Fig. 2: Waveform alignments of S-wave phases of doublet AI-0009 at other seismic stations except IU.SDV.

a Great-circle ray paths (gray lines) from doublet AI-0009 (star) to seismic stations (triangles labeled with station names; seismic stations from the CI network are marked collectively as “CI….” due to their close locations), with those sampling the lowermost 380 km of the mantle marked by thick lines (green for IU.SDV path and black for the other paths) and the SKS entry and exit points at the core-mantle boundary (CMB) marked by dots (orange for IU.SDV path and black for the other paths). The regions of detection and non-detection of ultra-low velocity zones (ULVZs) at the base of the CMB³⁵ are shown as shaded and light blue patches, respectively. b Superimposed S-wave waveforms of doublet AI-0009 after corrections for the effect of relative doublet location and origin time, with the radial and transverse components marked on the left top as R comp. and T comp. respectively, and station names and epicentral distances marked on the left. Waveforms are bandpass filtered from 0.4 Hz to 2.0 Hz and are aligned along the theoretical travel times based on the Preliminary reference Earth model⁴⁴ (PREM) (Time = 0 s). Because of the data quality, the waveforms with their station names marked with an asterisk are further filtered from 0.5 Hz to 2.0 Hz.