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Published August 1979 | Published
Journal Article Open

Shear-wave velocity at the base of the mantle from profiles of diffracted SH waves


Profiles of SH waves diffracted around the core (Sd) for three deep events at stations across North America and the Atlantic (Δ = 92° to 152°) are used to determine the properties of the lower mantle in the vicinity of the core-mantle boundary (CMB). The S-wave velocity above the CMB is found to be β_c = 7.22 ± 0.1 km/sec, in agreement with gross earth models, but higher than previously reported values from direct measurements of Sd. The frequency imdependence of the Sd ray parameter argues strongly against the possibility of a low-velocity zone immediately above the core mantle boundary. We compute synthetic seismograms for Sd by summing normal modes. A comparison of the present data with a synthetic profile for earth model 1066A gives excellent agreement at periods greater than 45 seconds. Synthetics for other models are used to substantially constrain the possibility of significant rigidity of the uppermost layer of the core.

Additional Information

Copyright © 1979, by the Seismological Society of America. Manuscript received January 30, 1978. We thank Katsuyuki Abe, Don L. Anderson, John Cleary, Yoshio Fukao, Anton Hales, Don Helmberger, Hiroo Kanamori, and Seth Stein for discussion. The normal mode eigenfunctions for model 1066A were kindly made available by Freeman Gilbert and Ray Buland. This work was supported at Caltech by National Science Foundation Grant EAR 77-14675 and at Stanford by National Science Foundation Grant EAR78-03653.

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