## 3 Results  
The obtained body-wave travel-time residuals are shown in Figure 4. The initial P-wave travel time residuals were mainly distributed in the range of \(\pm\)0.75 s, and the average travel time residuals were 0.26 s. In comparison, the P-wave travel time residuals were mainly distributed in the range of 0.5 s after relocation, and the average residuals were reduced to 0.17 s. Figure 4b shows the residuals of the initial and the final S-wave travel times (Table 1). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Iteration** & **P-Residual (s)** & \
\begin{tabular}{c} **P-Residual** \\ **Reduction (\%)** \\ \end{tabular} & **S-Residual (s)** &
\begin{tabular}{c} **S-Residual** \\ **Reduction (\%)** \\ \end{tabular} \\ \hline
1 & 0.227 & 0.00 & 0.573 & 0.00 \\
2 & 0.184 & 18.79 & 0.349 & 18.10 \\
3 & 0.175 & 22.89 & 0.302 & 20.56 \\
4 & 0.171 & 24.75 & 0.286 & 22.66 \\
5 & 0.169 & 25.54 & 0.278 & 23.95 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mean residuals in L1 norm and their variance reductions during the iterative inversion procedure. In Figure 5, the eight (8) performed cross-sections are projected in map view (seven in NE-SW orientation and the last one in WNW-ESE orientation). Figure 4: Histograms of (**a**) P-wave and (**b**) S-wave travel-time residuals prior to (green) and after (red) the inversion procedure. Figures 6-8 depict the distribution of body-wave velocity perturbations (%), as well as the V\({}_{\rm P}\)/V\({}_{\rm S}\) ratio in four horizontal slices at depths 5, 10, 15, and 20 km. Additionally, eight (8) cross-sections, seven (7) perpendicular to the Parnitha faults and one (1) parallel to these tectonic features, are shown in Figures 9-13. The interpretation of the obtained velocity anomalies is limited to model portions with satisfactory reconstruction of the checkerboard and ray coverage. For P-waves, velocity perturbations range from \(\pm\)10% in the shallow depth layers, while S-waves show changes of up to \(\pm\)15%. The V\({}_{\rm P}\)/V\({}_{\rm S}\) ratio varies between 1.65 and 2.01 (as shown in Figure 8). The 3-D inversion results can be confidently interpreted as the velocity models have high resolution and dense ray coverage down to 20 km depth (as shown in Figures S3 and S4). Figure 5: On the map, there are the locations of the eight (8) cross-sections performed. The width of each section is 2 km (\(\pm\)1 km from the mark of the section). Seismic events are marked in red color, while the stations of HUSN are in blue triangles. Letters indicate the direction of each section. Hypocentral depths in Figure 3. Fault traces as in Figure 1.  
Figure 6: Tomograms of lateral \(\rm V_{P}\) (%) variations at 5, 10, 15, and 20 km depth. Regions with low resolution are masked. Black circles indicate the recorded seismicity. Toponyms as in Figures 1 and 2. Figure 7: Tomograms of lateral \(\rm V_{S}\) (%) variations at 5, 10, 15, and 20 km depth. Regions with low resolution are masked. Black circles indicate the recorded seismicity. Toponyms as in Figures 1 and 2. According to tomograms between 5 and 15 km depth, the negative body-wave (dV\({}_{\rm P}\), dV\({}_{\rm S}\)) velocity anomalies (%) were mainly found in the Thiassion and Megara basins, as well as the Athenian plain. On the other hand, the positive velocity anomalies were mainly associated with the areas that are located near the mountain masses of Parnitha, Aegaleo, and Geraneia mountains. This pattern was also observed in past studies in Attica [43; 44; 45], which adds to the model's credibility. In the tomogram of 5 km depth, near Thiassion basin, we can see relatively low V\({}_{\rm P}\)/V\({}_{\rm S}\) ratio values (~1.68), while in deeper parts of the crust (depth slices of 10 and 15 km), these rise more than 1.80. The same distribution appears in the Athenian plain and Megara basin. Strong lateral body-wave velocity anomalies were primarily identified at depths of 5 and 10 km. This is likely due to the concentration of seismic events in this area, which could be introducing larger anomalies.