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Published July 30, 1999 | public
Journal Article

Low-velocity structure beneath Africa from forward modeling

Abstract

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.

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]

Additional details

Created:
August 22, 2023
Modified:
October 20, 2023