abstract

The source depths of 12 reservoir-induced seismic events at Lake Kariba were determined using the short- to long-period spectral ratios of multi-modal Rayleigh data recorded at two African stations. Source depths of less than 10 km were confirmed through the analysis of short-period pP-P time differentials. Two separate source-to-receiver crustal path models were employed in the analysis. An African shield model, with low-velocity surficial layers and a low-shear velocity zone between 7 and 18 km, was utilized to reproduce the observed multi-modal Rayleigh waves acquired from the Kariba to Nairobi path. The model is consistent with published evidence that the East African Rift System extends south to Lake Kariba. A conventional South African shield model was utilized for the Kariba to Pretoria path. Synthetic multi-modal Rayleigh waveforms were computed for a range of source depths for the two crustal models. The spectral ratios obtained from these waveforms provided the synthetic data base to which the observed Rayleigh wave spectral ratios were compared using linear discriminant analysis. Due to the limitations in the available data, the source depths could not be determined at a statistical level of confidence. However, the demonstration of the feasibility of the procedure should stimulate its application to many other problems associated with source parameter inversion.

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