## 5 Discussion  
Local and regional geological and tectonic characteristics around each station are important to define primary effects on the \(\kappa\) distribution within their azimuthal area subsets [1; 7; 42] estimated \(\kappa\) from the vertical component of ground motion and observed that the \(\kappa_{ver}\) values are slightly lower than the average \(\kappa_{hor}\), something that is also observed in our study. Generally, the vertical component of the ground motion is mainly controlled by the source effect and exhibits relatively less sediment-induced amplification than horizontal components (e.g., [43]). Under the assumption that the local site effects are more pronounced in the horizontal components than in the vertical components of ground motion, we present the spatial distribution of horizontal \(\kappa\) values (Figure 7) to determine if there exist different trends in the high-frequency regional attenuation between different azimuthal area subsets. The lowest \(\kappa\) values are spatially distributed within a few kilometres around the stations due to near-site effects and their values matching the \(\kappa_{0}\) value for horizontal components (PTJ: 0.0283-0.0360 s, ZAG: 0.0411-0.0475 s), particularly using constrained \(\kappa_{0}\) value based on hockey-stick model [36]. Station PTJ is situated on the top of the Medvednica Mountains as a part of the Pannonian Basin-External Dinarides-Alps transition zone. The station site area is composed of bedrock Palaeozoic-Triassic ortometamorphites and parametamorphites. In the lowland Zagreb area, where station ZAG is situated, Miocene semi-consolidated rocks to recent unconsolidated Pliocene and Quaternary sediments of the rivers Sava to the south and Krapina are present (Figure 1). Compared to [1], \(\kappa_{0}\) values for Zagreb area are higher than those for Kvarner region stations (Brijuni: 0.0249-0.0362 s, Rijeka: 0.0212-0.0239 s and Novalja: 0.0225-0.0235 s). The Kvarner stations sites and surrounding area are composed mainly of carbonates and the lowest \(\kappa\) values are regionally stabilized for hard rocks [11; 32; 37]. In an opposite way, the addition of softer rocks, within a few kilometres around the stations increases \(\kappa\) values [11; 30; 32]. The \(\kappa\) regionalization model schematically illustrated by [32] proposed division of near-site attenuation effects into upper local site effects (correlation with \(V_{530}\)), deeper site effects along near-field regional path (constrained model) and regional path effects from source. The \(\kappa_{0}\) values for Zagreb stations (PTJ and ZAG) are similar to those of Ozalj station (0.0377-0.0412 s) that is situated in a zone consisting mainly of consolidated rocks that are covered by thin alluvial sediments along the near Kupa River valley [1]. According to the model illustrated by [32], surrounding areas of Hrvatsko Zagorje and Sava Basins, with sediments depth up to 2 km (Figure 1), have influence on the increase of the high frequency attenuation and local site attenuation due to the addition of the deeper site attenuation. The older and more competent carbonate rocks in the top few kilometres of the crust  
Figure 7: Regional \(\kappa\) dependence around the PTJ and ZAG seismological stations shown as a spatial distribution of individual \(\kappa\) values and plotted using the natural-neighbour interpolation method. The red lines represent the possible seismogenic surface faults in Croatia. Possible active faults are marked with thick lines [28; 29]. The Zagreb city area is marked with a thick black polygonal line and seismic zones Novo Mesto-Krsko, Karlovac-Metlika, Pokupsko-Petrinja and Zagreb are marked with colours as shown in Figure 2. The M5.5 earthquake of 22 March 2020, with the epicentre 7 km north of the Zagreb city centre, is marked with a black star. are expected to have a lower kappa [37] as the situation is in the Kvarner region [1], than younger and less competent Miocene softer rocks in the uppermost crust surrounding the Medvednica Mountains.