Whole Mantle Vp/Vs Tomography

Interpreting seismic tomographic models in terms of geodynamics requires knowledge of the physical causes of velocity anomalies. A common approach is to compare VP and VS (or Vκ and Vμ) models though it is generally difficult to tell how much of the differences between VP and VS anomaly patterns originate from real differences in Earth's structure or from the difference in resolution and accuracy of the models. To circumvent this problem, we derive the three-dimensional distribution of the VP/VS ratio by directly inverting S-P differential travel times. This approach can bypass some of major difficulties associated with the comparison of independent VP and VS models. Use of S-P differential travel times is advantageous not only because they link directly VP to VS anomalies through common ray paths, but they also are little affected by the uncertainty of origin time or source time function. The resulting VP/VS tomographic model can be used to determine if the VP and VS anomalies in the lower mantle can be consistently interpreted as due to temperature anomalies or not. We converted the resultant VP and VP/VS perturbations to temperature perturbations dTP and dTS-P (= dTS-dTP), using mineral physics data. We find that lateral temperature variation can be the primary cause of the velocity anomalies through most of the lower mantle, though chemical heterogeneity is clearly needed in some regions near the base of the mantle.