Tomographic inversions using 1985 DELP Wakashio cruise explosive shot data for P-wave crustal structure beneath the Yamato Basin, Japan Sea, show 55% average variance reductions for each instrument and final overall rms travel time residuals of 0.07 s. Careful evaluation of model resolution and robustness maximized the volume of well-constrained velocities; anomalies up to 20 km from the shot lines were found. Velocity increases steadily to 6 km/s in the uppermost 2-3 km. Resolved portions from 3-8 km depth show more seismic heterogeneity, but are small in magnitude. The structure is consistent with that characterizing oceanic seismic layers 2 and 3, with layer 2 subparallel to the acoustic basement. The thickness of layer 3 is significantly larger, agreeing with thickening trends of layer 3 with increasing crustal thickness found at other oceanic locations. The observed crustal thickness, 4 km greater than predicted by passive upwelling, suggests the presence of an additional melt-producing mechanism, Convective partial melting at the onset of spreading, combined with rapid basin formation, can provide additional amounts of melt necessary for a thickened crust.