Geoscience Reference
In-Depth Information
Table 20.2
Elastic coefficients of equivalent transversely isotropic models of
the upper mantle.
Olivine Model
Petrofabric
PREM
(1)
(2)
(3)
(4)
(5)
(6)
Mbar
A
2.416
2.052
2.290
2.208
2.251
2.176
C
2.265
3.141
2.202
2.365
2.151
2.044
F
0.752
0.696
0.721
0.724
0.860
0.831
L
0.659
0.723
0.770
0.790
0.655
0.663
N
0.824
0.623
0.784
0.746
0.708
0.661
km/s
V
PH
8.559
7.888
8.324
8.174
8.165
8.049
V
PV
8.285
9.757
8.166
8.460
7.982
7.800
V
SH
5.000
4.347
4.871
4.752
4.580
4.436
V
SV
4.470
4.684
4.828
4.889
4.404
4.441
Mbar/Mbar
1.250
0.861
1.018
0.944
1.081
0.997
ξ
0.937
1.531
0.961
1.071
0.956
0.939
φ
0.686
1.151
0.963
1.153
0.914
0.977
η
g/cm
3
3.298
3.298
3.305
3.305
3.377
3.360
ρ
(1) Olivine based model;
a
horizontal,
b
horizontal,
c
vertical (Nataf
et al.
,
1986
).
(2)
a
-vertical,
b
-horizontal,
c
-horizontal.
(3) Petrofabric model; horizontal flow (Montagner and Nataf, 1986).
(4) Petrofabric model; vertical flow.
(5) PREM, 60 km depth (Dziewonski and Anderson, 1981).
(6) PREM, 220 km depth.
symmetry plane, are faster than velocities per-
pendicular to the layers. No such restrictions
apply to the general case of crystals exhibit-
ing hexagonal symmetry or to aggregates com-
posed of crystals having preferred orientations.
In a laminated medium, with a vertical axis
of symmetry, the P and SH velocities decrease
monotonically from the horizontal to the verti-
cal. A cracked solid with flat aligned cracks, or
an asthenosphere with sills, behaves as a trans-
versely isotropic solid. If the oceanic lithosphere
is permeated with aligned dikes, it will behave
a hexagonal crystal, with a horizontal symmetry
axis.
Oriented cracks are important in crustal seis-
mic studies and indicate the direction of the pre-
vailing stress or a paleostress field. The orienta-
tion of the shear waves and their velocities will
be controlled by the orientation of the cracks.
The magnitude of the velocity difference will be
controlled by the crack density and the nature of
the pore fluid. Cracks may form and open up as
a result of tectonic stresses.
Global maps of transverse isotropy
as a function of depth
There are numerous studies of
upper mantle
shear wave velocity and anisotropy
.
Azimuthal
anisotropy
can
reach
10%
in
the
shallowest
mantle.
Polarization
anisotropy
up
