## 4 Discussion and Conclusions  
In this study, 80 single station ambient noise measurements were performed in the city of Durres within the period of 18-21 February 2020, following the disastrous earthquake of 26 November 2019, that hit the city and inflicted heavy damages to the built environment. Seventy-six (76) out of eighty (80) measurements were processed for the purpose of this study. The unique strong motion recording in the city's center (DURR accelerometric station) showed PGA \(\sim\) 0.20 g and high spectral values (>0.3 g) in a broad range of periods (0.2 s \(\leq\) T \(\leq\) 2.0 s), implying existence of a low fundamental frequency (down to f\({}_{\rm o}\)\(\sim\) 0.5 Hz). From the geologic and bedrock iso-depth contour maps, it is evident that the city has been developed on a basin of soft geologic layers, with an elongated north-south direction, reaching a thickness of 130 m in its center. Surface geologic layers down-town exhibit low shear-wave velocity, with V\({}_{\rm S30}\)\(\sim\) 200 m/s. Data processing and analyses based on the ambient noise Horizontal-to-Vertical spectral ratio (HVSR) method showed that an extended part of the Durres city correlates to a low fundamental frequency, f\({}_{\rm o}\)\(<\) 1.0 Hz. This is in satisfactory agreement with the characteristics of the strong motion recording in the city's center, thus validating the findings of the applied methodology. This part is mainly in the center-west side of the city, while fundamental frequency decreases smoothly towards its eastern part (the eastern edge of the basin. The vast majority of examined sites (95%) exhibited amplitude of fundamental and dominant frequencies greater than 2, reaching a value of \(\sim\)6. Based on the consensus that HVSR peak amplitude may represent a lower threshold of actual ground amplification than that estimated by the Standard Spectral Ratio method, one would expect equal and/or higher amplification due to basin excitation by the seismic wavefield. A second, higher mode frequency peak, predominantly found in the western edge of the basin, may either indicate a thinner surface geologic layer of high velocity contrast interface with a deeper one, or a more complex site response behavior due to edge effects of the basin. Such an observation needs further investigation using theoretical 2D/3D modeling. A satisfactory correlation between fundamental frequency and thickness of the soft sediments is observed, though with large dispersion. This may be due either to errors of the bedrock depth estimation or to possible existence of additional superficial strata (e.g., soft colluvium, man-made deposits) which were not included in the subsoil profile. Targeted geological, geophysical and geotechnical data collection is needed to shed light on this issue and refine our understanding with respect to seisxmic risk of the city of Durres. The four site class categories proposed in this study indirectly indicate spatial variation of amplification, in linear soil behavior. Categories 1 and 2 include zones with higher average HVSR amplitude (>3.0) and are mainly extended in the central and western part of the city. It is remarkable that moderate to heavy earthquake damage was concentrated in the central and western part of the city (Figure 4a), potentially indicating a correlation of damage to site effects. Consequently, this categorization may form the backbone of any detailed and updated microzonation study of the city of Durres. For each category, an average fundamental frequency (\(\pm 1\) standard deviation) can be safely assigned along with its average amplitude. The average HVSR amplitude could be converted to actual S-waves horizontal spectral amplification using appropriate correction factors of the vertical amplification (VACFs) proposed either for other regions of the world ([72; 73]) or even better based on Albanian data.