## 3 Materials and Methods
### Seismological Data and Methods  
The 2021 seismic crisis in the wider area of Arkalochori began in the form of an earthquake swarm in early June 2021. The situation, however, changed dramatically after the occurrence of the \(M_{\mathrm{w}}=6.0\) mainshock of 27 September 2021. Although several permanent stations of the Hellenic Unified Seismological Network operate on the island of Crete, at the time of the mainshock, the closest station to the epicentral area was KNSS of the Hellenic Mediterranean University (HC network at formerly Technological Educational Institute of Crete [50]), at a distance of about 12-22 km (Figure S1). As a result, the seismicity of the first stage of the sequence (foreshock swarm) could not be determined with high precision, due to the lack of data from stations at local distances, which would help constrain the focal depths. Resolving hypocentral locations with data from more distant stations is further problematic due to the inhomogeneity of the velocity structure, because of the proximity of the area to the Hellenic subduction zone. The day after the mainshock, four temporary seismological stations (CRE1-4) were deployed by GI-NOA (HL Network [51]) around the aftershock zone (Figure S1b). The inclusion of data from these stations drastically improved the hypocentral locations, even with the use of a generic velocity model. We collected the available catalogues, including manually analyzed phase data from SL-NKUA and GI-NOA at all operational stations, and compiled a dataset of \(\sim\)2500 events for the period between 1 June and 18 October 2021. We divided the data to be processed in two periods, one before (Period A) and one after (Period B) 28 September, when the temporary local network was deployed. We used the HypoInverse code [52] to locate the sequence, initially with a generic model used by routine analysis at SL-NKUA, as well as with the local velocity model of [29] for Central Crete. The latter model was preferred, as it yielded lower root mean square (RMS) travel-time residuals and locations errors, while providing a better general image regarding the distribution of epicenters, in terms of scattering, especially between events that form spatial clusters. Another issue concerns the distance weighting parameters for the two periods, i.e., the maximum epicentral distance of stations to be used. After several tests, the maximum distance was set to \(\sim\)280 km for the first period and \(\sim\)120 km for the second one. A \(\mathrm{V_{P}}/\mathrm{V_{S}}\) ratio of 1.78 was measured using the Chatelain [53] diagram for the first period, whereas a \(\mathrm{V_{P}}/\mathrm{V_{S}}=1.76\) was preferred for the second period with the shorter distance limit. The application of station corrections, especially for the first period, plays an important role to reduce artifacts of the initial epicentral distribution, where spurious epicenters of small magnitude events seemed to spread northwards of the main seismicity cloud. To further improve the hypocentral distribution, with emphasis on the second period, we applied a relocation procedure using the HypoDD code [54]. The algorithm works by minimizing the double difference between calculated and observed travel times in combinations of event pairs with neighboring hypocenters, at inter-event distances much smaller than their respective hypocentral distance from a station. This method enhances the hypocentral distribution by minimizing relative location uncertainties between correlated events, caused by discrepancies between the velocity model and the real structure. Furthermore, waveform cross-correlation data are used to minimize uncertainties due to arrival-time reading inconsistencies in groups of events with similar waveforms, called multiplets. We used station KNSS (Figure S1b) as the main reference station for the identification of multiplets, as this is the closest station that was operational during both periods, with the exception of its N-S component that was damaged after the occurrence of the 27 September mainshock.