## 5 Interpretation and Discussion  
PS-InSAR techniques are a powerful tool to measure displacements of moving ground objects from time series of SAR images to millimetric precision [2]. Although the lower resolution of ERS and Envisat makes it harder to exactly identify individual objects acting as persistent scatterers in the resolution cell. Therefore, for regional studies, the displacements of the objects reflecting the radar signal can be assumed to represent the ground movements of the earth surface and not only the object on top of it. Identifying the underlying cause of regional displacement patterns can be challenging and different hypotheses should be tested. In the present study, we verified whether the observed slow, decelerating uplift in Brussels can be explained by tectonic processes or whether it is the result of human activity. Concerning the tectonic setting, Belgium is located in an intraplate zone and the seismic activity is considered as moderate [30; 31]. According to the records of historic earthquakes held by the Belgian Royal Observatory [32], the study area has not been affected by significant earthquakes during the last 25 years. Given the absence of any recent seismic activity, the hypothesis of induced tectonic uplift of Brussels can be safely rejected. Regarding the second hypothesis, many studies have demonstrated that extraction of groundwater can have major impacts on ground stability [1; 2; 3; 4; 5; 6; 7]. To get a clear view of the evolution of groundwater levels in the study area, piezometric data were harvested from the Brussels Environment Agency database and analyzed. Figure 7 shows the location of six selected piezometers providing continuous and reliable measurements from the main aquifers located in the studied area (Figure 7). The evolution of their groundwater levels through time are reported and subdivided into the different aquifers encountered for each piezometer site (Figure 8). Figure 6: Histograms based on the annual average line-of sight (LOS) velocities of the PS for ERS (**A**); Envisat (**B**); TerraSAR-X (**C**); Sentinel 1 (**D**). Figure 7: Location of the selected piezometers and simplified geological cross-section through the piezometers highlighting the different geological layers (from Quaternary to deep basement). The black rectangle of each piezometer corresponds to the deep extension of the strainer. Four of the selected piezometers are placed in the main aquifers below the thick aquitard of the Saint-Maur Member (Kortrijk Formation): piezometers Elis and Coca in the Paleozoic-Cretaceous aquifer, Coca in the Upper Paleocene aquifer (Hannut Formation) and Wash Express in the Lower Eocene (Kortrijk). Two piezometers are placed in the main aquifers above the Kortrijk Formation: PZ20 in the Middle Eocene aquifer (from Diest to Moen) and Car Wash 2000 in the Quaternary aquifer. A synthetic cross-section through the axis of the Senne River is presented at the Figure 7. The piezometers that are not exactly located in this axis were projected on this axis in order to simplify the understanding of the succession of aquifers/aquitards. No piezometers in perched aquifers were selected for this study as their importance in terms of areal extent and volume is limited. The evolution of the groundwater levels through time clearly shows a recharge of the deeper aquifers in Brussels since the early/mid-1990s. In the Elis piezometer the groundwater has risen by circa 30 m between 1995 and 2014. In the two Coca piezometers, rising groundwater levels were observed since the mid-1990s. Piezometer Wash Express shows a recharge in the Lower Eocene since 1992; after 2006 the recharge rate has reduced. In the two shallow aquifers no clear trends of rising groundwater can be observed. Piezometer PZ20 in the Brussels aquifer shows an initially rising groundwater level of circa 1 m between 1992 and 1994, which may be related to the end of a local groundwater extraction within the zone of influence around the piezometer.