ON THE PRESENCE OF INTERMEDIATE-SCALE HETEROGENEITY IN THE UPPER-MANTLE

Power spectra of tomographic models of the entire earth and of models of Europe and the Mediterranean area are compared. This comparison rev eals a discrepancy (a factor 15-30) between the power of both kinds of models at intermediate spherical-harmonic degrees (around l = 30). Tw o possible explanations for this discrepancy are given: first, the fac t 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 multip athing observed in the recordings of these waves, which manifests itse lf as an increasing complexity of their amplitude spectra due to incre asing interference of spectral components with increasing epicentral d istance, cannot be explained by present-day global earth models. The g radients in the models are simply not strong enough. However, random m odels with more power at intermediate scales, and consequently stronge r gradients, are able to explain the observed multipathing. It is conc luded that the global models underestimate the power in the heterogene ities of intermediate-scale structure in the earth. Finally, an explan ation is given why current global models, although they do not contain sufficient power at shorter scales, do explain the traveltime and dis persion data used for the construction of the models.