Browsing Faculty of Science by Author "Simon, R.E."
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Simon, R.E.; Wright, C.; Kwadiba, M.T.O.; Kgaswane, E.M. (Elsevier Science Ltd. www.elsevier.com/locate/lithos, NaN, 2003)[more][less]
Abstract: Average one-dimensional P and S wavespeed models from the surface to depths of 800 km were derived for the southern African region using travel times and waveforms from earthquakes recorded at stations of the Kaapvaal and South African seismic networks. The Herglotz–Wiechert method combined with ray tracing was used to derive a preliminary P wavespeed model, followed by refinements using phase-weighted stacking and synthetic seismograms to yield the final model. Travel times combined with ray tracing were used to derive the S wavespeed model, which was also refined using phase-weighted stacking and synthetic seismograms. The presence of a high wavespeed upper mantle lid in the S model overlying a low wavespeed zone (LWZ) around 210- to - 345-km depth that is not observed in the P wavespeed model was inferred. The 410-km discontinuity shows similar characteristics to that in other continental regions, but occurs slightly deeper at 420 km. Depletion of iron and/or enrichment in aluminium relative to other regions are the preferred explanation, since the P wavespeeds throughout the transition zone are slightly higher than average. The average S wavespeed structure beneath southern Africa within and below the transition zone is similar to that of the IASP91 model. There is no evidence for discontinuity at 520-km depth. The 660-km discontinuity also appears to be slightly deeper than average (668 km), although the estimated thickness of the transition zone is 248 km, similar to the global average of 241 km. The small size of the 660-km discontinuity for P waves, compared with many other regions, suggests that interpretation of the discontinuity as the transformation of spinel to perovskite and magnesiowu¨ stite may require modification. Alternative explanations include the presence of garnetite-rich material or ilmenite-forming phase transformations above the 660-km discontinuity, and the garnet– perovskite transformation as the discontinuity. URI: http://hdl.handle.net/10311/265 Files in this item: 1
Simon_L71_2003.pdf (2.274Mb) -
Kwadiba, M.T.O.G.; Wright, C.; Kgaswane, E.M.; Simon, R.E.; Nguuri, T.K. (Elsevier Science Ltd. www.elsevier.com/locate/lithos, NaN, 2003)[more][less]
Abstract: Pn arrivals from mining-induced earthquakes on the edge of the Witwatersrand basin show that the P wavespeeds in the uppermost mantle are almost constant throughout most of the Kaapvaal craton. The presence of only small wavespeed variations allows the use of a simple method of estimating crustal thicknesses below the stations of the Kaapvaal broad-band network using Pn times that has been compared with results from receiver functions. One thousand three hundred thirty-seven Pn arrivals were used to derive crustal thicknesses at 46 stations on the Kaapvaal craton. The average crustal thicknesses for 19 centrally located stations on each of the northern and southern regions of the craton that yielded well-constrained thicknesses were 50.52 ± 0.88 km and 38.07 ± 0.85 km, respectively. In contrast, the corresponding average thicknesses determined from receiver functions were 43.58 ± 0.57 km and 37.58 ± 0.70 km, respectively. The systematically lower values for receiver functions in the northern part of the Kaapvaal craton that was affected by the Bushveld magmatism at 2.05 Ga, suggest that the receiver functions do not enable the petrological crust mantle boundary to be reliably resolved due to variations in composition and metamorphic grade in a mafic lower crust. The Pn times also suggest pervasive azimuthal anisotropy with maximum wavespeeds of about 8.40 km/s at azimuths of about 15° and 217° in the northern and southern regions of the craton, respectively, and minimum wavespeeds of about 8.25 km/s. URI: http://hdl.handle.net/10311/281 Files in this item: 1
Kwadiba_L71_2003.pdf (2.747Mb) -
Wright, C.; Kgaswane, E.M.; Kwadiba, M.T.O.; Simon, R.E.; Nguuri, T.K.; McRae-Samuel, R. (Elsevier Science Ltd. www.elsevier.com/locate/lithos, NaN, 2003)[more][less]
Abstract: Events induced by deep gold-mining activity on the edge of the Witwatersrand basin dominate the seismicity of South Africa. The deployment of 54 broad-band seismic stations at 84 separate locations across southern Africa between April 1997 and April 1999 (Kaapvaal network) enabled the seismicity of South Africa to be better defined over a 2-year period. Seismic events located by the South African national network, and by localized seismic networks deployed in mines or across goldmining areas, were used to evaluate earthquake location procedures and to show that the Kaapvaal network locates mininginduced tremors with an average error of 1.56 ± 0.10 km compared with 9.50 ± 0.36 km for the South African network. Travel times of seismic events from the mines recorded at the Kaapvaal network indicate regional variations in the thickness of the crust but no clearly resolved variations in seismic wavespeeds in the uppermost mantle. Greater average crustal thicknesses (48–50 km compared with 41–43 km) are observed in the northern parts of the Kaapvaal craton that were affected by the Bushveld magmatism at 2.05 Ga. Estimates of average crustal thickness for the southern part of the Kaapvaal craton from receiver functions (38 km) agree well with those from refracted arrivals from mining-induced earthquakes if the crustal thicknesses below the sources are assumed to be 40–43 km. In contrast, the average crustal thickness inferred from refracted arrivals for the northern part of the Kaapvaal craton is larger by about 7 km (51 km) than that inferred from receiver functions (44 km), suggesting a thick mafic lower crust of variable seismic properties due to variations in composition and metamorphic grade. Pn wavespeeds are high (8.3–8.4 km/s), indicating the presence of highly depleted magnesium-rich peridotite throughout the uppermost mantle of the craton. Seismic Pg and Sg phases indicate that the upper crust around the Witwatersrand basin is comparatively uniform in composition when averaged over several kilometres. URI: http://hdl.handle.net/10311/280 Files in this item: 1
Wright_L71_2003.pdf (3.483Mb)
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