Fig. 1: Bathymetry and marine gravity anomalies of the western part of the Pacific Plate.

Bathymetry (a) and marine gravity anomalies (b) are created using the data in ref. ⁷². Four petit-spot volcanic fields are here referred to as Sites A–C and SE of Minamitorishima Island; yellow squares indicate the areas shown in Fig. 3a, b (Sites A, C) and Fig. 3c (SE of Minamitorishima Island). Red ellipses filled with yellow indicate the approximate locations of petit-spot eruption estimated from the eruptive ages of the lavas and assuming constant plate motion at the present-day velocity (10.29 cm yr⁻¹; ref. ⁷³)^{1,10,14,31,32,41}; the two yellow ellipses filled with red atop the outer rise indicate DAP eruption sites. The thick dotted line in the left panel delineates the present hinge line of the concavely flexed outer rise of the western Pacific Plate trench-subduction systems, as defined by a gravity anomaly of ~0 mGal (right panel). Loading by seamount chains disturbs the positive gravity anomaly of the outer rise, as indicated by thin dotted lines. Small white arrows off NE Japan designate the azimuths of lithospheric structures on the oceanward slope of the trench that are sub-parallel to the neighboring Nosappu and Kashima Fracture Zones (‘F.Z.’)²⁹.