Seismological Research Letters Volume 80, Number 1 January/February 2009 71 doi: 10.1785/gssrl.80.1.71 INTRODUCTION In South Africa the demand for energy resources, water, and infra- structure has grown signiicantly in recent years. Furthermore, many coal reserves that are currently being exploited will be depleted within the next 20 years. Consequently, plans to pro- vide alternative sources of energy are underway. Energy provid- ers are slowly moving away from traditional coal-ired stations to gas-powered facilities, nuclear power plants, and portable peb- ble bed modular reactor (PBMR) units. Several dams within the country have also been constructed to accommodate the grow- ing demand for water. In South Africa, no regulatory guidelines for seismic design of such critical facilities exist; hence engineers make use of international guidelines such as Regulatory Guide 1.208, published by the U.S. Nuclear Regulatory Commission (U.S. Nuclear Regulatory Commission 2007). Engineers also need to assess the seismic risk to formulate emergency evacua- tion procedures and for insurance assessment purposes. he irst step in assessing the seismic hazard and risk for any site is to develop a seismotectonic model. he area under investigation is divided into smaller zones/regions of similar tectonic setting and similar seismic potential (Cornell 1968). hese zones are then used in a seismic hazard assessment model to determine return periods of certain levels of ground motion at a given site in the area in question. For example, U.S. Nuclear Regulatory Guide 1.208 (U.S. Nuclear Regulatory Commission 2007) states that regional seismological and geological investi- gations should be undertaken to identify seismic sources and describe the Quaternary tectonic regime. he investigations should include a comprehensive literature review (including topographic, geologic, aeromagnetic, and gravity maps, as well as aerial photos), plus focused geological reconnaissance based on the results of the literature study. Once the regions of active faults have been identiied, more detailed explorations such as geologic mapping, geophysical surveying, borings, and trench- ing should be undertaken. Finally, the Quaternary history should be reviewed; surface and subsurface investigations of the orientation, geometry, sense of displacement, and length of ruptures should be conducted; and the possibility of multiple ruptures ought to be assessed. Seismotectonic models have not yet been developed for South Africa. he delineation of seismotectonic zones of Africa as part of the Global Seismic Hazard Assessment Program (GSHAP) in 1999 was based on an analysis of the main tec- tonic structures and a correlation with present-day seismicity. Because of the large scale of the GSHAP project, only regional structures were accounted for in the preparation of the source zones. Our study was initiated to remedy this knowledge gap. We have completed four steps, which are described in this paper. Compilation of a catalog of earthquake activity that has 1. been documented in historical records or instrumentally recorded. Synthesis of geological mapping, magnetic, and gravity 2. surveys, and evidence of neotectonic activity. Correlation of the seismicity data with the geological, geo- 3. physical, and neotectonic data. Identiication of any other data that could help to better 4. deine the boundaries of seismotectonic provinces. EARTHQUAKE CATALOG South African National Seismological Database We used earthquake records from the South African National Seismological Database (SANSD) to map seismicity. he SANSD is a compilation of seismological data from the South African National Seismograph Network (SANSN), operated by the Council for Geoscience (CGS). Historical data origi- nates largely from the work of Fernandez and Guzman (1979) and De Klerk and Read (1988), updated recently by Brandt et al. (2005). Instrumental data recorded by the SANSN has been published in regular seismological bulletins since 1977. Figure 1 shows the distribution of earthquakes above mag- nitude 3 in the SANSN database to June 2008. Note that earth- quakes in the database are limited to South Africa and Lesotho. here are in excess of 27,000 earthquakes in the database from 1620 to June 2008, ranging from M L 0.2 to M L 6.3, with vary- Seismotectonic Models for South Africa: Synthesis of Geoscientific Information, Problems, and the Way Forward Mayshree Singh, 1 Andrzej Kijko, 2 and Ray Durrheim 3 1. Council for Geoscience, South Africa 2. Benield Natural Hazards Centre, University of Pretoria, South Africa 3. University of Witwatersrand and Council for Scientiic and Industrial Research, South, Pretoria, South Africa