## 4 Data Analysis and Results
### Seismological Results
After creating the clustering hierarchy with [PERSON]'s linkage by setting an appropriate threshold to the fusion level, we divided the spatial distribution into three clusters (i.e., groups #2, #3, and #4) to aid the description. The evolution of the earthquake sequence is examined through the spatiotemporal projection of Figure 9, along the SW-NE-oriented, 16 km-long profile A-B of Figure 8b. The foreshock swarm began in June 2021 with a few small events (\(M_{\rm L}\leq 3.0\)) and an \(M_{\rm L}=4.2\) event on 4 June which did not, however, cause any significant outbreak. Minor bursts occurred on 8 June and then on 18 July including three \(M_{\rm L}=4.1\)-4.2 events. During that period, a slow migration of seismicity towards the southwest can be observed in Figure 9. Then on 24 July 02:07:37 UTC, an \(M_{\rm L}=4.8\) occurred, triggering a subsequence. Afterwards, apart from a few more minor bursts, the seismicity rate gradually diminished, notably to the point where only 17 events were located in the period between 16 and 25 September, with a complete absence of detectable events for about 41 h preceding the \(M_{\rm w}=6.0\) mainshock of 27 September 2021, 06:17:21 UTC. The whole foreshock group #1 is roughly distributed in an area of 3.0 km \(\times\) 1.7 km, elongated in an N-S direction, with its centroid \(\sim\)2.5 km WNW of Arkalochori. The main event's epicenter is located adjacent to the foreshock swarm, near its eastern edge. The occurrence of the mainshock, near the middle of profile A-B, caused seismicity to spread rapidly nearly through the whole length of the aftershock zone, both towards the southwest (group #4) and the northeast (group #3). The aftershocks appear to have little overlap with the foreshock zone, specifically with the sparser middle group #2 (cyan) and the northern margin of the southern group #4 (green). The two major groups, #4 south-west of the foreshocks and group #3 (blue) towards the NE, the latter with epicenters near the Kastelli airport, are separated by the less populated group #2 (cyan). This deficit  
Figure 9: **(Top) Stacked histogram of the number of seismic events per day in the study area between 1 June and 18 October 2021. (Bottom) Spatiotemporal projection along the SW-NE-oriented profile A-B of Figure 8b. Colors and numbers (CLID) represent groups 1–4 of Figure 8b. Events with \(M\geq 4.5\) are depicted as stars. **  
of aftershocks between groups #3 and #4 is likely associated with an area of maximumoseismic slip during the mainshock. The rupture of a large asperity in that region apparently caused redistribution of stress towards the northern and southern margins of the main fault, while the central part was relaxed due to stress drop. The largest aftershock was an \(M_{\rm w}\) = 5.3 event that occurred on 28 September 04:48:08 UTC, about 4 km west of the mainshock. The epicenter of the main event is located at the NE edge of group #4, whereas the largest aftershock occurred towards its western part, near the southwestern tip of the foreshock group. The major aftershocks include 11 more events with \(M_{\rm L}\geq\) 4.0 between 27 and 29 September, with only smaller events being detected through the rest of the sequence during the period of study (up to 18 October 2021). Regarding the period after 18 October, routine monitoring of seismicity at SL-NKUA detected five more events with 4.0 \(\leq M_{\rm L}\leq\) 4.5 between 20 and 22 of October, two associated with cluster #3 and three with cluster #4, while another \(M_{\rm L}\) = 4.2 event occurred on 29 December associated with the latter cluster. However, no significant changes in the overall spreading of the aftershocks' distribution were observed, besides a small, isolated cluster that was detected \(\sim\)10 km NNW of Kastelli, activated between 16 and 18 January 2022, with the largest event having \(M_{\rm L}\) = 3.6.