Extended Data Fig. 6: Seismic P-wave velocity models of the oceanic crust from the traced-normalised FWI.

A Gaussian smoothing operator (smoothing lengths of 2 km and 0.4 km in the horizontal and vertical directions, respectively) was applied for regularising the velocity update. (a) The velocity model from the trace-normalised FWI, (b) the velocity anomaly (the difference between the velocity models from the FWI and the tomography), and (c) the vertical velocity gradient (the derivative of velocity with respect to depth). The lithospheric age is calculated using a spreading rate of 16 mm/year²⁶. Black triangles in (a) mark the locations of OBS. The velocity contours in (a) are from 5 to 7 km/s with an increment of 0.2 km/s. The coloured parts in (b) and (c) start from the basement (the top of Layer 2). The horizontal distance starts with 0 km from the MAR.