Macroseismic survey of the ML5.5, 2014 Orkney earthquake

Abstract

On 5 August 2014 at 1222 hours (local time), an earthquake of local magnitude ML = 5.5 occurred in the Orkney area in the North West Province, South Africa. The earthquake shaking was felt widely in South Africa as far as Cape Town as well as in Maputo, Mozambique, and Gaborone in Botswana. One person was killed when a wall collapsed on him, and more than 600 houses were damaged. Following the earthquake, many people submitted reports to the Council for Geoscience (CGS) through an online questionnaire which recorded their experience, whilst others reported the event and its effects on social networks like Twitter and in newspapers. The CGS also sent out a team of scientists to further assess the effects of the event in the community by interviewing members of the public and completing additional questionnaires. A total of 866 observations were collected. Analysis of the collected macroseismic data produced 170 intensity data points which showed that a maximum intensity of VII was experienced in communities located in the epicentral area. The observed attenuation of intensity values was comparable to that observed on the French stable continental region especially in the area of 600-km radius from the epicentre. Airborne geophysical data were used to try and identify the fault along which the earthquake occurred. This was necessary as there was no surface expression of the earthquake and no previously identified fault near the epicentre. The interpretation of the data showed a fault located about 500 m from the epicentre appearing to form a boundary to the east of located aftershocks.