## 3 Seismotectonic Setting, Data Selection, and Analysis
### GNSS Data Analysis  
The epicentral area of the 2015 Lefkas earthquake, located to the southern part of the island, is remarkably close to the local continuous GNSS station PONT [32]. This station has been operating in the area since 2007. The GNSS data are processed daily, using 30 s RINEX files, in order to calculate the daily coordinates of the station (Figure 4). From the time series of PONT station coordinates, the station velocity was estimated for the period 2009-2015. Prior to the 2015 strong event, the station exhibited the anticipated regional motion of the area, and the velocity was calculated as V\({}_{\text{East}}\) = (20.45 \(\pm\) 0.03) mm/yr, V\({}_{\text{North}}\) = (6.81 \(\pm\) 0.02) mm/yr, and V\({}_{\text{Up}}\) = (-0.71 \(\pm\) 0.08) mm/yr with respect to 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))) (accessed on 30 December 2020). The earthquake of 17 November 2015 caused a co-seismic displacement of the PONT station of 360 mm to south, 201 mm to the west, and a subsidence of 73 mm (Figure 4).  
Figure 4: Time series of PONT continuous GNSS stations located in Lefkas island. Red line indicates the date of the 2015 seismic event.  
The files from PONT station were processed using the Bernese Software version 5.2 [55]. The software allows the estimation of epoch wise receiver coordinates in precise point positioning mode (PPP), as well as in the double difference mode. For the present research, coordinates were calculated after the earthquake of 2015 on a static mode. To get the best results, the PPP technique was used as a first step to get a priori coordinate values, which were, consequently, introduced in the more precise double difference method. In the processing procedure, GNSS data from 25 IGS and EUREF cGNSS stations from Europe, Greece, Cyprus, and Turkey were used to define the local reference frame. Ambiguities were solved using several resolution strategies depending on the baseline length between the stations: the wide-lane and narrow-lane for the medium baseline (<10-200 km) and the quasi-ionosphere-free (QIF) strategy for long baselines (<1000-2000 km). For the troposphere modelling, the dry and wet Vienna mapping function (VMF) of [PERSON] et al. (2006), the dry and wet Niell model [56], as well as the Saastamoinen model [57] were the models used to analyze the GNSS microwave data in different stages of the processing. Effects of the solid earth tides were taken into account and corrections of the ocean tide loading were applied based on the FESS2004 model. In analogy to site displacements caused by ocean tide loading, the loading effect of the atmospheric tides were taken into account in the processing of the GNSS data. The latter was done by introducing a specific file in the processing containing atmospheric tidal loading coefficients from a global grid-based model [58]. Moreover, the absolute antenna phase center corrections were used in the processing and to better improve the final solutions files containing the precise orbital parameters; the Earth orientation parameters and the satellite clock corrections were taken from the Center for Orbit Determination in Europe (CODE) (DOI:10.7892/boris.75876.4). On a weekly basis, the calculated coordinates were evaluated for the repeatability error, and in case of large deviations from the weekly solution, the values were excluded. The latter resulted in an unweighted RMS error, in mm, for each component that varied from 0.5 to 1.01 in the east component, 0.8 to 2.1 in the north one, and 1.8 to 4.6 for the up component. However, that was not applied for the first ten days after the main event. The daily coordinates were estimated with respect to IGb08 reference frame.