Recently, the field of geophysics has seen increasing recognition of the unique character of deformation and seismicity in stable continental regions (SCRs). However several important questions remain understudied. What controls the locations of earthquakes in SCRs? How well do observations, in SCRs, of elastic strain accumulation and release correlate with each other? How well do they correlate with stresses and geological proxies for rheological variation? The ultimate goal of this study was to better understand stable continental regions like southern Africa, where large earthquakes occur despite not being near plate boundaries, for example the 2017 Mw 6.5 earthquake in Moiyabana, Botswana. One way of studying the stress and strain in stable continental regions is by understanding the surface deformation of the region. This deformation is easily studied using global navigation satellite system (GNSS) velocity data. One of the biggest difficulties when it comes to GNSS data is that it isn't collected on a regular grid, but rather as irregular data points that need to be interpolated. This research investigated multiple interpolation methods and recommended two methods that best replicate the original velocity field (using a well populated dataset from Southeast Asia). These interpolated GNSS data can then be used to determine deviatoric strain in a region, which can in turn be fed into numerical stress models. However, limited GNSS data exist across southern Africa, and therefore topographic data was used to calculate the gravitational potential energy, and in turn the body stress and deviatoric stress for the region. This study also investigated how this deviatoric stress (or deviatoric strain) can be more accurately calculated on a spherical rather than a flat surface, which is particularly important over large study areas. Across southern Africa, data show that deviatoric stress lined up with stress data within mobile belts. This suggests that in these weaker mobile belt crust (such as the Namaqua-Natal and Damara-Chobe belts), gravitational collapse is the dominant driver of deformation, which is in line with conclusions that have been made in previous literature. In other regions, deviatoric stress vectors and stress data do not coincide and therefore there are other forces at play. These observations are obviously restricted by limited data coverage; it remains an open question if areas that have increased deviatoric stress due to gravitational collapse, which are also aligned with the orientation of weak zones, will have elevated strain in the long term.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uct/oai:localhost:11427/37697 |
| Date | 12 April 2023 |
| Creators | New, Thomas |
| Contributors | Sloan, Alastair |
| Publisher | Faculty of Science, Department of Geological Sciences |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Master Thesis, Masters, MSc |
| Format | application/pdf |
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