Abstract
EVIDENCE for heterogeneities at the core-mantle interface based on seismic observations of the position of the core shadow and scatter in body wave travel times at epicentral distances greater than 96° has been available for several years1. This, together with the variation in times of body waves reflected from the core, has been interpreted as due to irregularities on the shape of the core2. Phinney and Alexander3 interpreted spectral difference in long period diffracted P waves as due to vertical and lateral inhomogeneities in the lower mantle near the core. Waves diffracted underneath the Atlantic Ocean off Africa required no gradient but for some Pacific paths they found it necessary to introduce an average gradient of 0.2 km s−1 per 100 km in the lower mantle. They found an anomaly for waves diffracted under Hawaii but did not find a successful model.
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