As a supplement to seismic profiling surveys, crustal thicknesses have been estimated for 1 1 Fennoscandian seismograph stations equipped with three-component long period instruments, using the so-called spectral ratio technique of Phinney. The largest Moho depths, of the order of 45 km, were found for stations located in the north-east areas of Norway and Sweden and in Finland, with a local maximum in the Bothnian Bay. The coastal area of south-east Norway and Zealand, Denmark exhibit crustal thicknesses in the range 28-33 km. The agreement between our results and those obtained by conventional refraction profiling is good, when this comparison is restricted to profiles of lengths 300 km or more, and when the associated crustal thickness estimate is averaged over the central parts of the profiles in question. Also, a comparison between our results and other available geophysical information gives that the oldest tectonic provinces of the Baltic Shield also are characterized by relatively modest heat flow, and exhibit the greatest crustal thicknesses. Post-glacial uplift data and large wavelength free air gravity data appear to be uncorrelated with crustal thickness. The same partly applies to Bouguer gravity anomalies, thus implying that the isostatic compensation mechanism in Fennoscandia is of both Airy and Pratt type.hboduction For a long time there has been and still is a considerable interest among geophysicists in Pennoscandia aimed at detailed mapping of local crustal structures. In this respect several Seismic profiling surveys have been undertaken, and the corresponding data and results have been repeatedly reinterpreted in terms of two, three and even four-layer crustal models. The various profiling surveys have also been summarized repeatedly (

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S U M M A R YDuring the period 1961-1985, hundreds of nuclear explosions from the area of the Soviet Union and China were well recorded at Finnish seismological stations. The seismic waves recorded on the Baltic (Fennoscandian) shield penetrated through the mantle of the Siberian platform, the Ural Mountains and the East European platform. From the known crustal models, the functions describing the increase of the average velocities of sedimentary cover and crystalline complex of the crust with thickness were found. The corrections for sedimentary cover and crustal thickness were included. The large amount of data permitted the calculation of traveltimes of P waves for three sectors up to distances of about 5000 km. In all mantle models, the boundaries '400' and '700'km were found. Comparison of the results shows a difference in the traveltimes of the order of 5 s at a distance of about 4000 km, which reflects the mantle structure differentiation for depths greater than 700 km (lowest velocities for latitudinal direction and highest velocities for longitudinal direction). The average S-wave velocity model of the mantle was obtained using the traveltimes of S-wave first arrivals. High values of the V,/V, ratio were found in the depth interval 200-400 km, while in other depth intervals they were close to 1.73. Our 1-D models are compared with and discussed in connection with other models of the East European and Siberian platforms as well as with global tomographic solutions.

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