## 5 Summary and Discussion  
There was a lack of instrumental data at short distance during the 1963 Skopje earthquake and compiled reports were mainly based on macroseismic data depicting intensity, the 2016 earthquake was recorded at several local and regional seismic stations. The seismological network data were used to locate the hypocenter of the earthquake and its magnitude of M\({}_{\text{L}}\)5.3. In this study, three strong motion records were analyzed in more details and the response spectra presented in relation to their site geology. In earlier analysis [20], such ground effects were correlated with the reported damage. The 22 March 2020 Zagreb earthquake of magnitude M\({}_{\text{L}}\)5.5 was located using data from the Croatian national seismological network and regional seismic stations. In this study, we calculated for the first time the response spectra from the mainshock recorded on the two closest stations and applied the Nakamura method in order to explore the characteristics of that recent moderate sized earthquake. The differences in the values of the station spectra can be primarily attributed to the types of surface and locations where the instruments were installed. The dominant frequencies of 5 Hz or 0.2 s found on the SKO Observatory record of the instrument installed  
Figure 10: Horizontal vs. vertical ratio of the 22 March 2020 Zagreb magnitude M\({}_{\text{L}}\)5.5 earthquake at stations QUHS—top; QARH—bottom. on bedrock and similarly at IZIIS Institute may indicate that the stations are on the flank of the same geological block that generated the strongest earthquake on 11 September 2016. That is consistent with the value of 0.9V\({}_{\text{S}}\) velocity that is a general rate for rupture propagation on faults [3]. The prevailing frequency of 1 Hz (period 1 s) found on the records from the instruments installed over the sedimentary layer may indicate that the vibration at the station site is dominated by guided waves created in the subsurface and influenced by the relative site position in respect to the fault system activated by the main earthquake. Response spectra for the Zagreb event showed the peak values mainly between 0.1 and 0.2 s, equivalent to frequencies of 5-10 Hz, which corresponded with observed building damage in the Zagreb city [5]. The spectral peak for stations QUHS and QARH is broad, and high spectral accelerations (above 0.2 g) are observed between 0.5 and 0.7 s equivalent to frequencies of 1.4-2 Hz, the results comparable with the dominant H/V frequency peaks. The main principle of the microtremor spectral response methodology is that ambient noise measurements are performed without an earthquake influence and, therefore, the fundamental soil frequency peak amplitude only represents a site response in the natural state. Otherwise, site response analysis provides possible amplification for given ground motion propagated from the bedrock level through the local soil profile. The change from site response of natural state and the site response of the local soil when subject to an earthquake influence is subject to different scenarios, including nonlinearity. These recorded data at short epicenter distance are valuable information for improvement of the seismic design codes. The selected stations were at very close hypocentral distances, so the ground shaking exceeded the design criteria across certain frequency ranges. Therefore, old and new unreinforced masonry buildings might suffer damage when rare shallow, moderate-magnitude earthquakes occur nearby. The response spectra of these two moderate size earthquakes, in the Skopje and Zagreb areas, should be considered in the Design Spectra for the Building Codes, which are subject of continuous work for the committees formed by the countries' standardization institutions. In this research, our aim was to combine only the results of the SM analysis and the Nakamura method and prove that the site substructure can be fairly inferred to the first degree, which is very useful in evaluation of the regional velocity models.