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.