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