## 4 Nakamura Processing  
The accelerograms from the 2016 seismic sequence in the Skopje area were further analyzed using the complete records in raw format and applying the Nakamura method [13; 14]. The theory predicts that on soft ground, the horizontal motion is larger than the vertical motion. However, on rock, the horizontal and vertical motions should be similar, both in the maximum amplitudes and in the waveform content. Here, we used the software to automatically select the parts/windows of the waveform and calculate the maximum values of the H/V ratio and compare them with the ground characteristics for the selected stations. The basic assumptions upon which the Nakamura method is based then, as a first approximation, the resonance frequency, f\({}_{0}\), of the superficial layer and the amplification level A(f\({}_{0}\)), are given by the equations:  
\[\text{f}_{0}=\frac{\text{V}_{1}}{4\text{H}}\]  
Figure 8: Response spectra of the Zagreb 2020 M\({}_{\text{L}}\)5.5 earthquake records on the stations: left—QUHS; right—QARH. \[\text{A}(\text{f}_{0})=\frac{\text{V}_{2}\text{r}_{2}}{\text{V}_{1}\text{r}_{1}}\]  
where V\({}_{1}\) and V\({}_{2}\) are the S-wave velocities in the upper layer and the bedrock, respectively. The corresponding densities of the materials are r\({}_{1}\) and r\({}_{2}\), usually read from lab tests, and H is the thickness of the superficial layer. Thus, the amplification characteristics of horizontal motions by surface layers can be estimated from the ratio of horizontal-motion spectra measured at soft soil and bedrock sites. However, it is often difficult to determine the resonance frequency corresponding to the surface layer because the spectral ratio can have several peaks [15]. That phenomenon was recognized to be caused mainly by surface waves. Therefore, although uncertain, the method can assist to distinguish the amplification caused by multiple reflections of the vertically polarized waves in the top layers, which is important during forming the velocity models for further computational analysis. According to the results of surface layer investigation, the depth estimates depend on the velocity of the top material. The amplitudes of the peaks then match certain combinations of layers with given velocities, and the solutions might be ambiguous. It is a common practice to discard the extreme solutions that are not physically possible and keep the most plausible. Horizontal vs. vertical ratio of the 11 September 2016 magnitude M\({}_{1}\)5.3 earthquake at locations of Seismological observatory SKO, IZIIS Institute, and FCE (Faculty of Civil Engineering--Gradezen Fakultet) is shown on Figure 9. The predominant peaks and frequency found on the seismic station SKO record, where the instrument is installed on rock, are at 2, 2.5, and 3.75 Hz, while some spikes below 0.5 Hz could be artifacts of the processing. Using the general values for velocities in soft rock, the thickness of the top cover can be approximated to 70 m, which is in line with the Eurocode-8 classification of type A. Similar peak distribution was noticed when records from local explosions and ambient noise were analyzed ([PERSON], personal communication, 2021). The main peaks and frequency found on the seismic station IZIIS record, where the instrument is free-standing, are at 2.5, 3.75, and 6 Hz, probably a coupling effect with the base block. Using the average values for velocities in porous materials, the thickness of the top soil cover is between 40 and 50 m, and is according to Eurocode-8, classified soil of type B. The highest peak and frequency found on the seismic station FCE record, where the instrument is in the sub-basement of the faculty on ground that consists of sand and gravel sediments, is at 2.25 Hz.