V. Midzi, B. Manzunzu, T. Mulabisana, B.S. Zulu, T. Pule, S. Myendeki, G. W. Rathod
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The seismic hazard model for South Africa (ZAF) was developed by scientists from the Council for Geoscience, South Africa and the Indian Institute of Technology, Jammu, India. The model is described by Midzi et al. (2019). The seismic source model was developed using a newly updated catalogue and consists of zones of distributed seismicity, while the ground motion logic tree consists of two existing ground motion prediction equations (GMPEs) derived for active shallow crust. The model was originally created for the OpenQuake (OQ) engine.
Information about the OQ model versions and input files can be found on the Results and Dissemination page.
The viewer below depicts the seismic sources and hazard results in terms of PGA for a return period of 475 years. Click on the menu in the upper right corner to select the layer.
South Africa sits at the southern passive margin of the African continent, where it transitions into oceanic crust. It is bounded to the northeast by the East African Rift System. The basement geology of the region is dominated by Archean cratons and mobile belts. No plate boundaries are present in the region and deformation is slow. Nonetheless, several moderate to large earthquakes have occurred in the country with the largest recorded being the 1969 MW 6.2 Ceres earthquake. The Cape Fold Belt is a dominant structural domain along the coast of South Africa and displays some paleoseismic evidence for Holocene faulting (e.g., Goedhart and Booth, 2016).
The seismic source model was developed using a newly updated earthquake catalogue in conjunction with a seismotectonic model for Southern Africa. The catalogue incorporates data predominantly from the South African National Seismograph Network (SANSN) and also includes events generated by gold mining activity around the Witwatersrand Basin. The seismotectonic model was derived through analysis of available structural, neotectonic, and seismological data. A full description of the datasets used for developing the hazard model can be found in Midzi et al. (2019).
Seismic Source Characterisation
The seismic source characterisation (SSC) consists entirely of zones of distributed seismicity. Specifically, 22 seismic source zones are used to model areas assumed to be uniform in terms of their seismicity characteristics. The seismicity of each of these zones is assumed to follow a truncated exponential (Gutenberg-Richter) distribution. The specific seismicity parameters for each zone can be found in Midzi et al. (2019). Although major faults in Southern Africa have been identified, uncertainty associated with characterising the faults were deemed too large to confidently use them as unique sources in this study.
The OQ implementation uses the Area Source typology to model these zones. The OQ sources are depicted in the interactive viewer.
The PSHA for ZAF thoroughly accounts for epistemic uncertainties associated with the seismicity parameters. Uncertainties are large in the region because seismic and fault data is limited. Alternative seismicity parameters are considered through the use of a logic tree, which considers alternative values of earthquake recurrence (a- and b-values), maximum magnitude (Mmax), and depth.
Ground Motion Characterisation
South Africa is generally considered a stable continental region (Johnston et al., 1994) due to its position relative to plate boundaries, low level of earthquake activity and slow rates of crustal deformation. However, the current tectonic regime of South Africa also shows evidence of extensional tectonic stresses with dominant normal faulting. For this reason, two GMPEs are used for modelling ground motion attenuation in active crustal regions: Boore and Atkinson (2008) and Akkar et al. (2014), the latter having the ability to consider style of faulting.
Using both GMPEs results in two alternative and complete hazard models, which are combined together through the use of a logic tree. The table below shows the logic tree with the GMPEs and their associated weights. Note that although the tectonic setting in South Africa is modelled as active shallow, the sources are labelled in the OpenQuake input as Stable Continental Crust, which is generally considered to be the predominant tectonic setting of South Africa.
|Stable Continental Crust||Weight|
Table 1 - GMPEs used in the ZAF model.
Hazard curves were computed with the OQ engine for peak ground acceleration (PGA) and spectral acceleration (SA) at 0.2s, 0.5s, 1.0s, and 2s. The computation was performed on a grid of 17277 sites (spaced at approximately 10 km) with reference soil conditions with shear wave velocity in the upper 30 meters (Vs30) of 760-800 m/s. Midzi et al. (2019) computed hazard maps for a return period of 475 years for three spectral ordinates (PGA, 0.15s and 2.0s).
The hazard map for PGA corresponding to a 10% probability of exceedance in 50 years (475 year return period), can be seen using the interactive viewer. For a more comprehensive set of hazard and risk results, please see the GEM Visualization Tools.
Akkar S, Sandıkkaya MA, Ay BÖ (2014) Compatible ground-motion prediction equations for damping scaling factors and vertical-to-horizontal spectral amplitude ratios for the broader Europe region. Bulletin of Earthquake Engineering 12: 517-547
Boore DM, Atkinson GM (2008) Ground-motion prediction equations for the average horizontal component of PGA, PGV, and 5%-damped PSA at spectral periods between 0.01 s and 10.0 s. Earthquake Spectra 24(1): 99-138
Goedhart ML, Booth PWK (2016) A palaeoseismic trench investigation of early Holocene neotectonic faulting along the Kango Fault, southern Cape Fold Belt, South Africa–Part II: earthquake parameters. South African Journal of Geology 119: 569-582
Johnston AC, Coppersmith KJ, Kanter LR, Cornell CA (1994) The Earthquakes of Stable Continental Regions. Electric Power Research Institute Report (EPRI), Palo Alto, CA
Midzi, V., Manzunzu, B., Mulabisana, T., Zulu, B.S., Pule, T., and Myendeki, S. (2020) Probabilistic seismic hazard maps for South Africa. Journal of African Earth Sciences, Volume 162, February 2020