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Analysis of exceptionally large tremors in two gold mining districts of South Africa (1989)

McGarr, A. ; Bicknell, J. ; Sembera, E. ; [et al.]

Springer

Pure and applied geophysics 129 (1989), S. 295-307

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ISSN: 1420-9136

Keywords: Large mine tremors ; source parameters ; ground motion parameters ; Klerksdorp mining district ; Carletonville mining district

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Notes: Abstract An investigation of ground motion, recorded using broad-band, wide dynamic-range digital seismographs, of large mine tremors from two South African mining districts with different geologic settings, reveals some essential differences in both seismic source and ground motion parameters. In the Klerksdorp district where the strata are offset by major throughgoing normal faults, the largest tremors, with magnitudes ranging as high as 5.2, tend to be associated with slip on these pre-existing faults. Moreover, the seismic source and ground motion parameters are quite similar to those of natural crustal earthquakes. In the Carletonville district, by contrast, where substantial faults do not exist, the large-magnitude tremors appear to result from the failure of relatively intact rock and cause seismic stress drops and ground motion parameters higher than normally observed for natural shocks. Additionally, there appears to be an upper magnitude limit of about 4 in the Carletonville district. Detailed analyses of an exceptionally large event recorded locally from each of these districts serve to highlight these contrasts.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00874511

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2

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Changes in the early part of the seismic coda due to localized scatterers: The estimation ofQ in a stope environment (1989)

Cichowicz, A. ; Green, R. W. E.

Springer

Pure and applied geophysics 129 (1989), S. 497-511

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ISSN: 1420-9136

Keywords: Seismic coda ; quality factorQ ; localized scatterers ; fractured rock ; stope environment

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Notes: Abstract A single scattering model was used to analyse the temporary changes in the mean density of scattered waves in a discrete random medium. The model of the mean energy density, originally proposed bySato (1977) for spherical radiation and isotropic scattering, has been modified and applied to a medium in which the scatterers are confined to a specified volume. The time variation of the early part of the mean energy density function for the different source durations was investigated. The dominant effect on the theoretical mean energy density is caused by the specified volume containing scatterers. The duration of the source pulse influences the early part of the coda fort/t 0〈1.2, wheret is the lapse time measured from the source origin time, andt 0is arrival time of the body wave. The analysis of the coda signal of micro-events occurring immediately in front of the face enables us to estimate the size of the fracture zone induced by the stope. The model of the mean energy density of coda for a medium containing scatterers close to the seismic source was used to analyse a large number of events recorded close to an advancing mine face in a deep level gold mine in South Africa. The coda decay rate has two trends: the first, with a steep decay of coda, is produced by a larger deviation of rock parameters and/or larger size of the scatterers; the second trend, which decays more slowly, has the corresponding mean-free path ranging from 20 m to 200 m. The analysis indicates that the rock mass about 15–20 m from the stope contains a large proportion of fractured and blocked rock, which is the source of scattering. The scattering of theS-wave was much stronger and more stable, with the mean-free path varying from 11 m to 45 m. This is due to the shorter wavelength of theS wave in comparison with theP wave. The quality factor for theP coda wave varies from 30 to 100 in the fracture zone of stope and outside this zone it has a value of 300. The quality factor of theS wave varies from 20 to 78 in the equivalent volume. For rock surrounding the stope the ratioQ sp −1 /Q ss −1 varied from 0.31 to 0.69. This suggests that the radii of scatterers are smaller than 3.5 m.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00874521

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A seismic refraction investigation of the Archaean Kaapvaal Craton, South Africa, using mine tremors as the energy source (1992)

Durrheim, R. J. ; Green, R. W. E.

Oxford, UK : Blackwell Publishing Ltd

Geophysical journal international 108 (1992), S. 0

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ISSN: 1365-246X

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: The structure of the central Kaapvaal Craton of southern Africa has been investigated by deep seismic sounding, using mine tremors as energy sources. Seismometers were deployed at approximately 10km intervals on two profiles stretching between major mine tremor source regions. Mine tremors are rich in shear energy enabling joint interpretation of P- and S-waves and produce substantial energy at frequencies as low as 1 Hz. Record sections are presented for both P- and S-waves, and the traveltimes and amplitudes interpreted using 2-D ray-tracing techniques. Synthetic seismograms computed for a 1-D velocity model by the reflectivity method compare well with the observed data. A generalized seismic model of the Kaapvaal Craton has the following features: supracrustal strata 0–10 km thick; upper crystalline basement with P-wave velocities of 6.0–6.2 km s−1; the boundary between upper and lower crust at a depth of 14–18 km; a lower crust with a relatively uniform seismic velocity in the range 6.4–6.7 km s−1; and the crust/mantle transition over 1–3 km with the Moho at a depth of about 35 km. The lower crust is found to be seismically attenuating and has a Poisson's ratio of about 0.28. It is also known to be electrically conductive. These observations are in accord with the presence of hydrated mantle rock at the base of the crust.The velocity–depth model of the Kaapvaal Craton is similar to models derived for other Archaean cratons. The Proterozoic provinces adjacent to the Kaapvaal craton are significantly thicker, and have an intermediate- to high-velocity layer developed at the base of the crust. This is interpreted to indicate a change in the process of crustal growth, with basaltic underplating becoming more important since the Archaean. This change is attributed to a change in the composition of the upper mantle. The higher temperatures in the Archaean mantle led to the eruption of komatiitic lavas, resulting in an ultradepleted mantle unable to produce significant volumes of basaltic melt. Proterozoic crust developed above fertile mantle, and subsequent partial melting resulted in basaltic underplating and crustal inflation.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1365-246X.1992.tb03472.x

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Recent deformations of the deep continental root beneath southern Africa (1995)

Vinnik, L. P. ; Green, R. W. E. ; Nicolaysen, L. O.

[s.l.] : Nature Publishing Group

Nature 375 (1995), S. 50-52

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ISSN: 1476-4687

Source: Nature Archives 1869 - 2009

Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics

Notes: [Auszug] Shear-wave splitting in the mantle is related to the lattice preferred orientation of olivine, which depends on finite strain. Axes [100], [010] and [001] in olivine tend to become aligned with the longest, shortest and intermediate axes, respectively, of the strain ...

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1038/375050a0

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5

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Seismic refraction observations in the Transkei Basin and adjacent areas (1977)

Chetty, P. ; Green, R. W. E.

Springer

Marine geophysical researches 3 (1977), S. 197-208

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ISSN: 1573-0581

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Notes: Abstract Some seismic refraction observations undertaken during the IGY are reported here together with a summary of other refraction studies carried out within the Transkei Basin, the Mozambique Ridge and the South African continental shelf area. A 2.5 km section of Cretaceous and younger rocks is associated with profiles observed on the continental shelf; directly below this group are rocks with velocities in the range 4.0–5.5 km s-1, probably representatives of the Karroo and Cape supergroups. The basement material velocity variations were from 5.3 to 6.5 with an average of 5.9 km s-1, and is correlated with granite or Malmesbury Formation plus granite. This crustal structure is similar to that found on the eastern continental shelf of southern South America. The profiles in the Transkei Basin show a thick layer of sediment with velocity range 1.50 to 3.50 km s-1, underlain by a refracting layer in which the average velocity is 4.5 km s-1. The velocity of 6.6 km s-1 obtained for the oceanic layer is similar to the velocities of the crustal layer measured in the Argentine Basin. The mantle velocity (8.1 km s-1) is consistent with the average mantle velocity for the Indian Ocean but significantly lower than the Pacific Ocean average of 8.20 km s-1. The depth to Moho is about 12.0 km and the crustal section is typical oceanic. A plate tectonic model of the early opening of the South Atlantic is used to describe the evolution of the Transkei Basin. On the Mozambique Ridge the thin sediments (0.7 km) are underlain by rocks with velocities averaging 5.6 km s-1. This is more than 1.0 km s-1 faster than the velocity for layer 2 from the Transkei Basin and the Agulhas Plateau, indicating rocks of a younger age or of a different type. Moreover the crustal section of the Ridge has a thickness in excess of 22 km and is in isostatic equilibrium when compared with the adjacent Transkei Basin and Agulhas Plateau. DSDP site 249, situated on the Ridge, penetrated basalt at a depth of 0.4 km. Whether this is continental or oceanic basalt is not known; when this site 249 basalt was compared to the cored basalts of the adjacent Mozambique Basin, inconclusive results were obtained. The essential constitution of the Mozambique Ridge remains an enigma, but solution of this problem is vital for the proper understanding of the Mesozoic history of this oceanic region.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00310110

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