Geoscience Reference
In-Depth Information
Figure 9.57. The geoid
height anomaly as
measured by the Geos 3
satellite altimeter, free-air
gravity anomaly and
bathymetry along a
profile perpendicular to
the Aleutian Trench.
(From Chapman and
Talwani (1979).)
20
GEOS 3 ALTIMETER
10
0
10
20
200
100
0
0
GRAVITY
100
1
200
2
100 km
3
4
5
6
7
NW
SE
Aleutian Trench
9.6.5 Seismic structure of subduction zones
Earthquake data and seismic-refraction and -reflection profiling are all used to
determine the seismic-velocity structure around subduction zones. The large-
scale and deep structures are determined from earthquake data. The subducting
plate, being a cold, rigid, high-density slab, is a high-velocity zone with P- and S-
wave velocities about 5%-10% higher than those in normal mantle material at the
same depth. The asthenosphere above the subducting plate, which is associated
with convection and back-arc spreading in the marginal basin, is a region with
low seismic velocities. Evidence from seismic modelling of deep earthquakes
has revealed that the subducting plate may penetrate into the lower mantle as
an anomalous high-velocity body, reaching depths of at least 1000 km. Figure
9.60 shows the deviations in velocity beneath three Pacific subduction zones:
it is clear that there is no standard structure; some plates extend into the lower
mantle, others do not. The central Izu-Bonin arc appears to be deflected at 670 km
depth and extends horizontally beneath the Philippine Sea as far as the Ryukuyu
subduction zone. However, to the south beneath the Mariana subduction zone the
high-velocity Pacific slab extends to about 1200 km depth.
The crustal seismic structure across Japan and the Japan Sea is shown in
Fig. 9.58(c). Japan has a 30-km-thick continental type of crust whereas the crust
in the marginal basin is only 8-9 km thick with velocities near those of oceanic
crust. Normal upper-mantle velocities are found beneath the oceanic plate, the
Search WWH ::




Custom Search