Abstract

Recent moderately sized earthquakes (18 November 2013 and 2 December 2013) occurred in the Johannesburg area (ML = 3.6) and southern Limpopo Province (ML = 3.9) respectively. The earthquakes vigorously shook the epicentral areas. Following the shaking, many people submitted reports to the Council for Geoscience (CGS) through an online questionnaire which recorded their experience, whilst others reported the earthquake and its effects on social networks like Twitter. Following both earthquakes, the CGS sent teams of experts into the surrounding areas to assess the effects of the earthquake by interviewing members of the public and completing additional questionnaires. Analyses of the collected macroseismic data produced intensity data points which showed that a maximum intensity of VI was experienced at Fleurhof for the 18 November 2013 earthquake. Maximum shaking with intensity of IV to V was also experienced at KwaMhlanga, Modimolle, and Bela-Bela during the 2 December 2013 earthquake. Surprisingly, strong shaking of intensity IV was also experienced in northern Johannesburg, about 150 km from the epicentre. Investigations showed that this apparent amplification of the ground motion in Johannesburg can be linked to topographic effects. By analyzing first motion polarities, fault plane solutions were obtained for both earthquakes, oblique normal faulting with strike of 72° dip 51° and rake of −77° for the 18 November earthquake; normal oblique left lateral faulting with strike of 135°, dip of 45° and rake of −82° for the 2 December 2013 earthquake.

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