ALBERT

Notes: We present the results of global measurements of spheroidal and toroidal fundamental mode eigenfrequencies as well as fundamental spheroidal mode attenuation made from a data set consisting primarily of records from the Geoscope network. Data processing includes variable filtering and account is taken of higher order effects along the dispersion branch. Local frequency and attenuation maps are first obtained, as a function of angular order, by a regionalization scheme based on the introduction of a correlation length. In a second step, after correction for shallow-layer effects, shear velocity maps at different depths in the mantle are obtained by simultaneous inversion of spheroidal and toroidal local frequency maps. The results confirm the correlation of shear velocity with large-scale tectonic features in the first 200 km of the mantle. Below that depth, the maps are dominated by a degree-2 pattern whose power is peaked around a depth of 300 km, shallower than inferred in the first 3-D global studies of the upper mantle, and correlated with a degree 2 in attenuation, thus confirming the results of our earlier study of degree 2 in the upper mantle. We also report estimates of spherically averaged eigenfrequencies for fundamental spheroidal and toroidal mode branches.

Notes: Reference earth models can be retrieved from either body waves or normal-mode eigenperiods. However, there is a large discrepancy between different reference earth models, which arises partly from the type of data set used in their construction and partly from differences in parametrization. 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 locations for the depth of seismic discontinuities, nor the associated velocity jump. Eigenperiods derived from reference models constructed using body-wave data together with classifical attenuation models differ significantly from the observed eigenperiods.The body-wave and normal-mode approaches can be reconciled. The V' and V, velocities given by body-wave models are considered as constraints, and an inversion is performed for parameters that cannot be extracted from body waves in the context of a radially anisotropic model, i.e. the density p, the quality factor Q, and the anisotropy parameters 5, (b and q. The influence of anelasticity is very large, although insufficient by itself to reconcile the two types of model. However, 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 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 based on these different models. The quality factor Q, is found to be much larger in the lower mantle than in previous models (e.g. prern). Anisotropy, in the form of transverse isotropy 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 660 km discontinuity and 1000 km depth) and in the D'-layer. Compared with prem there is an increase of density in the D'-layer and a decrease in the lower transition zone. The attenuation estimates have been 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.