yr.
We present a growth tectonic model of the Earth's
inner core and the resulting model of the seismic
anisotropy. The inner core grows anisotropically
if the convection in the outer core is of Taylor-column
type. The anisotropic growth produces a flow field of the
poloidal zonal order 2 type as a result of the isostatic
adjustment of the viscous inner core. Crystals in the
inner core align themselves under the stress field
produced by the flow. We model the anisotropic structure
of the inner core, using the theory of Kamb [1959]
and elastic constants of
Stixrude and Cohen [1995b].
We consider models for both hcp iron
and fcc iron, which are the probable crystal structures
for the inner core iron according to
Stixrude and Cohen [1995a].
We have found that the c-axis for hcp iron and [111]
direction for fcc iron
align in the polar direction. The alignment is consistent
with seismic observations, which have revealed that
the P wave velocity is
faster in the polar direction.
Our model predicts that the degree of the alignment
decreases near the inner core boundary
in accord with recent body wave observations.
The radial dependence of the alignment would result
from the following three effects;
(i) crystals near the surface have not undergone stressed
state long enough to acquire anisotropy after
precipitation.
(ii) stress near the surface is different from that in the
interior of the inner core due to shear stress free
boundary condition.
(iii) partially molten structure
results in transversely isotropic stress condition
near the inner core surface due to compaction.
Thus the application of Kamb's theory successfully
explains the seismic anisotropy in the inner core provided
that the crystals have been subjected under the same stress
condition for the time scale of the order of yr.
AGU Index Terms: 8115 Core processes; Mineralogy, Petrology, and Rock Chemistry; 5112 Microstructure; 7207 Core and mantle; 8125 Evolution of the Earth
Keywords/Free Terms: Inner core, anisotropy, anisotropic growth,
JGR-Solid Earth 96JB02700
Vol. 101
, No. B12
, p. 28,085