Geoscience Reference
In-Depth Information
the difference between vertically (SV) and horizontally (SH) polarized S-wave
velocities determined by tomography show flow directions in the upper mantle:
horizontal flow beneath the shields and vertical flow beneath mid-ocean ridges and
subduction zones. It has been suggested that the longstanding debate on how far
into the mantle the continental roots or keels extend (
200-250 km on geochem-
ical, thermal and isostatic evidence, but as much as 400 km from some seismic-
velocity models) may be, in part, reconciled when seismic anisotropy is taken into
account.
The lower mantle is generally isotropic, but the D
zone is locally anisotropic
for S-waves. Beneath Alaska and the Caribbean the D
zone is transversely
anisotropic, with SH faster than SV. This anisotropy could be due to the pres-
ence of a stack of thin horizontal layers in the upper part of D
or could arise
from hexagonal crystals with their symmetry axes aligned vertically. Beneath
the central Pacific the anisotropy is very variable but seems to be confined
to the lowermost levels of D
. Anisotropy in the D
zone could be caused by
structural laminations (perhaps oriented inclusions of partial melt or subducted
oceanic crust) or could result from a change in deformation in the boundary
layer relative to the lower mantle. Data are sparse, but there may be some cor-
relation between the form of anisotropy and the presence or absence of the
ULVZ.
The seismic velocity of the inner core is anisotropic with an amplitude of
2-4%. It has a cylindrical symmetry about an axis that is approximately aligned
(tilted at 8-11
◦
) with the Earth's north-south spin axis. In early 1996 the inner-
core symmetry axis was at 79
◦
N, 169
◦
E. The inner-core anisotropy has been
determined from measurements of the travel times of body waves: paths parallel
to the spin axis are fastest. Additionally, normal modes (Section 4.1.4), which
have significant energy in the inner core, undergo some splitting, indicating that
the inner core is anisotropic. The anisotropy is, however, not completely uniform -
while the Western hemisphere is strongly anisotropic, the outer half of part of
the Eastern hemisphere is only weakly anisotropic. The anisotropy is thought to
be caused by preferential alignment of the hexagonal close-packed (h.c.p) phase
of iron (Section 8.1.5)inthe inner core. The reason for the development of the
anisotropy is not understood, but it is possible that convective flow in the inner
core could preferentially align iron, just as flow in the mantle leads to alignment
of olivine. Another possibility is that shear forces due to the axially aligned
corkscrew-like magnetic-field lines (Fig. 8.25)may cause a preferential crystal
alignment in the inner core.
Repeated measurements of the difference in travel time between P-waves that
penetrate the inner core and those on close ray paths that only pass through the
outer core have shown that, over three decades, the position of the inner core's fast
axis has moved with respect to the crust and mantle. This movement is a rotation:
the inner core is rotating faster than the rest of the Earth. Estimates of the rate
of rotation are varied, but it is probable that the inner core is rotating relative to
∼
