Illuminating fault complexity of the Kefalonia Transform Fault Zone: A study of microseismicity in Lefkada Island under the influence of the 2015 MW6.5 earthquake

The central Ionian Islands area, including Kefalonia and Lefkada Islands constitutes an area with the highest seismic hazard in Greece. Lefkada Island has experienced two strong (M > 6.0) earthquakes in 2003 (M W 6.2) associated with a fault segment in the northwestern part of the Island, and in 2015 (M W 6.5), associated with the adjacent southern fault segment, both aligned along the western coastline. The close temporal proximity along with the ongoing high seismic activity motivated our investigation. Microseismic data were relocated over a period of more than 4 years, following the 2015 mainshock resulting in a comprehensive seismic catalog and the determination of 110 focal mechanisms for the stronger events. A density-based clustering algorithm (DBSCAN) was applied to the relocated hypocenters and focal mechanisms, enabling improved association of seismicity patterns with the kinematic properties of the fault zone. The effects of the 2015 mainshock on regional seismicity were assessed through coseismic static stress changes, calculated using a variable slip model derived from a joint inversion of regional and teleseismic seismic data, combined with geodetic displacement estimates. Stress inversion of the calculated focal mechanisms identified faults prone to instability, which were used as receiver faults in the stress change analysis. The results reveal that post-2015 seismicity was strongly influenced by the mainshock, with clusters aligning along the primary fault zone as well as secondary structures within a rather narrow faulting zone. These findings enhance understanding of the seismic behavior of the Kefalonia Transform Fault Zone and its secondary faults, providing critical insights for seismic hazard assessment in this tectonically active region. • Relocated microseismicity reveals active fault structures in Lefkada Island. • Clustering of earthquakes and focal mechanisms aids structural interpretation. • Joint inversion yields a variable slip model for the 2015 Mw6.5 mainshock. • Static stress changes explain post-seismic clusters along fault structures.