## 1 Introduction
This is why, despite the constraints of the database, we will use the scaling equation described here for the probabilistic hazard analyses in Banja Luka. It will be simple to update the GMPEs and rerun the PSHA analyses as the number of acceleration records increases. Regional Seismicity, Geological Site Surroundings, and Official Vertical PGA Estimates for Banja Luka  
Banja Luka is located between the Pannonian basin and the Dinaric Alps fold-and-thrust belt. According to a recent study, the contracional reactivation of Late Palaeogene to Middle Miocene normal faults, which postdates the deposition of the youngest strata in the Pannonian Basin of the Pontian age, may be the cause of earthquakes in the Banja Luka epicentral area [28]. Figure 1 depicts the epicenters of earthquakes with \(M_{W}\geq 3.0\) detected in the region between 1900 and April 2025 [29]. Also visible are the epicenters of two recent major earthquakes in Croatia and a few of the largest regional historical earthquakes. The strongest earthquake close to Banja Luka occurred on 27 October 1969, as part of the seismic activity from 1968 to 1969, with an epicenter around 10 km northeast of Banja Luka (see Figure 1) and a magnitude of \(M_{W}=6.1\), while the intensity in Banja Luka was predominantly VIII (\({}^{\circ}\)MCS). On 22 March 2020, a \(M_{W}=5.3\) earthquake struck the Croatian capital, Zagreb, and this was the strongest earthquake near Zagreb since the \(M_{W}=6.3\) earthquake near Zagreb in 1880 [30]. On 29 December 2020, a destructive \(M_{W}=6.4\) event devastated the city of Petrinja, with the epicenter located 40 km south of Zagreb and 100 km northwest of Banja Luka [31]. Both earthquakes had a focal depth of around ten kilometers. [PERSON] et al. [28] proposed reverse faulting along ESE-WNW striking nodal planes after analyzing fault plane solutions for the two 1969 Banja Luka earthquakes. Furthermore, [PERSON] et al. [28] discovered evidence of an ongoing shortening in the interior Dinarides, which accounts for a component of the current Aria-Europe convergence, based on comparable findings for a number of minor earthquakes in the area. To learn more about Banja Luka seismotectonics, readers are referred to this exceptionally comprehensive study. In this study, we will base our insights into the geological settings around the analyzed site on the geological map compiled by [PERSON] et al. for Banja Luka and neighboring areas [33]. In a previous microzonation study for Banja Luka [32], this map was divided into 30 \(\times\) 30 s cells for the area shown in the bottom plot of Figure 1. The geology of each of these 1200 cells was then defined using the major lithost stratigraphic formations and their depths. Site geology was evaluated for each cell per [PERSON]'s classification [10]. Each cell's geology categorization was assigned as \"alluvial and sedimentary deposits\" (\(s=0\)), \"intermediate sites\" (\(s=1\)), or \"basement rock\" (\(s=2\))--see Figure 1. We have deliberately chosen the location in Banja Luka with coordinates \(44^{\circ}46.5^{\prime}\) N, \(17^{\circ}15^{\prime}\) E (see blue circle in Figure 1) as this location is surrounded by all three types of deep geology. To the west, there is a lower terrace of the river Vrbas atop of marl, clay, and sand, and we defined this formation with \(s=0\). To the north, there is a diabase-cherty formation (or metamorphic ophiolitic melange), which we defined with \(s=1\). To the east, there is a spilite formation, which we designated by \(s=2\). In 2018, Bosnia and Herzegovina introduced new seismic hazard maps as part of the National Annex to Eurocode 8 [34; 35]. The horizontal PGA values represent the seismic hazard, whereas the vertical PGA is obtained from the horizontal PGA and the ratios provided by Eurocode 8. The two 2018 maps were generated using the PSHA approach, for ground type A (rock), one for the 95-year return period and the other for the 475-year return period.