On the presence of intermediate-scale heterogeneity in the upper mantle

SUMMARY Power spectra of tomographic models of the entire earth and of models of Europe and the Mediterranean area are compared. This comparison reveals a discrepancy (a factor 15–30) between the power of both kinds of models at intermediate spherical-harmonic degrees (around l = 30). Two possible explanations for this discrepancy are given: first, the fact that Europe is more complex than the global average; and second, the lack of resolution in the inversions. Another approach to the problem of the power spectrum of heterogeneities of intermediate length scale is the study of multipathing of long-period surface waves. The multipathing observed in the recordings of these waves, which manifests itself as an increasing complexity of their amplitude spectra due to increasing interference of spectral components with increasing epicentral distance, cannot be explained by present-day global earth models. The gradients in the models are simply not strong enough. However, random models with more power at intermediate scales, and consequently stronger gradients, are able to explain the observed multipathing. It is concluded that the global models underestimate the power in the heterogeneities of intermediate-scale structure in the earth. Finally, an explanation is given why current global models, although they do not contain sufficient power at shorter scales, do explain the traveltime and dispersion data used for the construction of the models.