Selection of signals based on phase association and observation-synthetic correlation. Candidate time windows for the specified SH phases: (a) observation; (b) synthetics. Highly correlated (c.c. ≥0.8) time windows: (c) observation; (d) synthetics. Theoretical time–distance curves are superimposed as coloured dotted curves. Traces are aligned with the theoretical S/S_diff arrival time. All signals are normalized using eq. (5).