A seismic investigation of crustal structure in the Western Transvaal*

Abstract

Travel times are given as obtained from seismograms of the Witwatersrand earth tremors at distances up to 500 km. Five stations of several types were operated, which were moved as required. One method used continuous recording for a few hours daily; another used a triggering technique operated by radio from Johannesburg, where a network of six stations permitted the focus of each tremor to be located in time and place. Readings from the 200 seismograms obtained give velocities V₁ = 6.09 km/sec. and V_n = 8.27 km/sec. for the P₁ and P_n phases, and v₁ = 3.68 km/sec., v_n = 4.83 km/sec., for the corresponding S phases. Simply interpreted, these lead to a depth of 36 km. to the Mohorovičić discontinuity, including 5 km. of sediments confined to the epicentral region. There is evidence of a phase corresponding to an intermediate layer (the inclusion of which brings the total depth to 39 km.), but the data can also be interpreted to imply a regular increase of velocity with depth in the upper layer, as the presence of a local anomaly.