Macroseismic survey of the ML5.5, 2014 Orkney earthquake
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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.
KeywordsSouth Africa Earthquakes Intensity MMI scale Questionnaires Macroseismic field Epicentre
We extend our sincere appreciation to the Council for Geoscience for funding the macroseismic intensity survey. The authors are also grateful to M. Grobbelaar and D. Birch for giving their time to proofread this manuscript as well as to the reviewers whose input significantly improved our article.
- Allen TI, Wald DJ (2009) Evaluation of ground-motion modelling techniques for use in Global ShakeMap: a critique of instrumental ground-motion prediction equations, peak ground motion to macroseismic intensity conversions, and macroseismic intensity predictions in different tectonic settings. US Geological Survey Open-File Report 2009-1047Google Scholar
- Ashford SA, Sitar N (1994) Seismic response of steep natural slopes, Report No. UCB/EERC 94-05, Earthquake Engineering Research Center, College of Engineering, University of California at BerkeleyGoogle Scholar
- Boore DM (1972) A note on the effect of simply topography on seismic SH wave. Bull Seismol Soc Am 62:275–284Google Scholar
- Celebi M (1987) Topographic and geological amplification determined from strong-motion and aftershock records of the 3 March 1985 Chile earthquake. Bull Seismol Soc Am 77:1147–1167Google Scholar
- Faccioli E, Vanni M, Frassine F (2002) Complex site effects in earthquake ground motion, including topography. 12th European Conference on Earthquake Engineering 844Google Scholar
- Musson RMW, Cecić I (2002) Macroseismology. In: Lee WHK, Kanamori H, Jennings PC, Kisslinger C (eds) International handbook of earthquake and engineering seismology, International Geophysics. 81(A):807–822Google Scholar
- Richter C (1958) Elementary seismology. Freeman, San FranciscoGoogle Scholar
- Sibson R (1981) A brief description of natural neighbor interpolation. In: Barnett V (ed) Interpreting multivariate data. Wiley, Chichester, pp 21–36Google Scholar
- USGS (2006) M7.0 Mozambique earthquake of 22 February 2006. Poster, National Earthquake Information Center, US Geological SurveyGoogle Scholar