Global upper mantle structure from long-period differential travel times

Abstract

Over ten thousand measurements of PP-P and SS-S differential travel times have been made from long-period Global Digital Seismograph Network recordings of all events with mb ≥ 5.5 which occurred during the years 1976 to 1986. The residuals range from -5 to 5 s for PP-P and -12 to 12 s for SS-S and reveal a large-scale pattern of heterogeneity when plotted at the reflected phase's bounce point. Experiments indicate that lower-mantle structure and source-receiver structure can each contribute approximately ±0.5 s to the measured PP-P residuals so there is considerable signal to be explained. The pattern observed in the PP-P measurements is similar to the pattern observed in the SS-S measurements, with the SS-S residuals 2 to 4 times larger in magnitude. Comparisons of measured residuals to those predicted by mantle models are discussed. -from Authors