PKP differential travel times: Implications for 3‐D lower mantle structure

Differential PKP AB‐DF travel‐time measurements are compared directly with predicted residuals for mantle tomographic models to investigate the heterogeneity of the lowermost mantle. Much of the AB‐DF time is accumulated in the lowermost portions of the mantle, resulting in heavy weighting of 3‐D lowermost mantle structure on the differential travel‐time perturbations. Comparisons of the observations with tomographic models show strong model dependence. The best variance reductions are achieved using the recent P‐wave model of van der Hilst et al. [1997] (up to 20%), but the heterogeneity in the lowermost mantle may be 8 times larger than what shown by the tomographic model. The predictions for present S‐wave tomographic models correlate poorly with the observations if we assume P‐velocity anomalies are proportional to S‐velocity anomalies. We argue that S‐velocity anomalies in the lowermost mantle do not always track P‐velocity anomalies. Our study shows no obvious evidence of slab effect on the observed AB‐DF anomalies and, thus, the use of PKP phases is likely to provide a powerful tool in mapping the P‐velocity structure in the lowermost mantle.