## 2 Seismicity Relocation  
### Data Acquisition and Earthquake Relocation  
Since the occurrence of the 2003 Lefkada M\({}_{\text{w}}\)6.3 main shock, continuous seismic monitoring was intensified in the area of the central Ionian Islands, namely Lefkada and Kefalonia, by firstly installing temporary broadband and short period digital seismological stations, then by upgrading this network into a permanent one [6] ([[http://geophysics.geo.auth.gr/ss/](http://geophysics.geo.auth.gr/ss/)]([http://geophysics.geo.auth.gr/ss/](http://geophysics.geo.auth.gr/ss/))) and finally incorporating the stations in the Hellenic Unified Seismological Network (HUSN). For the purpose of this study, we used some additional stations located in the mainland, at distances not exceeding 90 km, in order to reduce the azimuthal gap in the events location. In order to identify the activated structure of the northern end of the Kefalonia Transform Fault Zone (KTFZ), we compiled a catalog of 662 earthquakes, for the period February 2018-October 2019. The starting time of the catalog was decided to be a year before the main shock in order to study any pre-seismic patterns because it was observed that the seismic activity was considerable. The locations were made by manual picking of 3560 P- and 3197 S- phases, using 14 broadband and short-period stations of the HUSN. Phase pickings for the earthquakes with M \(\geq\) 2.5 were performed manually by the analysts of the Geophysics Department of the Aristotle University of Thessaloniki (GD-AUTh). For the smaller magnitude earthquakes, phase picking was held manually by the authors of the present work, as an ongoing procedure of more deeply understanding the seismotectonic and geodynamic properties of the central Ionian islands. Relocation was performed with the HYPOINVERSE [7] program using the 1D model (Table 1) along with the v\({}_{\text{P}}\)/v\({}_{\text{S}}\) ratio for this region suggested by [8], for the relocation of the 2015 Lefkada aftershock sequence. Station delays were calculated for further refining the locations, following a procedure described in [9], to account for lateral crustal variations that are not considered in a 1D model. The medians of the hypocentral errors derived by HYPOINVERSE are of the order of 2900m for the horizontal and 1500 m for the vertical dimension. \begin{table}
\begin{tabular}{c c} \hline \hline
**V\({}_{\text{P}}\) (km/s)** & **Depth (km)** \\ \hline
5.850 & 0.0 \\ \hline
5.870 & 1.0 \\ \hline
5.980 & 2.0 \\ \hline
6.635 & 6.0 \\ \hline
6.490 & 8.0 \\ \hline
6.525 & 9.0 \\ \hline
6.560 & 11.0 \\ \hline
6.580 & 13.0 \\ \hline
6.625 & 21.0 \\ \hline
6.700 & 28.0 \\ \hline
8.000 & 40.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: 1D velocity model used for the seismicity relocation in the present study. Further relocation took place by applying the double difference (DD) algorithm [10], taking into account both catalog and cross-correlation differential times and employing the conjugate gradients method (LSQR) [11] with appropriate damping. For the cross-correlation differential times, the time domain cross-correlation technique was realized [12; 13]. Waveforms with a duration of 60 s, starting at the origin time of each earthquake, were prepared and band pass-filtered at 2-10 Hz with a second order Butterworth filter. Cross-correlation was performed for a 1-s window length starting at the arrival of the P and S phases, searching over a lag of 1 s. Differential times with a correlation coefficient of 0.8 or more (CC \(\geq\) 0.8) were kept being used in hypoDD. In the application of the DD algorithm, five sets of iterations with five iterations each were performed. The cross-correlation differential times were down weighted by a factor of 100 for the first 10 iterations, to obtain locations from the catalog data [8; 14]. The catalog data was downweighted by the same factor for the remaining 15 iterations to allow cross-correlation differential times to better define the active structures.