Regional analysis of D″ velocities from the ray parameters of diffracted P profiles

In examining the slownesses of linear profiles of diffracted P arrivals, we have discerned regional variations in the compressional velocity structure of D″, the base of the mantle. Three methods of determining the slownesses were tested, concluding that the waveform cross‐correlation (XC) technique is more reliable over a wide range of epicentral distances than methods involving a linear regression through either the first motion or first peak arrival times. XC slownesses suggest a statistically conservative range in D″ average velocities of 3.1%, and reveal regions of very fast velocities under Asia and the eastern Pacific basin and of very slow velocities under the New Guinea/Solomon Islands region. Slownesses were found to correlate well with the tomographic model V3 of Morelli and Dziewonski [1986], derived from a different technique and data set. Synthetic seismograms were generated using a summation of spheroidal normal modes, and at periods greater than 45 s the diffracted waves show much less variation and are very well modeled by PREM.