## 3 Materials and Methods
### GNSS Continuous Monitoring Data and GPS Benchmark Surveying  
A small network of two benchmark stations was established in 2006 in Methana Peninsula (Figure 1) [54]. These two stations were part of a larger GPS network established in the north-western part of the Hellenic Volcanic Arc (including the island of Aegina and the volcanic area of Susaki) aiming to study the ground deformation of this area. The stations in Methana were located in the northern (i.e., station MENO) and southern (MESO) parts of the peninsula and were reoccupied several times up to December 2019. All measurements took place approximately within the same period of the year (usually late October to December) in an effort to minimize seasonal phenomena. The Methana benchmark network was occupied using Leica receivers (SR9500 and AX1200). For each measurement period, the two benchmark stations were occupied for a period of 48 to 60 h, with time sampling of 15 s. Such time sampling was used for local-scale solutions with nearby continuous GNSS stations for which data were available with the same time sampling. While on regional-scale solutions, the data were resampled to 30-s data sets in order to be processed together with the freely distributed 30-s GNSS data available for European stations. In early 2015, a continuous GNSS (cGNSS) station was also established in the Lygourio (LYGO) close to Methana peninsula as part of the HxGN SmartNet ([[https://gr.nrtk.eu/](https://gr.nrtk.eu/)]([https://gr.nrtk.eu/](https://gr.nrtk.eu/))), that, ever since, has acted as local reference station. In addition, since August 2019, cGNSS data were available from a station (MTNA) located in the western part of Methana peninsula as part of the GNSS network of the Institute of Geodynamics, National Observatory of Athens (NOA). Horizontal velocity results from two more cGNSS stations were also available from the National Technical University of Athens (NTUA), i.e., station METH installed in 2004 in the southern sector of Methana peninsula, and from [55], i.e., station 010A located to the south-east of Methana peninsula, at Poros. The available 30-s data from the two cGNSS stations (i.e., LYGO and MTNA) that were included in this study were processed separately on a daily basis together with several other cGNSS stations located in Greece as part of a regional network. From the time series of the station's coordinates, the cGNSS station velocity was estimated in the IGb08 reference frame (see Figures A1 and A2 in Appendix A). The GPS data were processed using BERNESE software (v.5.2, University of Bern, Bern, Switzerland) [56]. The software allows for the estimation of epoch-wise receiver coordinates in Precise Point Positioning (PPP) mode as well as in the double difference mode. For the present research, the coordinates were calculated on a static mode. To get the best results, the PPP technique was used as a first step to get a priori coordinate values that were consequently introduced in the more precise double difference method. The GPS data from the benchmark stations and the other cGNSS stations in the broader area of Methana were processed together with several other EUropean REference Frame (EUREF) and cGNSS stations from Europe and Greece ([[http://www.epncb.oma.be](http://www.epncb.oma.be)]([http://www.epncb.oma.be](http://www.epncb.oma.be))) in order to define the local reference frame and to calculate the station coordinates. All measurement periods were processed with respect to the IGb08 reference frame ([[http://igscb.jpl.nasa.gov/network/refframe.html](http://igscb.jpl.nasa.gov/network/refframe.html)]([http://igscb.jpl.nasa.gov/network/refframe.html](http://igscb.jpl.nasa.gov/network/refframe.html))). Absolute antenna phase centre corrections were used in the data analysis. Several supporting files were introduced during the processing steps, aiming to define precise coordinates for the stations.