Abstract

BRANDT, M.B.C.. Local magnitude calibration of seismic events in the West Rand, Far West Rand, and Klerksdorp-Orkney-Stilfontein-Hartebeesfontein gold mining areas. J. S. Afr. Inst. Min. Metall. [online]. 2021, vol.121, n.11, pp.573-580. ISSN 2411-9717.  http://dx.doi.org/10.17159/2411-9717/1301/2021.

Richter magnitudes for seismic events were calibrated for use as a Local magnitude scale in the West Rand (WR), Far West Rand (FWR), and Klerksdorp-Orkney-Stilfontein-Hartebeesfontein (KOSH) gold mining areas. Richter magnitudes are currently calculated from seismograms recorded by local surface cluster networks using tabulated calibration values for Southern California, published in 1958. The Richter (1958) model is incorrect for distances of less than 30 km and should be applied only to crustal earthquakes in regions with similar attenuation properties to those of Southern California. When compared to the South African National Seismograph Network (SANSN), the cluster networks, on average, overestimate seismic event magnitudes by approximately 0.1 of a magnitude unit. A calibrated Local magnitude scale was derived by means of a multiple regression analysis between the Local magnitudes reported by the SANSN and the largest zero-to-peak trace amplitudes measured on the cluster network horizontal seismograms, after modelling the attenuation of the seismic waves as these progress from the epicentre to the station position. Magnitudes reported by individual stations for the same event show a significant scatter around the average magnitude owing to the near-surface amplifications of the seismic waves at the recordings sites. Average magnitude should be estimated using as many magnitudes of individual stations as possible to ensure an accurate estimate. Larger event magnitudes should be compared with those recorded by the SANSN.

Keywords : Richter magnitude; local magnitude calibration; seismic event; Witwatersrand Basin.

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