INHOMOGENEITIES OF THE EURASIAN MANTLE STRUCTURE FROM THE TRAVEL-TIMES OF THE NUCLEAR-EXPLOSIONS RECORDED BY THE FINNISH SEISMIC NETWORK DURING 1961-1985

During the period 1961-1985, hundreds of nuclear explosions from the a rea of the Soviet Union and China were well recorded at Finnish seismo logical stations. The seismic waves recorded on the Baltic (Fennoscand ian) shield penetrated through the mantle of the Siberian platform, th e Ural Mountains and the East European platform. From the known crusta l models, the functions describing the increase of the average velocit ies of sedimentary cover and crystalline complex of the crust with thi ckness were found. The corrections for sedimentary cover and crustal t hickness were included. The large amount of data permitted the calcula tion of traveltimes of P waves for three sectors up to distances of ab out 5000 km. In all mantle models, the boundaries '400' and '700' km w ere found. Comparison of the results shows a difference in the travelt imes of the order of 5 s at a distance of about 4000 km, which reflect s the mantle structure differentiation for depths greater than 700 km (lowest velocities for latitudinal direction and highest velocities fo r longitudinal direction). The average S-wave velocity model of the ma ntle was obtained using the traveltimes of S-wave first arrivals. High values of the V-P/V-S ratio were found in the depth interval 200-400 km, while in other depth intervals they were close to 1.73. Our 1-D mo dels are compared with and discussed in connection with other models o f the East European and Siberian platforms as well as with global tomo graphic solutions.