## 2 Seismological Data and Earthquake Sequence Analysis
### Data Analysis  
The Arkalochori earthquake sequence is divided into two main temporal groups, one that preceded the 27 September Mw = 6.0 mainshock, consisting of 620 events with a significant rise in numbers during July and August 2021, and the aftershock sequence, divided into three sub-groups, composed by 4130 seismic events (Figure 1). A major part of the sequence was recorded by local stations of the regional Hellenic Unified Seismological Network (HUSN), with the nearest stations being KNSS, PFKS (IFEGG), located \(\sim\)20 km to the SW and NE of the epicenter, respectively. On October 1, 2021, the Geodynamics Institute of the National Observatory of Athens (GI-NOA) installed 4 temporary stations (CRE1-4) around the aftershock zone, contributing to the depth accuracy and providing a preliminary hypocentral solution for this time period. A total of 4750 events of the 2021-2022 Arkalochori sequence that occurred during the period between 13 January and 31 January 2022 (Figure 2) were detected and manually analyzed using real-time waveform data from the Hellenic Unified Seismological Network (HUSN) in the SeisComP3 graphical user interface [25]. During the first stage of the sequence analysis, hypocenters were located in near real-time, by employing the Hypo71 single-event algorithm [26] and a custom regional 1D velocity model for the Hellenic peninsula [27]. In this study, two local 1D velocity models [28; 29] have been used through the second stage of the data processing, running the HypoInverse code [30]. Residual values from these models were compared (Table 1) with no significant differences, while the epicentral differences were less than 0.5 km, whereas the depths, which are more sensitive to the velocity model, differed by about 1 km on average. The [28] velocity model (Model 1) provided much shallower events than the respective ones of [29] (Model 2) and especially for the stronger events of the sequence (Mw = 6.0 and Mw = 5.3) that were located at a depth shallower than 5 km (2.7 and 0.7 km, respectively), which seemed unrealistic in terms of earthquake physics and the past of the area. Figure 1: Seismicity rate in terms of events per day (blue vertical bars) and cumulative number of events (solid black line) during June 2021–January 2022 in the area of Arkalohori. The occurrence of events with M\({}_{\rm L}\geq\) 4 is marked by red stars (M\({}_{\rm L}\) magnitude in the red axis to the right). The final hypocentral locations were obtained using the velocity model of [29]. The range of depth distribution is mainly between 5 and 15 km for the aftershocks of the 27 September Mw = 6.0 event. Figure 2 presents the distribution of 4750 events that were manually revised and relocated with HypoInverse code, along with the faults in the area as extracted from [31]. The seismic sequence was divided into five sub-groups according to its spatiotemporal occurrence (Figure 3):  
1. 13 January-27 September 2021 (period A), consisting of 620 events;
2. 27 September-28 September 2021 (period B), the first day of the aftershock sequence and just a few hours before the greatest aftershock (M5.3), composed of 90 events;
3. 28 September-12 October 2021 (period C), just after the occurrence of the M = 5.3 aftershock at 04:48 UTC, consisting of 803 events;
4. 12 October-31 October 2021 (period D), where the M = 4.0 event took place after a significant decay in the aftershocks number in Arkalochori;
5. 1 November-31 January 2022 (period E), which comprises small to moderate magnitude events in a deeper part of the crust (H > 20 km) located near Herakleion. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Model** & **Model 1** & **Model 2** \\ \hline Mean RMS (s) & 0.26 & 0.26 \\ Mean ERH (km) & 1.30 & 1.31 \\ Mean ERZ (km) & 4.41 & 4.52 \\ Mean Depth (km) & 9.43 & 13.94 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of the error statistics between Model 1 [28] and Model 2 [29] for the Arkalochori seismic sequence.