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Fig. 4 Comparison between observed and synthetic traveltimes for line IBr9. ( a ) Mean traveltime
residuals for each OBS. ( b ) Traveltime residuals for each shot at each OBS
On the other hand, lower crust beneath the western rear-arc (100-120 km), the Sofu
Trough and its eastern margin (160-260 km), the western Ogasawara Trough (310-
380 km), and off the northern tip of Ogasawara Ridge (400-420 km) has thicker areas
with Vp of 7.0-7.5 km/s (blue line in Fig. 3d ) than areas of 6.5-7.0 km/s (green line
in Fig. 3d ). Uppermost mantle velocities also vary, being slow (~7.5 km/s) beneath
the rear-arc, the volcanic front, and the western Ogasawara Trough. The velocity of
the uppermost mantle is also somewhat slow (7.6 km/s) beneath the northern tip of
Ogasawara Ridge. The Shikoku Basin has normal mantle Vp of 8.0 km/s.
We picked first arrivals for 87,224 traces and used these for the tomographic
inversion to model the crustal structure. Final traveltime residuals were approxi-
mately 25 ms and were quite small for each OBS (Fig. 4 ). According to the result
of the checkerboard test (Fig. 3c ), the resolved maximum depth ranges from 15 km
(beneath the Shikoku Basin) to 25 km (beneath the Ogasawara Trough). The
resolved area covers the entire region of the crust and uppermost mantle. The recov-
ery of the Vp perturbation was maximum 2-4% on the background velocity of
6.0-7.5 km/s, and the Vp accuracies were within 0.10-0.25 km/s.
Line IBr10
Line IBr10 is about 100 km south of line IBr9, running from the Pacific plate to the
Shikoku Basin. Figure 5 shows the final Vp image (Fig. 5a ), mapping of reflections
from the Moho on the Vp image (Fig. 5b ), the result of the checkerboard test (Fig. 5c ),
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