Seismic Hazard and Risk Assessment Based on the Parametric-Historic Procedure Case Study: Tulbagh, South Africa

This study provides an assessment of seismic hazard and seismic risk for Tulbagh, located about 90 km N-E from Cape Town, where the strongest and most damaging earthquake known in the earthquake history of South Africa took place. The study contains two parts: assessment of seismic hazard and seismic risk.The applied procedure for the assessment of seismic hazard is known as the “Parametric-Historic” procedure. This method permits the combination of both historical and instrumental data. The historical part of the catalogue contains only the strongest events, whereas the complete part can be divided into several subcatalogues, each assumed complete above a specified threshold of magnitude. In the analysis, the uncertainty in the determination of the earthquake was taken into account by incorporation of the concept of “apparent magnitude”.The technique has been developed specifically for the estimation of seismic hazard at individual sites without the subjective judgement involved in the definition of seismic source zones, when the specific active faults have not been mapped or identified, and where the causes of seismicity are not well understood. The Parametric-Historic procedure being parametric in nature, consists essentially of two steps. The first step is applicable to the area in the vicinity of Tulbagh and requires an estimation of the parameters specific to that area, otherwise known as the areaspecific parameters. In this case, these parameters include the mean seismic activity rate, the Gutenberg-Richter parameter, and the maximum regional magnitude. The second step is applicable to the Tulbagh site, and consists of parameters characteristic of the site and is referred to as the site-specific parameters which are the parameters that give the distribution of the amplitude of peak ground acceleration (PGA). The results of the hazard assessment are expressed as probabilities that specified values of PGA will be exceeded during the chosen time intervals. A worst case scenario sketches the possibility of a maximum PGA of 0.30 g.The second part of our study concentrates on the probabilistic seismic risk analysis. It includes a comprehensive introduction to the problems associated with the estimation of seismic damage to buildings. This is followed by the methodology for the estimation of expected damage from a probabilistic point of view. The work closes with an application of the described methodology to a site in the vicinity of Tulbagh.

Keywords:

Seismic hazard , seismic risk , South Africa , Tulbagh