A very slow basal layer underlying large-scale low-velocity anomalies in the lower mantle beneath the Pacific: evidence from core phases

Abstract

A multi-phase analysis using long-period World Wide Standardized Seismograph Network and Canadian Network data has been conducted using core-phases for deep focus events from the southwest Pacific. These include SKS, S2KS, SV diff, and SP dKS. The last phase emerges from SKS near 106° and is associated with a P-wave diffracting along the bottom of the mantle. Patterns in S2KS - SKS differential travel times ( T S2KS - SKS) correlate with those in SP dKS - SKS ( T SP dKS - SKS ). T S2KS - SKS values strongly depend on variations in V S structure in the lower third of the mantle, whereas T SP dKS - SKS values mainly depend on V P structure and variations in a thin zone (100 km or less) at the very base of the mantle. Anomalously large T S2KS - SKS and T SP dKS - SKS values (relative to the Preliminary Reference Earth Model (PREM)) are present for Fiji-Tonga and Kermadec events (recorded in North and South America), along with anomalously large SV diff amplitudes well into the core's shadow. More northerly paths beneath the Pacific to North America for Indonesian and Solomon events display both PREM-like and anomalous times. A model compatible with the observations is presented, and contains a thin very-low-velocity layer at the base of the mantle that underlies the large volumetric lower-mantle low-velocity regions in the southwest Pacific. A low-velocity layer of 20–100 km thickness with reductions of up to 5–10% (relative to PREM) can reproduce T SP dKS - SKS as well as SV diff amplitudes. Large-scale (more than 1000 km) lower-mantle V S heterogeneity (2–4%) can explain long-wavelength trends in T S2KS - SKS. The exact thickness and velocity reduction in the basal layer is uncertain, owing to difficulties in resolving whether anomalous structure occurs on the source- and/or receiver-side of wavepaths (at the CMB).