Data Sheet 1_Imaging the shear deformation zone along the Kefalonia transform fault zone.docx

Using high-resolution relocated aftershock seismicity for these sequences (1154 earthquakes, M 1.4–6.1, and 1291 earthquakes, M 0.2–6.5, respectively) and following the approach of Perrin et al. (2021), we estimate local best-fitting fault planes in moving windows along strike using principal component analysis (PCA), measure hypocentral distances orthogonal to each plane, and stack the normalized fault-normal distance profiles to determine the characteristic inner and outer bounds of the active shear zone (W_S1 and W_S2).

A regularized power-law function is fitted to the stacked distribution, marking W_S1 at the distance of maximum curvature, and W_S2 at the distance where the distribution departs from the power law. The two sequences yield W_S1 values of 0.83 km for the Kefalonia and 0.50 km for the Lefkada sequences and W_S2 values of 1.88 km and 1.49 km, respectively. Assuming approximate symmetry around the local best-fitting plane, these correspond to full active shear-zone widths of about 3.8 and 3.0 km, respectively.