## 2 Geological Setting
### Seismotectonic Setting  
The vast majority of the large events affecting Crete, before the 2021 Arkalochori sequence, have occurred offshore, either to the north or to the south, with the latter probably related to the intra-crustal graben system, i.e., the Ptolemy and Pliny trenches [39] (Figure 1b). The only knowledge regarding the local seismic activity in the study area comes from a temporary seismological network, which consisted of 10 analog stations, installed in the area of Heraklion during the period of September to December 1995 [29]. Data recorded by this network revealed the existence of shallow onshore activity. During that period, the seismicity was mainly concentrated along the eastern margin of the Heraklion Basin, decreasing from east to west, with epicenters also located in the vicinity of Arkalochori. Furthermore, a W-E-trending decrease in the sedimentary cover has been identified offshore, north of Crete [40].  
In a broader view frame, the location of Crete Island, north of the Hellenic Arc (Figure 1a), comprises an area of complex geodynamics, with transpressional tectonics dominating the south of Crete, in contrast with normal and oblique-slip faulting onshore, yielding a heterogeneous stress field [14; 41]. On the other hand, an extension has been observed to the north, between Santorini and Crete [42; 43; 44; 45]. The latter reveals extension in a general E-W direction, with NNE-SSW shortening observed in western Crete [46]. According to [14], the sub-horizontal S\({}_{1}\) axis is in an SW-NE (in the east) to the N-S direction (in the west), likely affected by the proximity of the region to the subduction interface and compression parallel to the direction of convergence, which becomes more oblique to the arc towards the east. However, the sub-horizontal S\({}_{3}\) axis also reveals E-W (in the west) to WNW-ESE (in the east) extension onshore, even in the region where the 2021 Arkalochori earthquake occurred, in accordance with the observed normal fault scarps, trending in a general N-S direction [19]. The resulting regional shear stress favors SW-NE left-lateral faults, mainly offshore, in agreement with [41; 47]. On the other hand, fault sources, determined by [11], indicate mainly E-W normal faulting to the north and south of Crete, incompatible with the resolved stress. It should be taken into account that the aforementioned stress field was determined using offshore earthquakes at crustal depths, both to the north and the south of Crete, as no large onshore events were located onshore until recently. The stress field in the region of Crete was characterized as transpressional by [14] due to the involvement of shallow events related to strike-slip (primary stress state) or reverse faulting (secondary state). In central Crete, due to the absence of focal mechanism data for shallow onshore events, the stress field is largely a result of interpolation from resolved nodes of neighboring regions. The stress shape value of 0.5 indicates instability in both S\({}_{1}\) and S\({}_{3}\) axes. However, the direction of S\({}_{3}\) seems to be generally stable in a WNW-ESE direction in the region from central Crete to the volcanic arc in the north.  
This complex stress regime is also revealed by the focal mechanisms, as all types (normal, strike-slip, and reverse) can be observed offshore, either to the north or to the south (Figure 1). Focal depths increase towards the north, due to the African slab rollback phenomenon. Onshore focal mechanisms at shallow depths are few. [PERSON] et al. (2018) worked on the western edge of Crete and obtained fault-plane solutions of shallow events, revealing either approximately N-S normal or strike-slip faulting. [PERSON] et al. (2019) constrained 29 fault-plane solutions close to the 2021 Arkalochori earthquake sequence, which also indicate normal or reverse motion with, in some cases, a significant strike-slip component.