Seismological evidence pertaining to the structure of the Earth's upper mantle

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Abstract
This review is concerned with the variation of compressional and shear‐wave velocities with depth in the earth's upper mantle. Information about the compressional velocities is obtained from a study of body waves of the P type. Most P‐wave studies have been limited to continental regions. These studies suggest that there is no pronounced shadow zone for P waves resulting from a low‐velocity channel in the upper mantle, except in tectonically active regions such as California‐Nevada, Japan, Andes, etc. Information about the shear velocities is obtained from studies of body waves of the S type and of Rayleigh and Love surface waves. Surface‐wave studies have been made for both continental and oceanic regions. The surface‐ and S‐wave studies suggest that there is an earth‐encircling, low‐velocity channel for shear waves in the upper mantle. The shear velocities in this channel are lower than average in oceanic regions, and higher than average in continental shield regions.