## 2 Seismological Data and Earthquake Sequence Analysis
### Hypocentral Relocation of the Earthquake Sequence  
Accurate earthquake hypocenter parameters are required in order to obtain a detailed image of the structural properties and processes that trigger seismic activity. The precision of hypocenter locations and their uncertainties depend on several factors, including the number and quality of available seismic phases, the accuracy with which arrival times are measured, the network geometry, the knowledge of the velocity structure, and the linear approximation to a set of non-linear equations, which is assumed in the inversion. hypoDD [32] is an algorithm that reduces residuals between observed and theoretical differences of travel times (or double differences) for pairs of neighboring events at each station that recorded both events, as can be seen from Equation (1). This way, errors due to unmodeled velocity structures are minimized without station corrections. A minimum 1D layered velocity model is used to predict the travel time differences and partial derivatives (Equation (2)). \[dr_{k}^{ij}=\left(t_{k}^{i}-t_{k}^{j}\right)^{obs}-\left(t_{k}^{i}-t_{k}^{j} \right)^{cal} \tag{1}\]  
\[\frac{\partial t_{k}^{i}}{\partial m}\Delta m^{i}-\frac{\partial t_{k}^{i}}{ \partial m}\Delta m^{j}=dr_{k}^{ij} \tag{2}\]  
where \(m\) = (x, y, z). Inter-event distance and misfit weighting are applied to catalog data after the end of each iteration, in order to optimize their quality during the relocation procedure. Horizontal and vertical relative spatial errors can be minimized by approximately one order of magnitude under certain conditions. In this study, more than 4500 events of M\({}_{\rm L}\)\(\geq\) 0.6, comprising 58,777 phases, were relocated with hypoDD. Among the main factors that had been taken into consideration during the relocation procedure were the following: (a) network coverage of the area, (b) the size of the dominant clusters, and (c) their maximum separation distance. This led to the formation of 98,070 P- and 43,812 S-phase pairs, respectively, for the whole volume, while in the central cluster (cluster #1), where the mainshock (M = 6.0) largest aftershock (Mw = 5.3) is situated, 98,070 P- and 18,673 S-phase pairs were formed. In the area of Central Crete, 4728 out of 4750 events of the initial catalog (M \(\geq\) 0.6), were relocated with hypoDD, giving a first result that could be rated as satisfactory. The mean temporal errors (rms) were reduced from 0.14 s to 0.11 s, while the spatial errors (erx, ery, erz) were decreased from 1.0, 0.8, and 2.0 km to 0.3, 0.3, and 0.4 km, respectively  
Figure 3: Temporal distribution of the 2021–2022 Arakalochori sequence for 4750 events. Periods A, B, C, D, and E are marked with orange, yellow, green, purple, and violet circles, respectively. The M \(\geq\) 4.0 earthquakes are depicted by yellow stars. Faults are marked as red lines (see text for details). (Table 1). The hypoDD-estimated errors in the final locations were calculated using the LSQR method, which may not be representative of the real ones [32]. The epicenter of the mainshock was located less than 3 km to the SE of Arkalochori (lat: 35.1416\({}^{\circ}\) N, long: 25.2736\({}^{\circ}\) E) at a depth of \(\sim\)9.6 km, obtained by the double difference algorithm procedure. The optimization of the final results leads to the clustering of the earthquake sequence into four main clusters. A dense cluster of events has occurred west of the mainshock, in an approximately 15 km long area associated with the foreshocks (cluster 1). The epicenters of cluster 2 were mainly distributed in the area between Amourgeles and Parthenio N-S oriented normal faults, in the region to the west of Arkalochori. Further to the NE, another significant cluster of events (cluster 3) was also observed, in the football of Agnos NE-SW striking normal fault, near Kastelli (Figure 4). Most seismicity is in a range of focal depths between 7 and 18 km.