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

Garnero, Edward J. and Helmberger, Donald V. (1995) A very slow basal layer underlying large-scale low-velocity anomalies in the lower mantle beneath the Pacific: evidence from core phases. Physics of the Earth and Planetary Interiors, 91 (1-3). pp. 161-176. ISSN 0031-9201. doi:10.1016/0031-9201(95)03039-Y.

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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, 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).

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Additional Information:© 1995 Elsevier Science B.V. Received 1 December 1994; revision accepted 8 March 1995. We thank Jeroen Ritsema, Annie Souriau, John Vidale, and Michael Wysession for helpful reviews and comments, and Steve Grand for software, data, and helpful discussions. This research was supported by NSF Grant EAR-9316441. E.J.G. was also supported by an NSF EAR Post doctoral Fellowship. This paper is Contribution 271, Institute of Tectonics, and Contribution 5476, Division of Geological and Planetary Sciences, California Institute of Technology.
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NSF EAR Postdoctoral FellowshipUNSPECIFIED
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Caltech Division of Geological and Planetary Sciences5476
Issue or Number:1-3
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Official Citation:Edward J. Garnero, Donald V. Helmberger, A very slow basal layer underlying large-scale low-velocity anomalies in the lower mantle beneath the Pacific: evidence from core phases, Physics of the Earth and Planetary Interiors, Volume 91, Issues 1–3, September 1995, Pages 161-176, ISSN 0031-9201, 10.1016/0031-9201(95)03039-Y. (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:35823
Deposited By: Ruth Sustaita
Deposited On:05 Dec 2012 19:35
Last Modified:09 Nov 2021 23:17

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