Pm. Mathews et Ii. Shapiro, CONSTRAINTS ON DEEP-EARTH PROPERTIES FROM SPACE-GEODETIC DATA, Physics of the earth and planetary interiors, 92(1-2), 1995, pp. 99-107
The amplitude of the Earth's nutation driven by a given component of t
he tidal potential is governed primarily by three parameters pi which
are composites of a larger number of 'basic' Earth parameters (ellipti
cities, compliances, moments of inertia, etc., of the Earth and its co
re regions). We obtain estimates of the pi by least-squares fitting of
nutation amplitudes estimated from very long baseline interferometry
(VLBI) data to theoretical expressions based on an analytical formulat
ion of nutation theory which incorporates the role of the solid inner
core. We show how the estimates obtained, as well as the overall fit,
vary with the ellipticity assumed for the inner core, and examine how
the results are affected when otherwise unmodelled effects of ocean ti
des and mantle anelasticity are taken into account. Considering two an
elasticity models, we find that the fit obtained with the use of one o
f them is noticeably worse than if the other is used or if no anelasti
city correction is made. Independent of the corrections applied, the X
(2) Of the fit is found to be smallest if the ellipticity e(s) of the
solid inner core is taken as about half that of the Preliminary Refere
nce Earth Model (PREM). Independent estimates of the ellipticity ef of
the fluid core and other basic parameters on which the pi depend cann
ot be obtained from the estimates of the pi alone. Nevertheless, with
certain assumptions that are less restrictive than those hitherto empl
oyed in the literature, we find ef to be 5.0% higher than the PREM val
ue if the best-fit value is assigned to e,, and 4.7% higher if e(s) =
e(s(PREM)); these values for ep are in accord with the estimate of Gwi
nn et al. (J. Geophys. Res., 91: 4755-4765, 1986), and correspond to a
nonhydrostatic flattening of about 465 m and 435 m, respectively, of
the core-mantle boundary. Our parameter estimates have implications fo
r the value of the static part k(0) of the second-degree Love number k
which seem to be hard to reconcile with information from other source
s. Observational estimates of the amplitudes of the 18.6 year nutation
s are also found to be not satisfactorily matched with theoretical exp
ectations. A careful re-examination of data analysis and theory is nee
ded to resolve these problems.