## 3 Spectral Analysis
### 11 September 2016 Skopje Earthquake  
The acceleration response spectra of the strongest earthquake from the September 2016 seismic sequence in the Skopje area recorded on the SKO, FCE, and IZIIS stations are shown in Figure 7 and summarized in Table 3. The spectral values of the magnitude M\({}_{1}\)5.3 event for the lower periods are about four times smaller than the design spectra curves in the Building Code, when normalized to 30 km, as engineers define near-source earthquakes. Consideration of further details regarding the design spectra and modifications are the subject of an on-going process in the country (Standardization Institute of the Republic of Macedonia. [MKC EN 1998-1:2012]).  
The maximum spectral peaks for the SKO station (Figure 7--top) appeared at 0.2 s on all three components, while secondary peaks were concentrated between 0.06 and 0.1 s, equivalent to a frequency of 5-16 Hz. For the IZIIS station (Figure 7--middle) the peak values are again in the same range, and with a dominant E-W component, having the largest peak at around 0.125 s or 8 Hz. For the FCE station (Figure 7--bottom) the peak values appeared at 0.08 s on all three components, equivalent to a frequency of 12.5 Hz, and with a dominant E-W component, the largest peak at around 0.125 s or 8 Hz.  
The dominant frequencies of 5 Hz or 0.2 s found on the SKO Observatory record of the instrument installed on bedrock and the top layers velocities using the Balkan model [10], lead to estimated value of 0.9 V\({}_{\rm S}\) velocity that is a general rate for rupture propagation on faults.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Station Code Name** & **Maximum Acceleration** & **Hypocentral Distance** \\  & **(g)** & **(km)** \\ \hline
**SKO** & 0.140 & 12.3 \\
**FCE** & 0.121 & 11.9 \\
**IZIIS** & 0.192 & 12.4 \\
**QUHS** & 0.225 & 12.5 \\
**QARH** & 0.200 & 14.0 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Maximum acceleration recorded and the hypocentral distance for the reporting stations.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Hypocentral** & **PGA** & **PGV (cm/s)** & **Sa (5\%)** & **Period Range** \\  & **Distance (km)** & **(g)** & **(g)** & **(s)** & **(s)** \\ \hline
**Skopje 2016** & 12.3 & 0.140 & 4.3 & 0.325 & 0.2 \\ \hline
**Skopje 2016** & 11.9 & 0.121 & 1.5 & 0.4 & 0.08 \\ \hline
**Skopje 2016** & 12.4 & 0.192 & 4.5 & 0.65 & 0.15 \\ \hline
**Zagreb 2020** & 12.5 & 0.225 & 7.2 & 0.4–0.65 & 0.1–0.2 \\ \hline
**Zagreb 2020** & 14 & 0.2 & 6.3 & 0.25–0.35 & 0.1–0.2 \\ \hline \hline \end{tabular}
\end{table}
Table 3: List of observed and computed seismic parameters for the selected events.  
The prevailing frequencies of 0.1 s or 10 Hz found on the FCE record of the instrument installed over sedimentary layer may indicate that the vibration at the station site is dominated by the guided waves created in the upper layers. Vibration with high frequencies might be of special interest to Civil Engineers due to possible resonance effects with structures of a similar natural frequency.  
Figure 7: The response spectra of the three components of the Skopje 2016 M\({}_{1}\),5.3 earthquake at the closest stations: top—Seismological observatory SKO; middle—IZIIS Institute; bottom—FCE (Faculty of Civil Engineering—Gradezen Fakultet).