Ambient Noise Tomography (Passive Seismic) to Image the Cape-Karoo Transition Near Jansenville, Eastern Cape

Abstract

During August and September 2015, a seismic network consisting of 17 stations was installed in the south-eastern Cape-Karoo region, near the town of Jansenville. Ambient seismic signals were continuously recorded for a three week period. In this chapter, we reconstruct estimates of the seismic Green’s functions between sensor pairs by cross-correlating the ambient seismic signals recorded in the vertical component of each station. The resulting Green’s functions contain clear direct Rayleigh wave arrivals. The measured group velocity dispersion curves of the Rayleigh waves were averaged in the period range from 2.5 to 5.5 s (approximately 2–5 km depth). The arrival times of the Rayleigh waves are picked at different periods and then inverted to compute 2-D group velocity maps. This resulted in a velocity model up to depths of 5 km. The results reveal two different velocity regions, broadly corresponding to the Cape Fold Belt and the flanking Karoo Basin. The higher group velocity anomalies (3–5 km in thickness) most likely represent the Carboniferous-Permian sequences of the Karoo Basin (Dwyka-Ecca-Beaufort Groups). A lowermost velocity region in the south-eastern study area could correspond to the Jurassic-Cretaceous sequences of the Algoa Basin that directly overlie the Cape Fold Belt.