Focal depths of South African earthquakes and mine events
MBC Brandt
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 minerelated
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