The asthenosphere low-velocity layer

Abstract

Results based on observations of various phenomena practically prove the existence of a world-wide low-velocity asthenosphere channel. Combination of new and earlier data on seismic wave velocities show that under continents the channel begins at the Mohorovicic discontinuity with velocities which are the smaller the greater the depth of the discontinuity. They are about 8.1 and 4.6 km/sec for longitudinal and transverse waves respectively, if the discontinuity is at a depth of 40 km. Longitudinal waves have a minimum velocity of about 7.8 km/sec near a depth of 80 km, transverse waves a minimum of about 4.4 km/sec near a depth of 150 km. The velocity which exists at a depth of 40 km under continents is reached again at a depth of about 190 km by longitudinal waves and at a depth near 270 km by transverse waves. The rate of decrease in velocity immediately below the discontinuity is in the average greater than the critical rate for formation of shadow zones. There is no indication of a discontinuity in the upper portion of the mantle. Poisson's ratio is nearly constant, 0.26, down to a depth of about 80 km, and increases in the asthenosphere channel to almost 0.29. Below the channel, it does not return to the value of 0.26 but remains nearly constant in most of the deeper portions of the mantle.

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