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Crustal thickness along line IBr10 varies between 5 and 20 km. The areas of thickest
crust, approximately 20 km and 18 km, are beneath the western Ogasawara Trough
and Ogasawara Ridge, respectively. Crustal thickness decreases gradually from 18 km
to 15 km from the volcanic front toward the rear-arc, except for basins between the
Sofu Trough and the Nishinoshima Trough. Beneath the Shikoku Basin, the crust is
thin, approximately 8 km. Crustal thickness increases toward Hakuho Seamount.
Upper crust with Vp of 5-6 km/s thickens from the unnamed seamount to the
Ogasawara Trough, and has a maximum thickness of 5 km. Middle crust with
Vp of 6.0-6.5 km/s (red line in Fig. 3d ) is found from beneath the rear-arc to
Ogasawara Ridge, and its thickness does not vary much. Lower crust has a relatively
constant thickness of 7 km between the rear-arc and the Ogasawara Trough; how-
ever, its velocities change laterally. The western Ogasawara Trough (410-430 km)
and Ogasawara Ridge (490-520 km) show relatively low velocities in the lower
crust and large areas with Vp of 6.5-7.0 km/s (green line in Fig. 5d ). The lower crust
between the unnamed seamount and the rear-arc has almost constant thickness of
5 km, with Vp of 6.5-7.0 km/s. On the other hand, the eastern Ogasawara Trough
(440-470 km), the volcanic front (390-400 km), the region between the rear-arc
and the Sofu Trough (200-280 km), and the eastern margin of the Shikoku Basin
(140-170 km) have large areas with Vp of 7.0-7.5 km/s (blue line in Fig. 5d ). The
uppermost mantle Vp is generally 7.8-8.0 km/s.
To construct the Vp image, we picked first arrivals for 19,743 traces and used them
for the tomographic inversion. Final traveltime residuals were approximately 60 ms
and those of each OBS were small (Fig. 6 ). The checkerboard test (Fig. 5c ) shows
Fig. 6 Comparison between observed and synthetic traveltimes for line IBr10. ( a ) Mean trav-
eltime residuals for each OBS. ( b ) Traveltime residuals for each shot at each OBS
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