Mapping of upper mantle structure from differential (PP — P) travel-time residuals

P and PP travel-time residuals have been collected from 23 quarters of the regional catalog of earthquakes of the International Seismological Centre (ISC). The difference in travel-time residuals between PP and P are used as an indicator of the P-delay time near the PP reflection point. Global maps of these differential travel-time residuals are used to study lateral heterogeneity in the crust and upper mantle. Two of the regions studied have mean differential travel-time residuals significantly different (at greater than the 95% confidence level) from the global mean of the differential travel-time residuals. In the central Pacific, southeast of Hawaii, differential PP — P travel-time residuals of ? 3.8 ± 0.36 s are observed. These observations imply an average P-delay time of ? 1.1 ± 0.38 s. Average P-delay times of 0.67 ± 0.20 s are observed beneath the Tibetan Plateau and the western portion of the People's Republic of China. These travel-time variations basically agree in location and sign with the surface wave results of Tanimoto and Anderson.