Fig. 6

a Horizontal projection of the dehydration gradient distribution on the plane 2 km vertically above the upper surface of the slab at present (0 Ma) obtained in this study. The dehydration gradient distribution is shown only in the region, where the temperature is higher than 200 °C, for which phase diagram values exist. The thin black box is the model region. The two black dashed lines are the profiles E–E’ and F–F’ shown in e and f, respectively. The black open triangles are volcanoes. The encircled green solid line indicates the tectonic tremor-generating area. b Same as a except for the upper surface of the slab. c Same as a except for the plane at a depth of 4 km from the upper surface of the slab. d Same as a except for the plane at a depth of 9 km from the upper surface of the slab. e Distribution of dehydration gradient in the vertical cross section of the slab along the current subduction direction E–E’ of the NZ plate with respect to the SA plate. The two solid black lines indicate the upper and lower surfaces of the oceanic plate. The green open circles indicate tectonic tremors with residual travel time errors of less than 0.5 s from Ide (2012). Tectonic tremors are plotted only within a width of 10 km (one-sided width of 5 km) along the profile E–E’. The red solid triangles indicate volcanoes within a width of 50 km (one-sided width of 25 km) along the profile E–E’. f Same as e except for the profile along the current subduction direction F–F' of the AN plate with respect to the SA plate