Lateral variations near the core‐mantle boundary

Differential travel times between S and SKS (Ts‐sks), and SKKS and SKS (Tskks‐sks), along with amplitude behavior have been used to study lateral variations at the core‐mantle boundary. The data were recorded by the World Wide Seismographic Station Network (WWSSN) and the Canadian Seismograph Network (CSN) from source regions in South America and the Fiji‐Tonga area. A core‐mantle boundary region traversed by SKS and S beneath the mid‐Pacific from the Fiji‐Tonga area to North American stations shows the most anomalous behavior in Ts‐sks. SKS and S are anomalously separated by up to 8 seconds. In addition to Ts‐sks anomalies, SKS and SKKS are anomalously separated by 2 to 3 seconds. A model fitting Ts‐sks for this anomalous region requires one or more of the following: (1) up to 5% increase in the top 300 km of outer core velocity; (2) a large scale high Vs region (2% increase) in the lower mantle NE of Tonga; (3) lateral variations in D” shear wave velocities of up to 3%. However, (1) above predicts a decrease in Tskks‐sks, contradicting the observations. This suggests that (2) and (3) are more likely explanations in modeling this data set.