A VERY SLOW BASAL LAYER UNDERLYING LARGE-SCALE LOW-VELOCITY ANOMALIESIN THE LOWER MANTLE BENEATH THE PACIFIC - EVIDENCE FROM CORE PHASES

A multi-phase analysis using long-period World Wide Standardized Seism ograph Network and Canadian Network data has been conducted using core -phases for deep focus events from the southwest Pacific. These includ e SKS, S2KS, SVdiff, and SPdKS. The last phase emerges from SKS near 1 06 degrees and is associated with a P-wave diffracting along the botto m of the mantle. Patterns in S2KS-SKS differential travel times (T-S2K S-SKS) correlate with those in SPdKS-SKS(T-SPdKS-SKS). T-S2KS-SKS valu es strongly depend on variations in V-s structure in the lower third o f the mantle, whereas T-SPdKS-SKS values mainly depend on V-p structur e and variations in a thin zone (100 km or less) at the very base of t he mantle. Anomalously large T-SPdKS-SKS and T-SPdKS-SKS values (relat ive to the Preliminary Reference Earth Model (PREM)) are present for F iji-Tonga and Kermadec events (recorded in North and South America), a long with anomalously large SVdiff amplitudes well into the core's sha dow. More northerly paths beneath the Pacific to North America for Ind onesian 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 southwe st Pacific. A low-velocity layer of 20-100 km thickness with reduction s of up to 5-10% (relative to PREM) can reproduce T-SPdKS-SKS as well as SVdiff 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 unc ertain, owing to difficulties in resolving whether anomalous structure occurs on the source- and/or receiver-side of wavepaths (at the CMB).