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

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, SVd_(iff), 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_(SPdKS-SKS)). T_(S2KS-SKS) values strongly depend on variations in V_S structure in the lower third of the mantle, whereas T_(SPdKS-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_(SPdKS-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_(SPdKS-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)