Mapping global D″ P velocities from ISC PcP-P differential travel times

Abstract

We analyze 78 793 differential PcP-P travel times reported by stations world-wide to the International Seismological Centre (ISC) during 1963–1987, to examine the seismic structure of the base of the mantle. Data were selected for earthquake-station pairs within a 30–85° distance range, and corrected for ellipticity and mantle path heterogeneity using the tomographic P-velocity model IASP3D-P of Vasco et al. (1994, J. Geophys. Res., 99: 13727–13755). The PcP-P paths sample a very small number of regions of the base of the mantle, limiting the amount of the lowermost mantle, D″, that could be examined. We find that the PcP-P differential times are on average 0.49s greater than the times predicted by the IASP91 model of Kennett and Engdahl (1991, Geophys. J. Int., 105: 429–465), suggesting that that the mean P velocity within a 300 km thick D″ may be 0.35% slower than IASP91 for the regions sampled by our data. We have attempted to create an image of some of the higher wavelength D″ heterogeneity to form a P-wave comparison for the ScS-S (and sScS-sS) study of Wysession et al. (1995a, J. Geophys. Res., 100: 8351). We have therefore used the similar technique of converting the travel time residuals into velocity anomalies in the lowermost 300 km of the mantle, and then stacking and smoothing them to create a geographical coherency plot for D″ lateral heterogeneity. There are five regions with data coverage good enough to allow inferences about D″ P-velocity structure relative to IASP91: (1) velocities at least 0.9–1.2% fast beneath Alaska; (2) velocities at least 0.5–1.0% slow beneath Hawaii; (3) velocities 0.65% fast beneath northern Asia, our region of greatest coverage; (4) a broad slow velocity region (up to −3.0%) beneath the western Pacific and eastern Asia; (5) a complicated region of fast velocity (+0.7%) surrounded by two slow velocity regions (−1.8% and −0.8%) within D″ beneath Central America. The transition from slow to fast D″ velocities moving from beneath the western Pacific Ocean toward Asia occurs in a different location for P velocities than for the S velocities of Wysession et al. (1995a).