Seismotectonic Models for South Africa: Synthesis of Geoscientific Information, Problems, and the Way Forward Mayshree Singh,1 Andrzej Kijko,2 and Ray Durrheim3 INTRODUCTION should be reviewed; surface and subsurface investigations of the orientation, geometry, sense of displacement, and length of In South Africa the demand for energy resources, water, and infra- ruptures should be conducted; and the possibility of multiple structure has grown significantly in recent years. Furthermore, ruptures ought to be assessed. many coal reserves that are currently being exploited will be Seismotectonic models have not yet been developed for depleted within the next 20 years. Consequently, plans to pro- South Africa. The delineation of seismotectonic zones of Africa vide alternative sources of energy are underway. Energy provid- as part of the Global Seismic Hazard Assessment Program ers are slowly moving away from traditional coal-fired stations to (GSHAP) in 1999 was based on an analysis of the main tec- gas-powered facilities, nuclear power plants, and portable peb- tonic structures and a correlation with present-day seismicity. ble bed modular reactor (PBMR) units. Several dams within the Because of the large scale of the GSHAP project, only regional country have also been constructed to accommodate the grow- structures were accounted for in the preparation of the source ing demand for water. In South Africa, no regulatory guidelines zones. Our study was initiated to remedy this knowledge gap. for seismic design of such critical facilities exist; hence engineers We have completed four steps, which are described in this make use of international guidelines such as Regulatory Guide paper. 1.208, published by the U.S. Nuclear Regulatory Commission 1. Compilation of a catalog of earthquake activity that has (U.S. Nuclear Regulatory Commission 2007). Engineers also been documented in historical records or instrumentally need to assess the seismic risk to formulate emergency evacua- recorded. tion procedures and for insurance assessment purposes. 2. Synthesis of geological mapping, magnetic, and gravity The first step in assessing the seismic hazard and risk for surveys, and evidence of neotectonic activity. any site is to develop a seismotectonic model. The area under 3. Correlation of the seismicity data with the geological, geo- investigation is divided into smaller zones/regions of similar physical, and neotectonic data. tectonic setting and similar seismic potential (Cornell 1968). 4. Identification of any other data that could help to better These zones are then used in a seismic hazard assessment model define the boundaries of seismotectonic provinces. to determine return periods of certain levels of ground motion at a given site in the area in question. For example, U.S. Nuclear EARTHQUAKE CATALOG Regulatory Guide 1.208 (U.S. Nuclear Regulatory Commission 2007) states that regional seismological and geological investi- South African National Seismological Database gations should be undertaken to identify seismic sources and We used earthquake records from the South African National describe the Quaternary tectonic regime. The investigations Seismological Database (SANSD) to map seismicity. The should include a comprehensive literature review (including SANSD is a compilation of seismological data from the South topographic, geologic, aeromagnetic, and gravity maps, as well African National Seismograph Network (SANSN), operated as aerial photos), plus focused geological reconnaissance based by the Council for Geoscience (CGS). Historical data origi- on the results of the literature study. Once the regions of active nates largely from the work of Fernandez and Guzman (1979) faults have been identified, more detailed explorations such as and De Klerk and Read (1988), updated recently by Brandt et geologic mapping, geophysical surveying, borings, and trench- al. (2005). Instrumental data recorded by the SANSN has been ing should be undertaken. Finally, the Quaternary history published in regular seismological bulletins since 1977. Figure 1 shows the distribution of earthquakes above mag- 1. Council for Geoscience, South Africa nitude 3 in the SANSN database to June 2008. Note that earth- 2. Benfield Natural Hazards Centre, University of Pretoria, South quakes in the database are limited to South Africa and Lesotho. Africa There are in excess of 27,000 earthquakes in the database from 3. University of Witwatersrand and Council for Scientific and Industrial 1620 to June 2008, ranging from M 0.2 to M 6.3, with vary- Research, South, Pretoria, South Africa L L doi: 10.1785/gssrl.80.1.71 Seismological Research Letters Volume 80, Number 1 January/February 2009 71 ▲ Figure 1. Map of Earthquakes 1620–2008 contained in the South African National Seismological Database (SANSD). Known clusters relating to natural and mining-induced seismicity are highlighted. The seismic recording stations are represented by triangles. ing levels of completeness relating to the different stages of and Joubert 1990; Joubert et al. 1991). Thermal springs are development and detection capabilities of the network. found here. • Lesotho Cluster—The rate of seismicity toward the north of Seismicity Clusters Lesotho increased significantly after the impoundment of Fernandez and Guzman (1979) first identified seismicity pat- the Katse Dam (Brandt 2000). Toward the west and south terns in South Africa. The most prominent clusters are: of Lesotho the seismicity is of natural origin. • Historical Earthquakes in the Cape Town area—Earth- • Witwatersrand Basin Cluster—South Africa has a num- quakes in this cluster were compiled from diaries, journals, ber of mining regions located in and around the country. and newspapers written from 1620 to 1902. The locations Probably one of the most difficult tasks in monitoring are given as Cape Town because this is where the effects earthquakes in South Africa is to distinguish between were felt, but the actual epicenter could be 100 km away earthquakes of natural origin and tremors and blasts from or more (Brandt et al. 2005). No earthquakes have been the gold, manganese, platinum, diamond, and coal mines. located in the Cape Town area since instrumental record- Most seismicity originates from the gold mining districts ing began in 1972. of the Witwatersrand basin. Within the basin, the clus- • Ceres Cluster—Earthquakes of ML 1–3 are recorded in this ters can further be classified into the Welkom, Klerksdorp, region six times per month, on average. The well-known Carletonville, West Rand, Central Rand, East Rand, and 1969 Ceres earthquake (ML 6.3) occurred on the west- Evander gold fields. Seismicity in these areas differs due to ern termination of the Kango-Bavianskloof fault (KBF). the different tectonic faults affecting the regions and dif- Ongoing research in this area has shown that it is possible ferences in mining activities (Singh and Pule 2007; Gay et that an Mw 7.0 earthquake occurred some 10,000 years ago al. 1995). in this region. To better understand earthquakes of tectonic origin, one needs • Koffiefontein Cluster—An ML 6.2 earthquake occurred to know to what extent the earthquakes recorded in the SANSN here in 1912. Paleoseismic studies show that a large earth- are mining related. In site-specific hazard investigations, care quake (Mw 8) occurred here some 50,000 years ago (Visser should be taken to carefully analyze the earthquake database. The 72 Seismological Research Letters Volume 80, Number 1 January/February 2009 history of mining in the area should be documented, and pos- quake and its associated effects. The earthquake was felt through- sible correlations made with mining-related tremors. One needs out Mozambique, as well as in parts of the neighboring countries to research the active mining areas, obtain blasting schedules and of Swaziland, Zambia, Zimbabwe, and South Africa, but caused records of mining tremors encountered, and correlate them with surprisingly little damage and a very small number of casual- the database. Similar studies were done by Kgaswane (2002) for ties. This is partly because the area immediately affected by the the mining regions. This is beyond the scope of this study and earthquake is sparsely inhabited. The earthquake was felt as far as is merely stated as a precautionary measure to be noted before Durban, where people were evacuated from high-rise buildings. drawing conclusions on the tectonic origins of earthquakes. Earthquakes with magnitudes exceeding 5 are listed in Table 1. The most striking feature of this list is that no earth- Largest Earthquakes quake exceeding magnitude 6.3 has been recorded since 1969, The earliest recorded large earthquake in South Africa occurred the start of the instrumental network. This could be due to a on 4 December 1809 (Von Buchenröder 1830). Many houses number of different reasons: 1) the magnitudes of historical in Cape Town suffered minor damage, and liquefaction features earthquakes have been overestimated; 2) the ML scale of the and fissures in the ground were observed in nearby Blauweberg network is underestimating earthquake magnitude (note that Valley. An ML 6.2 earthquake that occurred near Koffiefontein ML saturates at larger magnitudes, and other magnitude scales on 20 February 1912 was felt all over South Africa. The ML 6 like Ms or Mw should be used); 3) crustal stress might have been Cape St. Lucia event of 31 December 1932 (Krige and Venter released in the historical period; or 4) because South Africa is 1933) was located