Geoscience Reference
In-Depth Information
(a)
Figure 9.58.
Sections
across the Japan Trench
and Arc. Solid horizontal
bars denote land; ,
trench; , volcanic front.
(a) Topography, vertical
exaggeration 25 : 1. (b)
Free-air gravity anomaly.
(c) Crustal seismic P-wave
velocity (km s
−1
)
structure, vertical
exaggeration 10 : 1. (d)
Summary of the seismic
structure, true scale.
Shaded areas, seismically
active regions. Typical
earthquake focal
mechanisms are shown.
The low-
Q
, low-velocity
regions are the
asthenosphere beneath
the 30-km-thick overriding
plate and beneath the
80-km-thick subducting
Pacific plate. The
focal-depth distribution of
earthquakes beneath
Japan is shown in
Fig. 9.48. The thin
low-velocity layer at the
top of the subducting
plate is considered to be
the subducted oceanic
crust. (e) Heat-flow
measurements. The
dashed line is the
theorectical heat flow for
120-Ma oceanic
lithosphere, 120 Ma being
the age of the Pacific plate
beneath Japan. (After
Yoshii (1979), Matsuzawa
et al
.(1986) and van den
Beukel and Wortel (1986).)
W
E
Sea of Japan
NE Honshu
0
5
(b)
200
0
-200
(c)
0
5.9
6.6
6.7
5.8
7.0
8.0
8.1
20
6.7
6.6
8.1
8.1
7.5
40
1000
600
400
200
0
200
800
Distance from trench (km)
Intermediate v
Intermediate Q
(d)
0
Low v
Low Q
High v
High Q
100
Low v
Low v
Low Q
200
(e)
150
80
40
0
volcanic
line
trench
