## 3 Travel Time Tomography  
Local Earthquake Tomography (LET) techniques have been successfully applied to reveal the velocity structure in such cases of aftershock sequences. In this paper, the body-wave inversion was based on the LOcal TOmographic Software (LOTOS) by [33]. P- and S-phases of more than 800 earthquakes, recorded during the 2021-2022 time period by local and regional stations of the Hellenic Unified Seismological Network (HUSN) and the Hellenic Strong Motion Network (HSMN), located in Southern Greece, were used for the tomographic inversion. Checkerboard tests were performed to set the input parameter values that produced better resolution and increased the fidelity area. Regarding the 3D tomographic inversion, a dataset consisting of 12,236 P- and 9820 S arrival times was chosen, with at least 12 phases per event (Supplementary Material Figure S2). LOTOS code provides two alternative options: inversion for V\({}_{\rm P}\) and V\({}_{\rm S}\) (V\({}_{\rm P}\)-V\({}_{\rm S}\) scheme) using P and S travel-time residuals (dt\({}_{\rm P}\) and dt\({}_{\rm S}\)) and inversion for V\({}_{\rm P}\) and V\({}_{\rm P}\)/V\({}_{\rm S}\) ratio (V\({}_{\rm P}\)-V\({}_{\rm P}\)/V\({}_{\rm S}\) scheme) using dt\({}_{\rm P}\) and differential residuals, dt\({}_{\rm S}\)-dt\({}_{\rm P}\). In this work, inversion was performed for V\({}_{\rm P}\)-V\({}_{\rm S}\) and V\({}_{\rm P}\)-V\({}_{\rm P}\)/V\({}_{\rm S}\) schemes, in order to obtain additional constraints concerning the V\({}_{\rm P}\) and V\({}_{\rm S}\) anomalies [33; 34]. The stations' coordinates, their elevation, and the body-wave arrival times are essential as input data to the algorithm. The hypocenter locations and the origin times are not necessarily needed, given that their determination is performed during the execution of the calculations. However, if preliminary hypocentral locations are available, they are used to decrease the processing time. Moreover, the available initial 1D velocity model [29] and a set of input parameters, i.e., parameterization, grid dimensions, and damping parameters, are defined by the user [33]. A nodal representation was employed, given that the velocity field, reconstructed by a three-dimensional grid, does not assume a specific geometry of heterogeneities [35]. The grid spacing (\(\sim\)2 km) was kept considerably smaller than the expected resolution length, to reduce the bias of the resulting models due to the grid configuration. The optimal grid mesh has been determined considering the stations/events geometry. In addition, to further decrease the influence of the model parameterization on the solutions, the inversion was repeated using four grid orientations (0\({}^{\circ}\), 22\({}^{\circ}\), 45\({}^{\circ}\), and 67\({}^{\circ}\)). The inversion results, obtained for the previously mentioned grids, were stacked into one summary model, reducing the artifacts related to grid orientation, as described by [33]. The values of the P- and S-wave residuals during different iteration steps of the inversion procedure are presented in Table 2. For the P- and S-data, the reduction of the residual is \(\sim\)10% and 13%, respectively. The resulting P- and S-wave velocity anomalies with respect to the starting 1D velocity model are shown in horizontal depth slices (Figures 5 and 6). The interpretation of the obtained results is limited to the unmasked confidence regions, given that they are characterized by reasonable reconstruction of the checkerboard model. The mean computed P and S anomalies for the study area do not exceed \(\pm\)13%. \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 & 0.269 & 0.00 & 0.437 & 0.00 \\
2 & 0.211 & 21.50 & 0.248 & 43.14 \\
3 & 0.194 & 27.63 & 0.222 & 49.12 \\
4 & 0.188 & 30.08 & 0.218 & 50.10 \\
5 & 0.186 & 30.85 & 0.211 & 51.69 \\ \hline \hline \end{tabular}
\end{table}