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six underground nuclear explosions in Novaya Zemlya shows notable early
arrivals (oo4sec) at the Fennoscandia stations. These observations, together
with a marked linear dependence of the travel times on the epicentral distance
throughout the interval 8.5 < / J ° < 20.4. were evaluated with the aim of producing
a satisfactory model for the upper mantle of the Baltic zone. In particular,
there appears a considerable mantle zone with almost uniform velocity, or at
least, with not of increasing velocity. This mantle zone is delimited above by a
discontinuity at the 86 km depth (v = 8.45 km/sec), and delimited below by the
«20° discontinuity» (h = 507 km; v = 10.56 km/sec).
The proposed model is amply verified on the basis of the foreseen travel
times compared with the corresponding ones, observed from a fair number of
impulses associated with teleseismic Pn, noticeable as far as 40°, and from phases
reflected and refracted on the discontinuity at 507 km.
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