Large waves crash over a coastal lighthouse during rough sea conditions. The image is used to illustrate the potential coastal context of offshore seismic activity. Photo: Terra Guard

NNA News: An earthquake initially measured at magnitude 5.8 was recorded south of Africa late Sunday, with the epicentre located offshore at a depth of about 10 kilometres. 

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The GFZ German Research Centre for Geosciences recorded the earthquake at 8:54 p.m. UTC on Sept. 20, equivalent to 10:54 p.m. South Africa Standard Time, and initially placed the epicentre at 48.41 degrees south latitude and 31.47 degrees east longitude. The event was subsequently recorded by other seismic monitoring systems at slightly different parameters. The US Geological Survey put the magnitude at 5.7, with the epicentre at approximately 48.43 degrees south and 31.23 degrees east, also at a depth of 10 kilometres.

The difference in magnitude reflects the preliminary nature of earthquake measurements. Seismic agencies can revise the magnitude and location as additional data are processed. GFZ said its earthquake information service provides preliminary information from seismic networks around the world and that automatically determined earthquake parameters can be revised following further analysis by seismologists.

The coordinates place the earthquake south of the African mainland and offshore, rather than within South African territory. The event should therefore not be described as an earthquake that struck South Africa. The Council for Geoscience operates South Africa's National Seismograph Network, which monitors earthquakes occurring in and around the country. The network provides instrumental data used to assess seismic activity and earthquake hazards.

A map shows the preliminary location of the earthquake recorded south of the African mainland, offshore in the southern Indian Ocean. Photo Credit: TerraGuard

At the time of publication, NNA had found no verified reports of casualties, structural damage or other impacts in South Africa linked to the earthquake. The offshore location also places the epicentre a considerable distance from South Africa's populated coastline. The preliminary depth of 10 kilometres indicates the depth of the earthquake's focus below the Earth's surface, but does not by itself establish the level of hazard experienced by communities.

The event occurred in the broader southern African region, where seismic monitoring agencies continuously track earthquakes and other ground movements. GFZ's global Earthquake Explorer provides near-real-time information on seismic events and allows preliminary earthquake parameters to be reviewed as they are updated.