Extended Data Fig. 2: Seismic sections and travel-time ray tracing based on the forward model of OBSs 32 and 5 and forward P-wave velocity models.

a, A seismic section of the hydrophone component of OBS 32 in the across-axis profile. b, The recorded section with picked and calculated travel time overlaid. Red lines represent the predicted travel time. The coloured vertical bars represent the observed travel time in the same colour of rays in panel c. The size of the vertical bars indicates twice the uncertainty⁵². P2 and P3, refracted rays from oceanic layers 2 and 3, respectively; PmP, reflected rays at the Moho; Pn, turning rays in the upper mantle. c, A simulation of ray tracing using the final forward model. The dashed black lines represent the seabed, the sediment basement, the interface between oceanic layer 2 and layer 3 and the Moho discontinuity, from top to bottom. d–f, Seismic section of OBS 5. We select the two OBSs to show the good (OBS 32) and poor quality (OBS 5) of the raw OBS data. g,h, The forward seismic P-wave velocity structures. Thick black lines represent the seabed and sediment basement. Thin black lines indicate the contour of Vp in every 1 km s⁻¹. The dotted line represents the iso-velocity contour of 6.4 km s⁻¹. The thick dashed black lines show the Moho discontinuity, on which the sections constrained by PmP reflections are marked with thick white lines. The green rectangles along the interfaces represent the values of resolution for the velocity nodes of the top of layer 2 and the bottom of layer 3, and values greater than 0.5 are considered reliable.