Tomographic inversion of S-SKS times for shear velocity heterogeneity in D″: Degree 12 and hybrid models

Abstract

Differential travel times between 5 (or diffracted S) and SKS were measured to study the global distribution of shear wave velocity heterogeneity in the lowermost 250 km of the mantle (the D″ region). Commencing with -3000 S-SKS times with variable qualities, we minimize uneven path coverage by thinning redundantly sampled regions (e.g., the Fiji-Tonga to North America corridor) and collect additional data for sparsely sampled areas, especially in the Southern Hemisphere. About 1500 paths were retained, distributed to reconcile both high-density and homogeneous sampling. We compare (1) spherical harmonic and (2) equal-area block parameterizations of D″ shear velocity heterogeneity for identical minimum resolving lengths and mean model errors. We show that the two parameterizations result in indistinguishable patterns of heterogeneity and power distribution for resolution down to a block size of 4.5° × 4.5° and maximum spherical harmonic degree of 40. We demonstrate synthetically that a high-degree (L) inversion followed by a lower-degree (Ls) spherical harmonic synthesis effectively circumvents model contamination from expansion truncation effects. Tomographic inversion of this data set yields a global distribution of D″ heterogeneity robust up to degree 12. In our preferred model (L=40, Ls=12, rms ∼1%), surface hotspots and estimated lower mantle plume roots are located in or at edges of low-velocity regions (δVs