Coordinate stability is crucial for every RTK CORS network, especially when network- based RTK is supported. Network-based techniques like VRS, FKP and MAC model the error sources in order to eliminate the distance-dependent errors in relative GNSS geodetic positioning. The effective modelling of the error sources requires highly accurate coordinates of the reference stations. Tectonic movements should be considered, as they can lead to changes in the stations’ coordinates wi th respect to the coordinate reference frame in which they are expressed. Greece is situated on the boundaries of the Eurasian plate and has different tectonic characteristics compared to the majority o f European countries. Moreover, Greece is characterized by strong seismic activity. In certai n cases, earthquakes can cause abrupt changes in the station coordinates. The Hellenic Positioning System (HEPOS) consists of 98 permanent GPS reference stations distributed throughout Greece. During the first years of the operation of HEPOS, several strong earthquakes of different characteristics took place in certain areas of Greece. Data from the HEPOS stations are processed in order to detect possible permanent displacements induced by 8 different earthquakes. The paper presents the processing strategy and the obtained results.