Quantitative evaluation of source parameters of historical earthquakes in southern Africa

Abstract

Macroseismic intensity information is an important instrument in the evaluation of historical seismicity. In an effort to improve the current South African earthquake catalogue, available macroseismic information of historical and early instrumental period earthquakes were used to determine source parameters (epicentre and magnitude of events). Forty-six earthquakes with more than eight Intensity Data Points (IDPs) each were analysed. Of these earthquakes, 16 had 20 or more IDPs and three had more than 80. The IDPs were dominated by relatively low intensity values, mostly determined from human perception of shaking, rather than structural damage, which is consistent with a stable continental region. The source parameters of these earthquakes were computed using a suite of state-of-the-art 3rd generation techniques. The quality of obtained solutions was greatly influenced by the spatial distribution of the IDPs, which was not uniform for all the events, with some events having clustered IDPs whilst others were linearly distributed. As a result, a quality ranking system of the results was developed to give an indication of the confidence in the results. Of the 46 earthquakes analysed, 36 had good quality results. Twelve events analysed in this study are new events that were not in previous catalogues. The results obtained help in improving the earthquake catalogue of the region, as there is an increase in the number of events recoded in the historical and early-instrument period. This kind of work and results obtained highlight the value of seeking out additional contemporary sources in order to revise the source parameters of earthquakes for which no or only very limited instrumental information is available.