Composite images obtained through 2-D-GRT inversion for 48-high quality events (see Fig. 4 and Table 6 for details). All images were computed based on a 60° projection angle with respect to north shown on Fig. 1. The left images show perturbations in P-wave velocity δα/α and the images on the right show perturbations in S-wave velocity δβ/β. The colour shading indicates the magnitude of the velocity perturbations in per cent (see colour bar for scale), with blue and red representing fast and slow velocity perturbations, respectively. The top panels show only the raw images, while the bottom panels additionally contain our structural interpretations.