## 4 Discussion and Conclusions  
In this study, we present the results of the seismic tomography study in the Attica region in Greece, focusing on the 3-D distribution of P- and S-wave velocity anomalies and the V\({}_{\rm P}\)/V\({}_{\rm S}\) ratio. The data used come from a part of Greece that is densely covered by seismic stations of HUSN. The inversion results were evaluated in terms of their fidelity with checkerboard synthetic tests, and the trustworthy region was defined based on the ray coverage. The interpretation of the velocity anomalies and ratios focused on their relation to the region's known tectonic structures and seismic activity. The results highlight distinct velocity anomalies and patterns in different depths, which are interpreted concerning the region's tectonic structures and seismic activity. The shallow negative velocity anomalies obtained in the region are primarily found in sedimentary basins (Thriassion, Megara, Athenian plain), while positive anomalies are associated with mountainous areas (Parnitha, Aigaleo, Geraneia). The V\({}_{\rm P}\)/V\({}_{\rm S}\) ratio shows lateral variations, with lower values in the basins and higher values at greater depths. The 19 July 2019 Mw = 5.1 earthquake was in an area with a low body-wave velocity anomaly, near the border between high- and low-velocity zones. Ref. [43] reached similar results, since that area was also connected to the destructive 7 September 1999 Mw = 6.0 earthquake. This was caused by the aftershock zone located in the hanging wall of the fault, which was disconnected from the fault itself. The high fracturing of the region led to the existence of low velocities and could be observed in the distribution of events to the depth. The extension of the discontinuity between positive (north) and negative (south) body-velocity perturbations (\(\%\)) may be an indication of a possible continuation of either the TNF or FyNF towards the west, in the mountainous region near Erythres, before the final change in the overall fault orientation as we head to the Eastern Corinth Gulf (Figures 6 and 7). In the northern suburbs of Athens, seismic activity was located between 12 and 14 km depth, trending approximately in the WNW-ESE direction (Figure 14). Since the upgrade of HUSN in 2008, the detectability of such microseismic activity was increased, allowing us to have a richer earthquake catalog. In January 2015, an earthquake with an M\({}_{\text{L}}\) = 3.2 was felt by many residents of the metropolitan area, and it was particularly interesting. The same region that is identified through negative velocity perturbations of the initial V\({}_{\text{P}}\) model in this study lies between Kamatero (south) and Menidi (north) fault zones, as mapped in the work of [47], where the urban area with severe damages lies. Ref. [48] noticed a similar pattern while examining the postseismic displacement of the 1999 Mw = 6.0 earthquake. They observed that the fault rupture was becoming shallower inside the Athens basin, and it was assumed that this structure was ruptured in 1999 and continued at depth inside the Athens basin, crossing the alpine basement of both Parnitha and Aigaleo mountains at a right angle. The results of LET, though, lead to distinct fault zones to the west (Thriassion Normal Fault) and east (Kamatero Fault Zone) of Aigaleo mountain, respectively. In the Athenian plain, seismic activity and the distribution of body-wave anomalies and absolute velocities appear to be linked to a deeper (>9 km) subvertical structure, an observation that may be taken into account for the future plans of the Earthquake Planning and Protection Organization (EPPO). Figure 14: (**a**) Location of the seismic events with M\({}_{\text{L}}\)\(\geq\) 0.6 from January 2019 to March 2024, combined with the surface mapped faults [8; 47] and blind fault segments of TNF and KFZ [47]. (**b**) A morphological map of the study area showing absolute P-wave velocities along profile 8, which runs in a WNW–ESE direction.