Hence, we did not use a different scaling equation for the Vrancea region [61; 62]. Earthquakes that contribute the most to the seismic hazard can be assessed using the technique commonly known as seismic hazard disaggregation [63]. Figure 6 shows the seismic hazard disaggregation at 44\({}^{\circ}\)46.5\({}^{\prime}\) N, 17\({}^{\circ}\)15\({}^{\prime}\) E (refer to Figures 1 and 5) for the 95-, 475-, 975-, and 2475-year return periods. The four plots displayed in Figure 6 show the PSHA disaggregation of _PGAvert_ estimates for the location 44\({}^{\circ}\)46.5\({}^{\prime}\) N, 17\({}^{\circ}\)15\({}^{\prime}\) E (solid blue circles in Figures 1 and 5) and the probabilities of exceedance of 10% in 10 years, and 10%, 5%, and 2% in 50 years. These probabilities correspond to the so-called return period, Tr, with values of 95, 475, 975, and 2475 years. These plots show that the most contributing magnitudes increase with the return period, Tr.