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same scale as that of the checkerboard grid could be derived. We estimated Vp
accuracy using the differences between a given Vp perturbation (5% in this study)
and the result of the checkerboard test. If a Vp perturbation of 3% was obtained for
a Vp of 6-7 km/s, the accuracy is 0.10-0.15 km/s.
Description of Velocity Images
In this section, we introduce the Vp profiles of lines IBr9 and IBr10 and describe
the reliability of the Vp models. It is important for understanding the crustal char-
acteristics to specify the Moho. The major interfaces along the two profiles are the
Moho and the top of the subducting oceanic crust. For the Moho we selected a
continuous interface along contours between 7.5 km/s and 8.0 km/s, using a diffraction-
stack migrated image of the reflections on the Vp image.
Line IBr9
Line IBr9 runs from the Pacific plate to the Shikoku Basin. The four panels of
Fig. 3 show the final Vp image (Fig. 3a ), Moho reflections mapped onto the
Vp image (Fig. 3b ), the result of the checkerboard test (Fig. 3c ), and the 2-dimensional
volume of each crustal layer along the line (Fig. 3d ). This line crosses the Izu-
Ogasawara Trench, off the northern tip of Ogasawara Ridge (east of 390 km in
Fig. 3 ), the northern part of the Ogasawara Trough (310-390 km), the northern foot
of Nichiyo Seamount in the volcanic front (295 km), the Sofu Trough (160-
210 km), the southern foot of An'ei Seamount in the rear-arc (120-150 km), and
Kan'ei Seamount (90 km). This line crosses a volcanic front with relatively thin
crust as published by Kodaira et al. ( 2008 ).
The crust is thickest (18-19 km) beneath the volcanic front and the southern foot
of An'ei Seamount (Fig. 3d ). Crustal thickness decreases toward the fore-arc to
about 12 km beneath the northern tip of Ogasawara Ridge. Between the volcanic
front and the rear-arc, it is 15-17 km thick, with almost constant thickness beneath
the Sofu Trough. Beneath the Shikoku Basin, the crust thins to approximately 8 km,
similar to that of typical oceanic crust (White et al. 1992 ).
Upper crust with Vp of 5-6 km/s is about 3-4 km thick between the fore-arc and
the rear-arc. Middle crust with Vp of 6.0-6.5 km/s is thickest beneath the fore-arc
basin (380-390 km), the volcanic front (280-310 km), a seamount at the eastern edge
of the Sofu Trough (220-260 km), and the southern foot of An'ei Seamount in the
rear-arc (120-160 km), as shown by the red line in Fig. 3d . The middle crust is thin
beneath the western Ogasawara Trough, the Sofu Trough, and the Shikoku Basin.
Lower crust with Vp of 6.5-7.5 km/s is thick beneath the rear-arc and the volcanic
front. Although the thickness of the lower crust is almost constant from the rear-arc
to the volcanic front, Vp in this layer varies. In the lower crust beneath the rear-arc
(120-160 km) and the volcanic front (280-310 km), areas with Vp of 6.5-7.0 km/s
(green line in Fig. 3d ) are thicker than areas of 7.0-7.5 km/s (blue line in Fig. 3d ).
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