Evaluating the ratio of S-wave to P-wave velocity variations for the Pacific LLVP based on long-period traveltime data

The large-low-velocity-provinces (LLVPs) are broad seismic anomalies in the lowermost mantle beneath Africa and the central Pacific Ocean. For these structures, the ratio R of S-wave and P-wave velocity perturbations appears to be higher than 2.5-3.0. This is one of several seismic characteristics used to infer that LLVPs are compositional distinct structures.

We evaluate the estimates of R for the Pacific LLVP based on traveltime delays of P-waves (ΔT_P) and S-waves (ΔT_S). We explore how well R can be resolved using ΔT_S/ΔT_P recorded in the same seismograms by using ray (RT) and finite-frequency theories (FF).

Our calculations indicate that RT predicts a higher ΔT_S/ΔT_P than for FF with a strong epicentral distance-dependence when R varies with depth. On the other hand, FF predicts that ΔT_S/ΔT_P varies only weakly with epicentral distance in a global dataset. This indicates that ΔT_S/ΔT_P based on long-period traveltime data is determined by the average value of R in the lower mantle and that the radial structure of R, particularly in D'', cannot be constrained from these data alone.

Waveform simulations show that the high ΔT_S/ΔT_P of the Pacific LLVP is also strongly affected by the velocity structure in the uppermost mantle. If R increases with depth in the mantle, ΔT_P due to the high-velocity lithosphere beneath eastern North America and ΔT_P due to the LLVP in the lower mantle are equally strong and of opposite sign, whereas the effect is small for ΔT_S. Consequently, the high ΔT_S/ΔT_P recorded for the Pacific LLVP can be explained without invoking an anomalously high R-value.