## 4 Methods
### SAR Data and Interferometric Processing  
Imaging geodetic techniques such as Synthetic Aperture Radar (SAR) Interferometry have been proven to be an irreplaceable tool for measuring surface motion in different contexts and environments, including land subsidence [109], slope instabilities [110; 111], volcanoes [112], and earthquakes [113; 114]. Interferometric SAR (InSAR) has evolved over the past years into an operational technique for monitoring surface motion, allowing wide area coverage (including remote areas) at relatively low cost. Towards this direction, the systematic availability of open and free SAR data from the Copernicus Sentinel-1 mission [115] played an important role, ensuring the applicability of InSAR techniques on a global scale without the requisite for planning or ordering SAR acquisitions for specific sites. However, the requirement for big data processing both in terms of computational power and storage capacity led to the devolvement of cloud-based solutions [116]. Among such solutions, the Geohazards Exploitation Platform (GEP) is an initiative setup by the European Space Agency (ESA), aiming to support further exploitation of satellite Earth Observation (EO) data for geohazards assessment [117]. GEP is a cloud-based environment that provides a set of processing tools and services that allow for the mapping of hazardous land surfaces and the monitoring of terrain motion. It aims to support scientists and EO practitioners to better understand geohazards and their impact while introducing innovative concepts, such as the exploitation of the e-collaboration environment. The service has been already applied in scientific and operational frameworks [118; 119]. For mapping the ground displacements of the Strymon basin, we utilized the Surface motioN mAPPING (SNAPPING) service on the GEP. SNAPPING enables the Persistent Scatterers Interferometric (PSI) analysis of Sentinel-1 SLC data for measuring average displacements as well as the investigation of their temporal evolution (e.g., displacement time series). The service is implemented to facilitate on-demand processing and the easier monitoring options of a user-defined area of interest. The SNAPPING medium resolution service is considered sufficient for wide area processing, followed by more detailed investigation when a relevant signal (in our case, fault-related motion) is detected. The wide overview of ground displacements at medium resolution, reducing relevant production costs, is a proper practice that it well demonstrated in relevant research and development activities [120]. Medium resolution refers to the merging of Persistent Scatterers (PS) point measurements within a 100 m radius. However, PS targets exhibiting different phase properties are not affected, thus maintaining local displacement patterns. In practice, InSAR time series measurements reach an accuracy of 1-2 mm/yr for average displacement rates and 3-5 mm for each displacement estimate in the time series [121]. This should be critically assessed when a region of relatively low interseismic motion is being investigated. However, the abovementioned accuracy level might be affected by the time span of the observation period and the amount of SAR imagery used in the processing. To ensure the robustness of our analysis, we expanded the InSAR observation period from April 2015 to December 2020. A period of approx. 6 years is acceptable for obtaining a millimeter level accuracy. In turns, Sentinel-1A and -1B satellite units were exploited, repeated every 6 days, to better address non-linear temporal signals while minimizing error. A total number of 164 acquisition dates from ascending track 102 (10 April 2015-27 December 2020) and 159 dates from descending track 7 were considered (4 April 2015-9 December 2020) following a fully unsupervised processing on the GEP platform. An area of about 10,200 km\({}^{2}\) (120 \(\times\) 85 km) was examined.