Although the presence of anisotropy in the inner core is generally
accepted, its
strength and depth dependence still vary considerably from model to
model. Our joint
inversions of normal-mode splitting-function coefficients and absolute
and
differential body-wave travel times have demonstrated that a
significant part of the
data can be explained by a simple model with the symmetry axis aligned
with the
Earth's rotation axis.
This model has ~1.8% anisotropy throughout the inner core and
predicts almost a linear dependence of travel-time residuals on
cos2 , where
is the angle the ray makes with the symmetry axis.
The simple anisotropy model fits the PKIKP data trend well at all
distance
ranges except
between 173° and 180° where the data exhibit strong
curvature as a
function of cos2.
Because the data from this distance range are most sensitive to the
deepest
300~km of the
Earth, one may expect the deviation in data to be an artifact of poor
sampling or
bias.
However, current data coverage is relatively good, and the curvature
is
consistently observed even if the data are divided into four
independent subsets,
suggesting that it is a robust global
feature unique to this distance range.
The parabolic
behavior of travel-time residuals at antipodal distances
becomes more evident when the
anisotropic signal associated with the upper part of the inner core
(between
300 and 1221~km radius) is removed.
When the best-fitting axis of symmetry is searched for this distance
range, we obtain
a tilted axis (77°N latitude and 30°W longitude) which
enhances the
parabolic nature of the travel-time residuals.
Anisotropy of the central inner core is
clearly different
from that inferred for the upper part of the inner core:
the inner-most inner core appears to be a seismically distinct region.
It has a strong
4 dependence,
in agreement with the experimental results of Mao et al.
(1998) for pure iron.
The existence of a seismically distinct IMIC has significant
consequences. The model
restricts development of anisotropy to mechanisms acting close to the
ICB. Mechanisms
involving the entire inner core, such as degree~one convection, would
preclude
distinct IMIC anisotropy. The model also suggests two distinct
episodes of inner core
evolution, presumably related to changes in core environment. For
example, at an
early stage of the Earth's history, the chemical composition of the
core may have been
different, or when the Earth differentiated, the core temperature and
pressure were
such that an inner core of a few hundred kilometer radius formed
relatively rapidly.
The second stage of inner-core evolution seems to be similar to today:
it is
characterized by an anisotropy which exhibits linear dependence on
cos2 with a
symmetry axis closely aligned with the rotation axis. The inferred
tilt of the IMIC
symmetry axis suggests that there was an event which misaligned
it from the rotation axis. The only evidence we have now of the
existence of IMIC is
its distinct anisotropy. The small size, corresponding to
~0.01% of the
Earth's volume, and its location, at the centre of the Earth, make it
inherently
difficult to observe.
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