## 2 Materials and Methods
### Data  
The placement of the instrumented building and the recording stations is shown in Figure 2c. From the available records in the Sofia area, we selected two three-component accelerograms: the SGL1 record was taken in the basement of the building, and the SGL2 record was taken on the sixth floor (Figure 3). The selection focused on the instrumental behavior of typical structures in the central area of Sofia. Afterward, we performed appropriate processing and analysis to determine the primary characteristics of ground motions suitable for engineering practice, which are presented below. The recordings used in this study were obtained using high-sensitivity digital accelerometers of ETNA type installed in the basement and on the top floor of the building. The duration of the strong-motion record varies depending on the station's proximity to the epicenter and the sensitivity of the recording equipment. In general, more distant stations register shorter durations due to attenuation laws, while closer stations capture longer records that include P-wave arrival, the strong seismic shaking phase, and the coda. The signal windows used for analysis were carefully selected to exclude background noise and instrumental artifacts. Acceleration time histories for the three orthogonal components of ground motion recorded in the basement of the building are shown in Figure 3. Significant ground accelerations occur between 6 and 15 s. Post-peak acceleration fluctuates, showing the dynamic  
Figure 3: EW, NS, and UD component accelerograms (after baseline correction) recorded in the basement (SGL1) (**a**) and on the sixth floor (SGL2) (**b**). Figure 2: Placement of the instrumented building and the recording stations: (**a**) location of the building on Google map; (**b**) view of the building from “Slaveykov square”, where the two digital accelerometers were installed (the red arrow shows the building where the equipment was installed); and (**c**) analytical model and instrumentation. ground response to seismic waves. These oscillations gradually decrease, indicating energy dissipation and damping. By the 40 s mark, the graph stabilizes with minor fluctuations, marking the end of major ground movement. EW (first graph) component acceleration remains relatively low initially but increases around the 5-10 s mark, where a clear seismic wave arrival is observed. The motion gradually diminishes after reaching peak amplitude, indicating energy dissipation over time. The response shows moderate intensity, with oscillations tapering off after approximately 15 s. Similarly to the EW component (upper graph) and the NS component (middle graph), the strongest motion occurs within the 5-12 s range. The amplitude is slightly different, indicating directional variation in ground shaking. The oscillation decay follows a pattern comparable to the EW component, suggesting consistent damping effects. The UD component (bottom graph) exhibits a more abrupt initial acceleration increase than the horizontal components. The peak acceleration occurs within the first 10 s, showing significant vertical shaking, which is crucial for evaluating structural response, especially for multi-story buildings. The vertical component generally has a lower amplitude than the horizontal components, but it still plays a significant role in seismic effects on structures. The sixth-floor record of the EW component, starting around the 5 s mark (Figure 3b, first graph), exhibits significant oscillations with high amplitudes, indicating intense ground motion from primary seismic waves (P-waves). The NS component in Figure 4b exhibits a more intense and dynamic response compared to the NS component in Figure 4a, with higher acceleration peaks and larger displacements. The NS component in the basement suggests a stiffer or better-damped system, experiencing lower acceleration and smaller displacements. The NS component on the sixth floor indicates a more flexible or excitation-sensitive system with greater dynamic amplification and energy absorption.