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Low-velocity structure beneath Africa from forward modeling

Ni, Sidao and Ding, Xiaoming and Helmberger, Donald V. and Gurnis, Michael (1999) Low-velocity structure beneath Africa from forward modeling. Earth and Planetary Science Letters, 170 (4). pp. 497-507. ISSN 0012-821X. doi:10.1016/S0012-821X(99)00121-1.

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Seismic waveforms observed in South Africa containing the first arrival crossover of S to SKS (70° to 110°) are analyzed. The data consist of analog records from the World Wide Seismographic Station Network (WWSSN) of deep events beneath South America. The S-waves arrive 2 to 3 s early relative to PREM at ranges from 70° to 95° and then become increasingly delayed, becoming 5 to 6 s late at 110°. The SKS phase is late by 3 to 5 s over the entire range. This pushes crossover between S and SKS, normally observed at about 81°, out about 2° to 3°, which is the most anomalous shift ever reported. To model such features, we modified Grand's tomography model [Grand et al., GSA Today 7 (1997) 1–7], and generated 2D synthetics to match the data. The overall shape and position of the lower mantle low-velocity anomaly proposed by Grand predicts good results if lower mantle anomalies are enhanced to a level of about 4%. This results in a complex tabular structure extending upward from the core–mantle boundary about 1500 km into the mantle. These features appear to be consistent with a large young plume which is erupting off the CMB.

Item Type:Article
Related URLs:
URLURL TypeDescription
Ni, Sidao0000-0003-2988-4850
Gurnis, Michael0000-0003-1704-597X
Additional Information:© 1999 Elsevier Science B.V. Received 24 December 1998; revised version received 24 May 1999; accepted 24 May 1999. This research has been supported by the National Science Foundation Grant Numbers EAR97-25808 and EAR96-29279. We thank Harmen Bijwaard and two anonymous reviewers for their helpful suggestions. Contribution Number 8626 of the Division of Geological and Planetary Sciences, California Institute of Technology. [RV]
Group:Seismological Laboratory
Funding AgencyGrant Number
NSFEAR 97-25808
NSFEAR 96-29279
Subject Keywords:core–mantle boundary; upwelling; plumes; D″ layer; SKS waves
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Other Numbering System NameOther Numbering System ID
Caltech Division of Geological and Planetary Sciences8626
Issue or Number:4
Record Number:CaltechAUTHORS:20121211-113907160
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Official Citation:Sidao Ni, Xiaoming Ding, Donald V Helmberger, Michael Gurnis, Low-velocity structure beneath Africa from forward modeling, Earth and Planetary Science Letters, Volume 170, Issue 4, 30 July 1999, Pages 497-507, ISSN 0012-821X, 10.1016/S0012-821X(99)00121-1. (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:35911
Deposited By: Ruth Sustaita
Deposited On:11 Dec 2012 21:16
Last Modified:09 Nov 2021 23:18

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