Earthquake fault parameters and tectonics in Africa

Fault plane solutions of earthquakes in southern Africa indicate that the least compressive stress is oriented approximately E-W nearly parallel to that in the northern part of the rift system. Seismic moments, source dimensions, and stress drops were determined for eight earthquakes from body- and surface-wave spectra by using the theory of Brune (1970). Spectral estimates of these quantities for the 1966 earthquake in the Republic of Zaire agree well with those observed in the field. Relatively higher stress drops are found for events not associated with rift faulting. If higher stress drops indicate higher rock strength, these higher stress drops, with other geological and geophysical data, suggest that the northern part of the rift system is similar to ocean ridges and behaves as a plate boundary but that the southern part is different and is not a plate boundary. The tectonics associated with the northern part of the rift system appear to be extending southward.