Journal of the Southern African Institute of Mining and Metallurgy
On-line version ISSN 2411-9717
BRANDT, M.B.C.. Focal depths of South African earthquakes and mine events. J. S. Afr. Inst. Min. Metall. [online]. 2014, vol.114, n.10, pp. 855-862. ISSN 2411-9717.
Focal depths of 15 tectonic earthquakes and 9 mine-related events were determined for South Africa using data recorded by the South African National Seismograph Network. These earthquakes and events were relocated by means of the Hypocenter program using direct P-waves (Pg) , critically refracted P-waves (Pn) , and first-arrival S-waves for the magnitude range 3.6 < ML< 4.4. Focal depths were first determined by means of the minimum root mean square (RMS) of the differences between the measured travel times and those predicted using the velocity model. The depths for tectonic earthquakes had a 2 km < D <10 km range and an average depth and standard deviation of 6.9 ± 2.3 km. Depths for mine-related events ranged over 0 km < D < 7 km, averaging 3 ± 2.3 km. Next, arrival times for the additional regional depth phases sPn, PmP, sPmP, and SmP were measured. Focal depths were re-determined for the relocated epicentres, with the minimum variance (i.e. spread) of the differences between the measured travel times and travel times predicted by means of the Wentzel, Kramer, Brillouin, and Jeffreys (WKBJ) method for synthetic waveform modelling. Depth ranges were 4 km < D < 7 km (average 5.9 ± 1.2 km) and 1 km < D < 4 km (average 2.4 ± 1.2 km) for tectonic and mine-related events, respectively. The derived depths were verified for one tectonic earthquake with synthetic-to-recorded-waveform fits using the WKBJ synthetic seismogram software for the abovementioned regional phases. The focal mechanism parameters for this earthquake source were obtained from the National Earthquake Information Centre. Focal depths were estimated for nine stations by visually comparing synthetic waveform phases with recorded waveforms, ranging from 5 km to 8 km
Keywords : focal depth; earthquake location; regional depth phases; waveform modelling.