Fig. 4: Detection of I-J in the synthetic and observed correlation wavefield.

a Synthetic correlogram estimated using PREM¹⁹. The inset marked by the black rectangle indicates the time and inter-receiver distance window predicted for I-J (highlighted by the dashed blue line near the blue arrow) used to calculate the slant stacks shown in (c) to (e). Negative amplitudes in the correlogram are in purple, and positive amplitudes are in green. b Waveforms of feature I-J (waveforms in dark yellow) extracted from the observed correlogram overlayed on the synthetics of the best-fitting model (waveforms in blue) calculated with an IC shear speed reduction of 3.4% relative to PREM values. Negative amplitudes of I-J are filled in dark yellow to facilitate the visual inspection of their arrival times. Note the prominence of the signals (in between the horizontal solid black lines at each interstation distance) in both observed and synthetic correlation waveforms. c–e Phase-weighted stacks of I-J detected in the observation (c), best-fitting model (d), and PREM (e). The orange circles on the slant stacks from (c) to (e) guide the comparison of features’ timing.