The P wave velocity structure near the base of the mantle

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Abstract
Travel times from 16 events recorded at two temporary networks of short‐period seismometers deployed across Australia have been used to study the P wave velocity structure of the mantle below depths of 1700 km. The slowness‐distance curve derived from these times shows small deviations from the slownesses predicted from the preliminary reference earth model of Dziewonski and Anderson [1981] that may be significant. The most important feature is a rapid decrease of more than 0.5 s/degree between distances of 85° and 88°. Velocity filtering of seismograms from 5 nuclear explosions in Kazakh recorded at these Australian networks indicates the presence of two arrival branches at distances between 83° and 91° whose slownesses differ by 0.40 to 0.55 s/degree and which intersect at a distance of 87°. Processed P wave seismograms from the Yellowknife array in Canada also indicate the presence of a similar phenomenon in the same distance range in two separate regions. The results can be explained by a widespread rapid or sharp increase in velocity of 2.5 to 3.0% about 200 km above the core mantle boundary.