P-Wave Velocities in the Upper Mantle Beneath Fennoscandia and Western Russia

A detailed and extensive record section constructed from recordings at the NORSAR array of presumed explosions in continental Russia exhibits two distinct (T, Δ) triplications. The reliable identification of these upper mantle travel-time branches is possible because of the dense areal sampling of the NORSAR configuration. A simple upper mantle P-velocity model which can account adequately for the data involves velocity discontinuities at depths of 420 km and 690 km, and fairly uniform velocity gradients elsewhere. For this model, the first arrival branch for Δ ≲ 21° extends as a second arrival to a distance of about 33°, at which distance it is terminated by the 420-km discontinuity. Rays bottoming between depths of 420 and 690 km span the distance range 16° ≲ Δ ≲ 28°, and give first arrivals in the range 21° ≲ Δ ≲ 24°. Rays which penetrate the 690-km discontinuity give rise to secondary arrivals in the range 19° ≲ Δ ≲ 25°, and first arrivals for distances Δ ≲ 25°.