PKP DIFFERENTIAL TRAVEL-TIMES - IMPLICATIONS FOR 3-D LOWER MANTLE STRUCTURE

Differential PKP AB - DF travel-time measurements are compared directl y with predicted residuals for mantle tomographic models to investigat e the heterogeneity of the lowermost mantle. Much of the AB - DF time is accumulated in the lowermost portions of the mantle, resulting in h eavy weighting of 3-D lowermost mantle structure on the differential t ravel-time perturbations. Comparisons of the observations with tomogra phic models show strong model dependence. The best variance reductions are achieved using the recent P-wave model of van der Hilst et al. [1 997] (up to 20%), but the heterogeneity in the lowermost mantle may be 8 times larger than what shown by the tomographic models correlate po orly with the observations if we assume P-velocity anomalies are propo rtional to S-velocity anomalies. We argue that S-velocity anomalies in the lowermost mantle do not always track P-velocity anomalies. Our st udy shows no obvious evidence of slab effect on the observed AB - DF a nomalies and, thus, the use of PKP phases is likely to provide a power ful tool in mapping the P-velocity structure in the lowermost mantle.