Low-velocity structure beneath Africa from forward modeling

• Published in: Earth and planetary science letters Vol. 170; no. 4; pp. 497 - 507
• Main Authors: Ni, Sidao, Ding, Xiaoming, Helmberger, Donald V, Gurnis, Michael
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 30.07.1999
• Subjects: core–mantle boundary; D″ layer; plumes; SKS waves; upwelling; D″ layer; core–mantle boundary; upwelling; plumes; SKS waves
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/S0012-821X(99)00121-1

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.