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P-wave velocity (km/s)
P-wave velocity (km/s)
P-wave velocity (km/s)
7.0
4.75
7.5
Depth = 40 km
8.0
8.5 7.0
7.5
8.0
8.5 7.0
7.5
8.0
8.5
Depth = 65 km
Depth = 90 km
4.50
4.25
1%
dInVs/dInVp
1%
dInVs/dInVp
1
1.5
2
1%
dInVs/dInVp
1
1.5
2
4.00
1
1.5
2
3.75
(a)
40 km
64 km
88 km
0
Δ
Vp (km/s)
1.5
0
Δ
Vp (km/s)
1.5
0
Δ
Vp (km/s)
1.5
(b)
Fig. 13.4 (a) shows V P versus V S in the mantle wedge beneath the NE Japan arc at depths of 40, 65, and 90 km
(Nakajima et al ., 2005). A white circle in each figure denotes the reference velocity, and hatched polygons represent
uncertainties of the reference velocity estimated in two different methods (see Fig. 2 of Nakajima et al ., (2005) for
more details). Reproduced with permission of Elsevier. (b) shows the predicted variations in V P versus V S at
depths of 40, 64, and 88 km based on the results of the numerical model (distribution of temperature, melt and
aqueous fluid) of Iwamori & Zhao (2000) and the theoretical relationship among φ , α , V P and V S (Takei, 2002). In
these regions no hydrous mineral is predicted to occur. V P and V S have been calculated assuming that the
predicted fluid fraction is partitioned into the cracks/dykes (90% of the total amount present in the local system)
and the grain boundaries (10%). It is noted that even if the partition proportion is changed, the predicted scatter in
V S /V P ratio does not significantly vary, although the respective range of V S and V P are modified. Reproduced
with permission of the American Geophysical Union.
technique, beneath the Japan arcs (Kawakatsu
& Watada, 2007). An abrupt decrease in shear
wave velocity ( V S )by
layer dehydrates at 150-200 km depth, and 0.1-
0.4 wt % H 2 O will subduct further by NAMs.
Beneath the adjacent Central Japan arc, where
the along-slab geothermal gradient is very small
due to overlapping subduction of the Pacific Plate
and the Philippine Sea Plate (Iwamori, 2000),
the HBL is observed as a low-velocity channel
atop the slab and continues down to as deep as
5-10% has been found
just above the subducting slab between 90 and
130 km depth, and is interpreted as a serpentinite
layer beneath the NE Japan arc. The arguments
in the previous section, as well as the geotherm
in Figure 13.1, suggest
that this serpentinite
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