## 5 Results
### EO-Based Terrain Motion Measurements  
Interferometric PSI processing revealed multi-temporal ground displacements over the entire Strymon basin, including the surrounding mountain ranges (Figures 8 and 9). Following SNAPPING PSI processing, approx. 50 k and 70 k point targets were detected for the ascending and descending tracks, respectively. This can be translated to an average density of 6-8 points per square kilometer, which is acceptable given the processing parameters used (low amplitude dispersion), the significant vegetation cover (i.e., agricultural lands and forested regions), and the medium-resolution PSI service used. PS points are located, as expected for the PSI technique, mainly over buildup areas and non-vegetated natural terrain. The overall agreement between the independent geodetic techniques (i.e., GNSS and InSAR) for the calculated annual motion rates is clearly shown in Figure S1. Correspondingly, observations from different SAR geometries show consistency in terms of recognized patterns and motion sign, which serves as an inter-verification of results by independent EO datasets. For the majority of point measurements (approx. 95%), displacement rates do  
Figure 8: Average LoS displacement rates for the period 2015–2020, derived by SNAPPING service on GEP using Sentinel-1 data from ascending track 102. White lines (a–e and f–k) correspond to spatial profiles shown in Figure 8. Please note that, for the selected color scale, rates higher than \(\pm\)4 mm/yr appear saturated to allow for the proper visualization of spatial variability of motion. not exceed \(\pm 2\) mm/yr, and are within the uncertainties of the PSI measurements (an order of a couple of millimeters per year; see Supplementary Figures S2 and S3). This signifies for the overall stability of the area or for the presence of rather low motion gradients. Regions of relatively higher displacement gradients, reaching \(-16\) mm/yr, correspond to local patterns (i.e., subsidence) that are mostly recognized on flat areas within Strymon basin (Figures 8 and 9). The circular to elliptical shape of these local patterns most probably relates to groundwater extraction phenomena. The temporal variability of ground displacements for these areas, as derived by the analysis of the corresponding time series (Figure 10), also highlights the relationship of these temporal deformations to water table variations in underground aquifers [124,125]. Positive motion rates, negligible in terms of magnitude, and corresponding to uplift at the highest parts of the mountains, can be attributed to a residual topo-dependent atmospheric signal that has not been compensated for. Figure 10: PSI displacement time series for ascending track 102 at the location of high subsidence rates within the basin. The calculated trend (low pass filtering) has been subtracted from observed LoS displacements to obtain the seasonal component. The annual seasonality and linear trend are consistent to water table variations in underground aquifers. Time series has been arbitrarily shifted for visualization purposes. Figure 9: Average LoS displacement rates for the period 2015–2020, derived by SNAPPING service on GEP using Sentinel-1 data from descending track 7. White lines (a–e and f–k) correspond to spatial profiles shown in Figure 8. Please note that, for the selected color scale, rates higher than \(\pm 4\) mm/yr appear saturated to allow for the proper visualization of spatial variability of motion. Focusing on more tectonic-related signals, the lack of linear patterns along the fault zones at the boundary of the basin does not allow for the correlation of any InSAR signal to the interseismic long-term slip-rates of these tectonic structures. This is more evident given that the PSI displacement rates do not exceed the obtained uncertainties nearby these fault zones and in the vicinity of regions of high strain accumulation, as indicated by the geodetic GNSS analysis.