## 1 Introduction  
Bulgaria is situated within the larger Balkan seismic belt, where active faulting and tectonic movements are common due to the convergence between the Eurasian and African tectonic plates. The country is among the European countries with the highest seismic hazard. The strongest earthquake in continental Europe occurred in southwestern Bulgaria in 1904, with a magnitude of M = 7.8. The capital city of Bulgaria, Sofia, is situated in one of the country's most active seismic zones and has experienced strong earthquakes, including a M = 6.8 earthquake in 1858, which produced ground motions with an intensity of up to 9 on the MSK scale [1, 2, 3, 4, 5]. The city faces significant earthquake risks, so assessingstructural response is critical. An earthquake with a magnitude of Mw = 5.6 (Ms = 5.8) struck Western Bulgaria on 22 May 2012, at 03:00:33 local time (00:00:33 UTC), 25 km southwest of Sofia (Figure 1). The earthquake hypocenter had a depth of 10 km, with epicenter coordinates of 42.66\({}^{\circ}\) N and 23.01\({}^{\circ}\) E. The maximum macroseismic effects in Pernik were classified as intensity VII-VIII on the MSK-64 scale [6; 7; 8; 9]. The earthquake caused significant damage, including structural damage to buildings, and resulted in several injuries; fortunately, there were no reported fatalities. The event also triggered increased public awareness of seismic risks in the region, prompting discussions on better preparedness and monitoring of seismic activity in the future. This shallow earthquake was strongly felt and instrumentally recorded across the Balkan region, including Greece, North Macedonia, Turkey, and Romania [10]. Developing earthquake-resistant building codes requires seismic engineering studies based on real-world records of strong ground motions. To calculate seismic parameters needed for assessing the dynamic response of structures being built or designed, making realistic selections and processing input records of strong motions during an earthquake event is necessary. Researching large ground motions is crucial in seismic engineering and risk assessment [11; 12]. The seismic risk in densely populated urban areas near seismic sources is very high [13; 14; 15; 16]. Monitoring strong ground motions in such areas is crucial for obtaining accurate data on the actual response of buildings, which can be used to develop effective seismic design codes and informed prevention policies. Local geological and tectonic settings can significantly influence the duration and intensity of strong ground motions, especially for shallow crustal earthquakes [17; 18; 19]. Factors such as the structure of stratigraphic soil layers, proximity to the seismic source, topographic effects, and the presence of sedimentary basins, such as the Sofia Graben, can, for example, amplify seismic responses [20]. These conditions underscore the importance of tailored monitoring and analysis in effectively addressing site-specific risks. In the early 1970s, [PERSON] and [PERSON] (KY) proposed a simple formula for computing the response of a building [21]. The developed model integrates stochastic processes and spectral analysis to evaluate the behavior of structures subjected to seismic forces using accelerometric data. The KY formula is based on the spectral representation of ground motion and structural response modeling, which links ground motion to the dynamic  
Figure 1: Location of accelerometric stations (marked with black triangles) in the Sofia area. The stars indicate the epicenters of the main earthquake with Mw = [PERSON] (00:00:33 UTC) and the subsequent aftershocks with Mw = 4.7 (01:30:51 h UTC) and Mw = 4.2 (02:13:29 h UTC). The SGL1 station used in this study, located in the center of Sofia, is marked with a red triangle. response of structures through transfer functions. Recent studies have revisited and expanded the KY formula, demonstrating its continued relevance in structural engineering. [PERSON] et al.