Fig. 1: Bathymetry map of the Northwest Pacific Ocean region based on a SRTM15 + V2.4 bathymetry grid65, showing the location of the four transects acquired during the 2018 and 2019 seismic experiments (Lines 1 and 2 Hawaiian Ridge; Lines 1 and 2 Emperor Seamounts).

The red dashed line shows the ‘trail’ along which the bathymetry profile in Figs. 3 and 6 have been constructed. Grey lines show the magnetic anomaly lineations used to construct the seafloor age grid13. Blue lines show the 83 and 121 Ma isochrons that define the boundaries of Cretaceous Normal Polarity oceanic crust. Bold numbers with black filled circles show approximate surface sample ages in Ma79,80,81,82,83,84. The inset shows a subset of the profiles (at 20 km spacing for better visualization) along which the gravity anomaly has been calculated and compared to observations. Thick purple lines locate the profiles shown in Fig. 2.