Abstract
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.
Highlights
Magnitude in seismology is a concept that describes the size of a seismic event based on instrumental measurements of some kind
The average values of –log10A0(D) in 5 km bins (Figure 8) compare well with Equation [7] except for the 40 km bin, which may indicate a significant near-site amplification at that distance station compared to most of the seismic events. This shows that good estimates of Local magnitudes by the cluster networks are possible if outliers are rejected and a sufficient number of measurements with appropriate weights for signal-to-noise ratios are averaged, as is currently the practice with Antelope 5.4 software (2018)
Tabulated calibration values –log10(A0) were derived for the cluster network horizontal components, such that the Local magnitudes reported by the cluster networks on average have the same sizes as the Local magnitudes reported by the South African National Seismograph Network (SANSN)
Summary
Magnitude in seismology is a concept that describes the size of a seismic event based on instrumental measurements of some kind. The average values of –log10A0(D) in 5 km bins (Figure 8) compare well with Equation [7] except for the 40 km bin, which may indicate a significant near-site amplification at that distance station compared to most of the seismic events This shows that good estimates of Local magnitudes by the cluster networks are possible if outliers are rejected and a sufficient number of measurements with appropriate weights for signal-to-noise ratios are averaged, as is currently the practice with Antelope 5.4 software (2018). Most of Richter’s (1958) tabulated calibration values are adjusted downward to account for the magnitude overestimate of approximately 0.1 of a unit established in Equation [6] when taking the incorrect model into consideration This is as expected, since seismic waves show less attenuation with distance in South Africa than is the case in Southern California (Brandt, 2015). The newly derived calibration values are similar to those in Equation [4] (Saunders et al, 2012) when adjusted for the constant difference between amplitudes recorded by horizontal seismographs and vertical seismographs
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