## 4 Data Analysis and Results
### Seismological Results  
Regarding the initial locations of earthquake hypocenters (Figure 8a) of the first period, before 28 September 2021, focal depths remain uncertain, due to the lack of local stations, mainly ranging between 3 and 20 km (Figure S3c), with the majority of events being  
Figure 6: Field photographs of (**a**) the Lagouta normal fault, which comprises the southern segment of the Kastelli fault zone, and (**b**) a pair of conjugate syn-sedimentary faults, within Tortonian deposits, compatible with left-lateral strike-slip faulting. Figure 7: Simplified 3D block diagram of the eastern marginal area of MB. Arkalochori village is located at the hanging wall of Lagouta fault but also at the hanging wall of Nipiditos fault, which is segmenting the Kastelli fault zone. concentrated near a discontinuity of the velocity model at \(\sim\)6 km [29]. On the other hand, more constrained focal depth values, between 5 and 13 km (Figure S3d), are determined for the second period. Location statistics for the two periods are presented in Table 2 and Figures S3 and S4. Average vertical location errors are nearly halved during Period B and horizontal location errors are reduced by \(\sim\)26%, as a result of the deployment of stations at local distances, which also reduced the azimuthal gap (Figure S4). In order to assess the 2021 Arkalochori earthquake sequence in more detail and to identify the activated tectonic structures, a relocation procedure was applied, as previously described in Section 3.2, to both periods (Figure 8b). The events distribution was divided into four groups. Group #1 (red) is a temporal cluster that concerns the first period of the foreshock swarm activity, whereas groups #2, #3, and #4 (cyan, blue, and green, respectively) were determined by spatial clustering of the events of the second period. This was achieved by applying [PERSON]'s linkage to the matrix of inter-event epicentral distances of the relocated hypocenters [72]. This procedure of agglomerative hierarchical clustering, also known as \begin{table}
\begin{tabular}{c c c} \hline  & **Period A** & **Period B** \\ \hline Number of events & 843 & 1657 \\ Mean RMS error (s) & 0.219 & 0.128 \\ Median ERH (km) & 0.530 & 0.390 \\ Mean ERZ (km) & 3.471 & 1.064 \\ Median ERZ (km) & 2.180 & 0.900 \\ Median Depth (km) & 7.120 & 8.370 \\ Median min.stat.dist. (km) & 17 & 6.3 \\ Mean azim. gap (\({}^{\circ}\)) & 138 & 110 \\ \hline \end{tabular}
\end{table} 
Table 2: Initial location statistics for Period A (1 June-27 September 2021, V\({}_{\rm P}\)/V\({}_{\rm S}\) = 1.78, long-distance weighting) and Period B (28 September-18 October 2021, V\({}_{\rm P}\)/V\({}_{\rm S}\) = 1.76, short-distance weighting). ERH, ERZ are horizontal and vertical location errors, “min.stat.dist” denotes the epicentral distance of the closest station with available arrival-time data (see also Figures S3 and S4 in the Supplementary Materials). Figure 8: Seismicity of the study region for the period between 1 June and 18 October 2021. (**a**) Initial locations with HypoInverse, (**b**) results of double-difference relocation with HypoDD. Colors and numerical labels in panel (**b**) represent the foreshock swarm activity (group #1, red) and the three spatial aftershock groups: #2 (cyan) middle, sparse group, #3 (blue) northern cluster near Kastelli airport, #4 (green) southern group, including the mainshock. Major events (\(M\geq 4.5\)) are depicted as stars. Profile line A-B in panel (**b**) is used for the spatiotemporal projection of Figure 9. The location of Arkalochori village is marked with a black square. the \"minimum variance method\", fuses two clusters into a new one, provided that the resulting increase in the sum of squares (i.e., its objective function) between the objects of the new cluster is minimized, compared to the outcome of any other potential combinations of cluster fusions.