Fig. 7

Results of the tomographic inversion along the X-axis. The black triangles and squares in each panel indicate the stations within 10 km of each section. The white and red dots indicate all faulting earthquakes and low-angle thrust-faulting earthquakes within 10 km of each section, respectively. The brown line indicates the depths of the upper boundaries of the PAC (Nakajima et al. 2009). PaM, NaC, NaM, PhC, and PhM indicate the crustal (C) and mantle (M) parts of the three plates: PAC (Pa), NA (Na), and PHS (Ph). The areas surrounded by white lines indicate low-velocity regions in the PHS. The magenta dashed lines on the sections north of Y = 40 km indicate the smoothed iso-velocity contour of Vp = 7.0 km/s in the NA. The black bold and dashed lines on the sections south of Y = − 40 km represent the iso-velocity contours of Vp = 7.5 km/s in the PHS and the top of the crust of the PHS, respectively (see the text for details). The red circles with bold black outlines indicate the low-angle thrust-faulting earthquakes used to estimate the upper boundary of the PHS. The bold red lines indicate the newly proposed upper boundary of the PHS. The black and light blue lateral bars at the top of each cross-section indicate slow slip areas with slips larger than 5 cm in the 2007, 2011, and 2014 SSEs (Hirose et al. 2014; Sato et al. 2017) and larger 1 cm in the 2014 SSEs (Sato et al. 2017)