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The aftershock sequence of the 5 August 2014 Orkney earthquake (ML 5.5), South Africa

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Abstract

More than 1000 aftershocks were recorded within a month after the occurrence of the ML 5.5, 5 August 2014 Orkney earthquake. The events were relocated using the double difference method as part of an effort to identify the fault which might be the source of the events. A north–south trend of seismicity was revealed by the relocated events, with a diffuse cluster to the north of the main event. A depth profile shows these two clusters: one at a depth of about 2 km to the north of the main event and the other at depth between 3 and 6 km south of the main event. Focal mechanism solutions of 18 aftershocks were determined using first motion polarities from seismic stations of the Council for Geoscience cluster networks. Stress inversion analysis results from the focal mechanism solutions show a dominant extensional stress field in the region; the main event had a strike-slip fault plane solution. This is consistent with the regional stress field which is predominantly related to the East African rift system. It is possible that the occurrence of the main event triggered seismicity on shallower faults within the mining horizons oriented in a different direction to the fault on which the main event occurred. The area has a complex heterogeneous faulting structure as indicated by the observed low p values and complex focal mechanism solutions.

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Acknowledgements

The authors would like to extend their gratitude to the Council for Geoscience for providing seismic data from the national network stations and cluster networks in and around Johannesburg, Carletonville and Klerksdorp. The stress inversion results were obtained using Win-Tensor, a software developed by Dr. Damien Delvaux, Royal Museum for Central Africa, Tervuren, Belgium. The authors are also very grateful for the insightful reviews given by Prof. Mustapha Meghraoui and two other reviewers.

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Authors and Affiliations

  1. Council for Geoscience, Pretoria, South Africa

    B. Manzunzu, V. Midzi & A. Mangongolo

  2. SiM Mining Consultants (Pty) Ltd., Potchefstroom, South Africa

    F. Essrich

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  1. B. Manzunzu
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  2. V. Midzi
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  3. A. Mangongolo
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Correspondence to B. Manzunzu.

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Manzunzu, B., Midzi, V., Mangongolo, A. et al. The aftershock sequence of the 5 August 2014 Orkney earthquake (ML 5.5), South Africa. J Seismol 21, 1323–1334 (2017). https://doi.org/10.1007/s10950-017-9667-z

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