Fig. 2

Initial velocity model and hypocenter distribution and grid configuration during tomographic inversions. a One-dimensional JMA2001 velocity model (Ueno et al. 2002) used for initial hypocenter determination and initial model for tomographic inversion. Bold and dashed lines show the velocities and V _p/V _s ratio of the model. b Relocated hypocenters based on the one-dimensional model shown in (a). Colored dots represent hypocenters. Colors indicate hypocenter depth. Black triangles and squares indicate station locations. Lower and right-hand panels show E–W and N–S vertical sections, respectively. PAC upper boundaries (Nakajima et al. 2009) are shown along the latitude of 35.4°N and longitude of 141°E, on the E–W and N–S vertical sections, respectively. c Configuration of grid nodes during tomographic inversions. Crosses represent locations of velocity nodes in horizontal section. Triangles and squares indicate the locations of stations. Black bold line shows the locations of refraction survey (Nakahigashi et al. 2012)