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

Edward J. Garnero, Donald V. Helmberger

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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, SVdiff, and SPdKS. 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 (TS2KS - SKS) correlate with those in SPdKS - SKS (TSPdKS - SKS). TS2KS - SKS values strongly depend on variations in VS structure in the lower third of the mantle, whereas TSPdKS - SKS values mainly depend on VP structure and variations in a thin zone (100 km or less) at the very base of the mantle. Anomalously large TS2KS - SKS and TSPdKS - 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 SVdiff 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 TSPdKS - SKS as well as SVdiff amplitudes. Large-scale (more than 1000 km) lower-mantle VS heterogeneity (2-4%) can explain long-wavelength trends in TS2KS - 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).

Original languageEnglish (US)
Pages (from-to)161-176
Number of pages16
JournalPhysics of the Earth and Planetary Interiors
Issue number1-3
StatePublished - Sep 1995


ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Geophysics
  • Physics and Astronomy (miscellaneous)
  • Space and Planetary Science

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