Constraints on the correlation ofP- andS-wave velocity heterogeneity in the mantle fromP,PP,PPPandPKPab traveltimes

Abstract

We investigate the correlation of large-scale P- and S-velocity heterogeneity in the mantle by determining how well 106,000 compressional P, PP, PPP, and PKPab traveltimes can be explained by S-wave velocity model S20RTS(scaled using a depth dependent factor) and by a model in which the lateral P-velocity variations are different. We first assess the assumption that P-wave traveltimes can be explained by a model in which lateral P-velocity variations (δν P ) are identical to S-velocity variations (δυ S ) in model S20RTS. For a given depth, we project δυ S from S20RTS into model S2P using a depth-dependent scaling factor R defined as: δυ S = R(z) x δυ P . We find, by grid search, that the highest reduction of data variance is obtained when R increases linearly from 1.25 at the surface to 3.0 at the core-mantle boundary. A comparison of S-wave (+SS) and P-wave (+PP) traveltimes for identical source-receiver pairs also indicates that R increases with depth. Significantly higher variance reduction is not obtained when R is parametrized with an additional degree of freedom. Therefore, the precise shape of R cannot be constrained by our data. P- and PP-wave traveltime anomalies with respect to the scaled model S2P yield coherent geographic variations. This indicates that there are large-scale lateral P-velocity variations in the mantle that are different from those in model S2P. We estimate these variations by inverting P-wave traveltime anomalies with respect to model S2P for a degree 12 model of P-velocity heterogeneity. This model, P 12 s 2 p , indicates where in the well-sampled mantle regions we need to modify model S2P to further improve the fit to the traveltime data. Anomalies in P12 s 2 p exist throughout the mantle. It is, therefore, not obvious that compositional heterogeneity is prominent in the lower 1000 km of the mantle only, as suggested previously. Low P-wave velocities in the upper mantle beneath oceans are the strongest anomalies in P12 s 2 p and explain better the delayed traveltimes of PP-wave phases with oceanic surface refection points. Lower mantle anomalies include high and low P-velocity structures beneath eastern Asia and North America, respectively. The high P-velocity anomaly in the lower mantle beneath the central Pacific is consistent with the assertion made by other researchers that large-scale lower mantle upwellings are not purely thermal in origin.