Test of tomographic models of D″ using differential travel time data

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Abstract
We compare local measurements of SH‐velocity in D″ under the Pacific Ocean with four recent S‐velocity models derived with different techniques. From the local measurements, we find evidence for both strong fast and slow anomalies with an amplitude sometimes exceeding 10%, as well as strong lateral velocity gradients. The tomographic models underestimate the magnitude of the observed anomalies by roughly a factor of 3. The model that best matches both the sign and the localisation of the anomalous structures is exclusively an SH model. This indicates, in particular, that the presence of anisotropy in D″ may not be ignored and that it is important to separate SH and SV contributions in tomographic studies of the lowermost mantle.