Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.651691
Title: An estimation of seismic anisotropy in the crust from three different time and distance scales
Author: Graham, Gerhard
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1994
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Abstract:
In this thesis shear-wave splitting is analyzed in shear-wave arrivals from three different time and distance scales to identify the occurrence and extent of anisotropy in the crust. The refracted shear waves recorded by the closely spaced seismic network of the third Turkish Dilatancy Project from regional earthquakes in western Turkey have similar polarizations to those previously observed above local earthquakes, but with less scatter and substantially greater time delays (up to 1.0 s) between the split shear waves. This is interpreted as indicative of the presence of anisotropy in the lower crust with similar anisotropic symmetry to the crack-induced anisotropy of the upper half of the crust. Mine tremors in the deep gold mine of South Africa have seismic magnitudes ranging from ML -4 to ML more than 5. An examination of three-component acceleration and velocity seismograms recorded on the surface above an active gold mine indicates that the shear-wave polarizations have a nearly uniform alignment. This combined with the measured time delays are consistent with shear waves propagating through the effective anisotropy of parallel, vertical microcracks throughout the rockmass. It is concluded that the dry fractures caused by the high stresses during normal mining processes have negligible effect on the shear-wave polarizations at the wavelengths at which shear waves are recorded at the surface. The anisotropy observed at the surface appears to be due to microcracks aligned by the regional stress regime rather than disturbances in the local stress regime due to mining operations. High frequency subsurface recordings of mine tremors that occur in confined volumes surrounding the work face of the mining excavations show shear-wave splitting over distances of a few hundred metres. Plate Carée cylindrical projections of the observed shear-wave polarizations and time delays were compared with theoretical patterns of polarizations and time delays from different synthetic models. From this comparison it became clear that a model containing fluid-filled, non-vertical cracks striking approximately north-south could be used to produce the observed polarizations and time delays to some extent.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.651691  DOI: Not available
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