Inhomogeneities of the Eurasian mantle structure from the traveltimes of the nuclear explosions recorded by the Finnish seismic network during 1961-1985

Abstract

SUMMARY During 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 VP/VS 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.