(1) IPGP, 4 Place Jussieu, Paris cedex 05, 75252 France
ph 33 1 4427 4895, fax 33 1 4427 3894 , jepm@ccr.jussieu.fr
poster/oral:
Reference Earth Models can be retrieved from either body wave travel times or from normal mode eigenperiods. However there is a large discrepancy between reference Earth models, which arise from the kind of dataset used in their construction, and partly from differences in parameterization. Reference models derived from body wave observations do not give access to density, attenuation factor and radial anisotropy. Conversely, reference models derived from normal modes cannot provide the correct location for the depth of seismic discontinuities, nor the associated velocity jump. Eigenperiods constructed using body wave reference models together with classical attenuation models differ significantly from the observed eigenperiods.
The body wave and normal mode approaches can be reconciled. The VP and VS velocities given by body wave models are considered as constraints and an inversion is performed for parameters which cannot be extracted from body waves in the context of a radial anisotropic model, i.e., the density
, the quality factor
,
and the anisotropic parameters 
,
and 
.
The influence of anelasticity is very large, though insufficient by
itself to reconcile the two types of models. But by including in the inversion procedure
the density and the three anisotropic parameters, body wave models can be brought into
complete agreement
with eigenperiod data. A number of different reference models
derived from body waves
were tested and used as starting models: iasp91, SP6 and
two new models ak303
and ak135. A number of robust features can be extracted from the inversions of these
different models.
The quality factor is found to be much larger
in the lower mantle
than in previous models (e.g. prem).
The anisotropy, in the form of transverse anisotropy
with a vertical symmetry axis, is significant in the whole upper mantle,
but very small in the lower mantle, except in the lower transition
zone (between the 660km discontinuity and 1000km), and in the D"-layer.
Compared with prem there is an increase of density in D"-layer and a decrease in the
lower transition zone.
The attenuation estimates were derived using velocity
dispersion information but are in agreement
with available direct measurements of normal mode attenuation.
Such attenuation data are still of limited quality
and the present results emphasize the need for improved attenuation measurements.