# Horizontal PGA Estimates for Varying Deep Geological Conditions--A Case Study of Banja Luka
## 2 Area Surrounding the Case-Study Site--Seismicity and Geology  
### Seismicity of the Area Surrounding the Analyzed Location in Banja Luka  
The area surrounding Banja Luka is characterized by intraplate seismicity. The city of Banja Luka is located between the south-west part of the Pannonian basin, with a rare occurrence of stronger earthquakes [27; 28] and the Dinaric Alps--see the top left plot in Figure 1. The Adriatic Platform and Dinarides collide, causing most regional earthquakes to occur along the Adriatic coast (see Figure 1). Moho depths in the western Balkans range from 25 km in the area of the Pannonian basin to 45 km below the Dinarides [29; 30]. The metropolitan area of Banja Luka encompasses the Banja Luka valley and surrounding hills. The area has deep fault zones that are visible in the terrain [31; 32; 33]. The largest MCS intensity assessed in Banja Luka was VIII, caused by the \(M_{W}\) = 6.1 Banja Luka earthquake on 27 October 1969. The 1969 earthquakes began with a series of small-magnitude earthquakes between late 1968 and early 1969. This activity resumed in September and culminated with the largest foreshock on 26 October 1969 (\(M_{W}\) = 6.1 and \(M_{L}\) = 6.0, epicent  
Figure 1: **(Top left)** Dinaric Alps and Pannonian basin, **(top right)** epicenters of the earthquakes with \(M_{W}\geq 3\) recorded between 1900 and 2025 [34], **(bottom)** the analyzed site (solid blue circle) and the epicenters of the largest historical earthquake in the vicinity of Banja Luka and two recent destructive earthquakes in neighboring Croatia. \({}^{\circ}\)MCS) and the main shock the next day (27 October 1969, \(M_{W}\) = 6.1 and \(M_{L}\) = 6.4, epicentral intensity of VIII-IX \({}^{\circ}\)MCS). The earthquakes' impact on Banja Luka (over 1117 people were injured and 15 were killed [32]) would have been even greater if each of the schools, firms, and private as well as public institutions had not been closed following the foreshock and if authorities had not prevented residents from returning to their homes [35]. The major earthquake was felt across a vast area. The VII \({}^{\circ}\)MCS isoseismal encompassed an area of 9000 km\({}^{2}\)[32], and 60% of all buildings were irreparably damaged, including the majority of schools. Over 76,000 people were rendered homeless, and financial losses were significant. In neighboring Croatia, an \(M_{W}\) = 5.3 earthquake hit Croatia's capital, the city of Zagreb, on 22 March 2020, with the epicenter only 7 km north [36] and a hypocentral depth of 10 km. This was the strongest event recorded near Zagreb in the past 140 years--an \(M_{W}\) = 6.3 earthquake was recorded in 1880. Later that very year, on 29 December 2020, yet another catastrophic earthquake hit Croatia, this time near the city of Petrinja, around 100 km northwest of Banja Luka and 40 km south of Zagreb, with \(M_{W}\) = 6.4 [37]. Although the maximum intensity of the 2020 Zagreb earthquake was not larger than VII, it still caused significant damage to the historical city center, with over 1900 buildings rendered uninhabitable as a result of the catastrophe. There were 27 people injured, and one of them died as a result of their injuries. Interestingly, even though the 1880 Zagreb earthquake was thought to be stronger (maximum felt intensity, VIII-IX) than the 2020 one, and the population within today's city limits increased tenfold (from 80,000 to 800,000), the effects of the 1880 event were strikingly similar: approximately 1800 buildings were damaged, 29 people were seriously injured, and one person died as a result of injuries. Today, the wider area of Banja Luka has a population of around 180,000 inhabitants. Although many buildings in the city center were built after the 1969 earthquake in compliance with earthquake-resistant design codes, in the wider area of Banja Luka, there are still many older and vulnerable buildings.