Seismic monitoring in Namaqualand/Bushmanland region

Abstract

The Namaqualand-Bushmanland region has numerous features that make it attractive for the storage of radioactive waste. In the late 1970s a programme to find a suitable site for low- and intermediate-level waste was launched and Vaalputs was identified. Such site has been in operation at Vaalputs, ca. 100 km S of Springbok since 1986, and seismicity is one of several key factors that are monitored as part of the on-going disposal operations. We evaluate the region’s seismic history with the aim of assessing its long-term stability and geodynamic setting. The two-station network of short-period seismometers that was installed in 1989 was replaced with a three-station network in 2012, which comprises one broadband and two short period seismometers. Data from these networks, the South African National Seismological Network, and the International Seismological Centre has been used to compile a catalogue of the general seismicity of the region. A previously known cluster of earthquake foci, with MMax = 5.8, termed the “Grootvloer cluster”, is found to consist of three distinct seismic source zones, namely: (1) the Springbok area source, which is attributed to mining activities that ended around 2000; (2) the Great Escarpment area source, which could be attributed to continental margin instability, and (3) the Bushmanland Plateau area source, which is not easily understood. However, we believe that such sub-cluster reflects the strain in the upper and middle crust in response to the transpressional force of a robust, horizontally–oriented s1, a vector known in the literature as the Wegener Stress Anomaly. The overall cumulative trend of the number of events displays a significant increase in the rate, from 13 events/year for the period 1989 to 1995 to 25 events/year for the period 2005 to 2009. These changes seem to coincide with periods of increased global seismic moment release.