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Seismic wave velocities and earth structure on the African continent

LAMONT-DOHERTY GEOLOGICAL OBSERVATORY COLUMBIA UNIVERSITY, PALISADES, NEW YORK 10964
DEPARTMENT OF GEOLOGY AND MINERALOGY UNIVERSITY OF MICHIGAN, ANN ARBOR, MICHIGAN 48104

Abstract

Phase and group velocities of seismic surface waves were determined for the African continent. Rayleigh wave phase velocities range from approximately 3.90 to 4.20 km/sec in the period range of 30.0 to 63.0 sec. Group velocities of fundamental mode Love and Rayleigh waves in the period range of 10.0 to 60.0 sec range from 3.35 to 4.15 km/sec and 3.00 to 3.85 km/sec respectively. As expected, for the shorter periods the velocities depend strongly upon regional structure. Group velocities of sedimentary Rayleigh waves and of higher mode Rayleigh waves have also been determined. Short period body-wave velocities were determined for this region as follows: P_n = 8.06 km/sec, S_n = 4.55-4.72 km/sec, and L_g = 3.48-3.60 km/sec. Due to insufficient observations, a P_g velocity was not determined. The observed surface wave velocities are similar to those reported for the Baltic Shield and differ somewhat from those velocities found for the Canadian Shield. A theoretical model that accounts for the observed velocities is derived for the African continent. Regional variations in S_n propagation indicated structural differences which may be explained by the existence of a gap in the mantle portion of the lithosphere beneath the northern part of the African rift zone.

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