Effect of velocity structure in D″ on PKP phases

Abstract

Differential travel times between various branches of PKP are used to study the P velocity structure in D″. Simple geometrical ray tests show that the ray path of the AB branch, which almost grazes the core‐mantle boundary (CMB), is much more sensitive to the velocity structure in D″ than the DF and BC branches, which have small incident angles at the CMB. Thus, the differential travel times of AB and DF and AB and BC provide an effective means of studying lateral variations in D″. In a systematic search of six years of the GDSN short period data on CD‐ROM, large anomalies in differential travel times of AB ‐ DF and AB ‐ BC were observed for different regions of D″. The scatter of BC ‐ DF data is half the scatter of AB ‐ DF and AB ‐ BC data, suggesting that the lowermost mantle has more influence on AB ‐ DF and AB ‐ BC data than the bottom of the outer core or the top of the inner core. The mean of the AB ‐ DF data set shows a 1 s offset relative to Preliminary Reference Earth Model (PREM) [Dziewonski and Anderson, 1981], in that (AB ‐ DF)prem is smaller than (AB ‐ DF)obs. Variations of the lower mantle PREM model that contain a discontinuity or a more distributed 1.5% velocity reduction in the lower 500 km can explain the discrepancy. The largest variations in the AB ‐ DF data are associated with paths sampling beneath the mid‐Pacific. The AB ‐ DF data along the mid‐Pacific paths are about 1.0 s smaller than the average of the AB ‐ DF from other paths and the differences are as large as 2.0 s at some distances.