## 5 Discussion
If we take a look at the gradual increase of \(\kappa\) with distance from the stations, the regional path effect from the source attributed to the \(\kappa\) is described by the slope regression \(\kappa_{R}\). It can be observed that the spatial \(\kappa\) distribution is not isotropic and different trends between estimated lower and higher attenuation of ground motion subsets are observed. According to [1], the source of anisotropy is most likely attributed to orientations of the cracks and fractures under the local tectonic stress field, waveguides along major faults and attenuation within fault zones, S-wave reflections from different parts of the shallower Moho discontinuity. Variations in observed \(\kappa\) distribution between lower and higher attenuation subset areas is clearly under influence of local and regional geological structures; to the north influenced by the Ivanscica Mt. and Kalnik Mt., to the south by Zumberak Mt., and in its centre by the Medvednica Mountains, that are distributed predominantly within regional active faults ([19], Figure 1) and main seismic zones (Figure 2). We limit spatial distribution of \(\kappa\) (Figure 7) within distance of 80-100 km from Zagreb area to avoid additional path complications, since ground motion ray paths from source to station show a wider range of \(Q\) (Figure 6, [37]), and to derive conclusions within seismic zones Novo Mesto-Krsko, Karlovac-Melika and Pokupsko-Petrinja (Figure 2), that influence seismic hazard of the Zagreb city area. As pointed in [1], observed lower attenuation properties may be caused by S-wave reflection from different parts of the shallower Mohorovicic discontinuity, which is less than 25 km in the Pannonian basin and thickens towards the Dinarides [44]. However, lower values of kappa along the SW-NE striking Medvednica structure (Figure 7) are probably caused by the shallow subsurface to surface position of the metamorphic and volcanic core of the mountain (Figure 1b). The core is situated below the major detachment and, thus, probably has not undergone thin-skin deformations that additionally cracked the rocks above the detachment. The attenuation of the earthquake ground motion is complex in nature, and high-frequency kappa physics is still unresolved [32; 33; 36]. The regional propagation difference in the transition zone between Dinarides and the Pannonian basin is observed from the kappa spatial distribution. It can be concluded that the regional difference in attenuation properties of the rocks in the transition zone between Dinarides and the Pannonian basin in this area are far from isotropic. In comparison to [1], the obtained results are similar and consistent. However, some differences are evident, pointing to effects such as local intrinsic attenuation anisotropy from different causes, e.g., scattering due to heterogeneity, fracturing, flow of fluids in rocks. The results from both studies taken together indicate a change in the attenuation properties from the Dinarides to the Pannonian basin. The main question is how the high-frequency attenuation observed along the propagation path affects seismic hazard of the Zagreb area. As shown in Figure 2, Zagreb city is under the influence of the near-field Zagreb seismic zone and close seismic zones Novo Mesto-Krsko, Karlovac-Melika and Pokupsko-Petrinja. It is also situated in the contact area of three major regional tectonic units (Figure 1). On 22 March 2020, Zagreb was hit by the strongest earthquake since 1880 [15], 5.5 on the Richter scale, with a shallow focal depth of \(<\)10 km. It occurred at 06:24 local time and hit the Croatian capital Zagreb as well as two neighbouring counties, namely Zagreb County (Sava Basin) and Krapina-Zagorje County (Hrvatsko Zagorje Basin). The maximal intensity of VII-VIII \({}^{\circ}\)EMS-98 scale was observed close to the earthquake epicentral area, just 7 km north of the centre of Zagreb, a city with a population of close to one million.