## 4 Co-Seismic Ground Deformation
### GNSS Data and Results  
GNSS data from four continuous stations (belonging to the private network of MET-RICA SA) on the central-eastern part of Crete were used in the current study to measure the co-seismic displacement. One of the stations (ARKL) is located almost above the hypocenter of the main event, in ARALochori. The other sites are located in the city of Heraclion (HERA station; \(\sim\)24 km NNW of ARALochori), in the village of Moires (MOI1 station; \(\sim\)38 km WSW from epicenter), and in the Ierapetra region (IERA station; \(\sim\)49 km ESE from the epicenter). Daily raw GNSS data from these four stations were processed for a period of several years before the 2021 seismic sequence up to 30 April 2022, using the Bernese v5.2 GNSS software [46]. In the processing procedure of the local GNSS data, several stations of the EUREF and IGS were included together with other auxiliary files. The absolute antenna phase center corrections were used, together with precise orbital solutions from the Center for Orbit Determination in Europe (CODE) and the Vienna Mapping Functions for the tropospheric modeling. For the coordinate estimation on static mode solutions, the precise double difference method was used. For ambiguity resolution, numerous strategies were applied, based on the length of the formed baselines between the GNSS stations. The processing resulted in the estimation of high-precision station coordinates. Time series were formed, annual velocities prior to the 2021 seismic event were calculated and co-seismic displacements were determined. The daily coordinates of the GNSS stations were estimated on the global ITRF2014 reference frame. Based on the formed time series of the stations' coordinates (Figures S5 and S6), for the pre-seismic period, all the stations show SE horizontal linear type of motion (with respect to ITRF 2014) and subsiding vertical motion, consistent with the velocity field of the area [47]. Calculating the change in the baseline distance between the stations prior to the seismic sequence it is observed that a small extension pattern occurred in the area, since lengthening on the baselines was recorded of small amplitude (\(\sim\)1.5 mm/year). The main seismic event on Arkarochori caused, as is expected, strong ground displacement in the epicentral station ARKL as well as in the HERA station (Table 4). The vector of the co-seismic displacement was calculated as the static shift of the station coordinates nine days prior to and three days after the 27 September 2021 event, to minimize the effect of possible post-seismic motions. The higher displacement was recorded on the ARKL station, while noticeable displacement occurred in the HERA site (Figure S5). The two other sites (MOI1 and IERA) have not shown any considerable co-seismic motion (Figure S6). The displacement vector on ARKL site shows a strong subsiding component of \(\sim\)16 cm and significant eastward and northward horizontal motion. Both the E-W and the vertical components deduced by the GNSS analysis agree with the interferometric results. Small discrepancies in the amplitude of the components deduced by the two techniques may be attributed to the incompetence of the DInSAR to define the north motional component that is inherent in the LoS vector, and in our case is quite noticeable (\(\sim\)8 cm). The displacement vector on HERA station shows NW horizontal co-seismic motion and a noticeable upward vertical component. The overall image of the ground deformation in the epicentral area, based on both interferometric and GNSS results, defines a strong subsiding pattern with substantial horizontal motional component compatible with normal faulting activated structure.