Fig. 4

a Horizontal projection of the current water content distribution on the plane 2 km vertically above the plate boundary obtained in this study, for which the ultramafic rock phase diagram was applied. The water content distribution is shown only in the region where the upper surface of the oceanic plate is shallower than the bottom of the model (depth of 200 km) and where the temperature is higher than 200 °C, for which phase diagram data exist. The thin black box represents the horizontal projection of the model region. The two black dashed lines (e) and (f) denote the profiles along the current subduction direction passing through the positions \((x, y)\) = (5 km, 200 km) and (5 km, -510 km), respectively. The black open triangles are volcanoes. The map was created by using Generic Mapping Tools (GMT) (Wessel and Smith 2016). b Same as (a), with the exception of the plate boundary, for which the turbidite phase diagram was applied. c Same as (a), with the exception of the plane at a depth of 4 km from the plate boundary, for which the MORB phase diagram was applied. d Same as (a), with the exception of the plane at a depth of 9 km from the plate boundary, for which the ultramafic rock phase diagram was applied. e Depth distribution of the water content in the slab in the vertical cross-section along the current subduction direction (e) in (a), passing through the position \((x, y)\) = (5 km, 200 km). The two solid black lines indicate the upper and lower surfaces of the oceanic plate. The red solid triangles indicate volcanoes within a one-sided width of 30 km. f Same as (e), with the exception of the profile (f) in (a), passing through the position \((x, y)\) = (5 km, -510 km)