Earthquake 500km from Marion Island poses minimal risk to Cape Town, says expert - IOL

FOLLOWING reports of an earthquake off the southern coast of Africa, causing alarm online, a lecturer in Geophysics at the University of Cape Town says South Africans have little to worry about in  terms of tectonic activity. 

Various geological agencies and societies recorded a 5.7 magnitude earthquake, with some viral reports saying that the seismic event “occurred 500km offshore”. 

But Dr Diego Quiros confirmed to IOL Media that there is no reason for the public to be shaking in their boots over earthquakes and tsunamis.

Firstly, he explained, the event was far too weak and remote.

He said: “The event location, depth, origin time and magnitude will have uncertainty, that been said three agencies located the event, USGS, GFZ/GEOFON, and EMSC and their solutions lie within 30 km of each other. 

“For earthquakes in faraway oceanic regions with remote seismic station coverage international agencies often report depths with a fixed value, e.g. 10 km. 

“The event occurred approximately 500 km from Marion Island, part of the Prince Edward Islands. Since Prince Edward Islands belong to RSA the event occurred approximately 500 km from South Africa, however it is also correct to say that the event was located between 1675 and 1690km from Gqeberha and 1930 and 1955km from Cape Town depending on which agency’s location you used.”

Quiros added: “5.7 would be a moderate earthquake in terms of magnitude alone, however given its remote location at a known tectonic plate boundary its impact for the South African mainland is very minimal.

“Even for any people present at Marion Island which is 500 km from the event the impact should be small, perhaps some might have felt the event.”

He explained that at this type of boundary, tectonic plates move apart in some sections and slide horizontally past each other in other sections.

Since 1970, roughly 50 events have been recorded along the boundary.

None have had a significant impact on SA and Quiros added that the boundary is unlikely to generate an event to worry South Africans. 

For those fearing a tsunami, Quiros said: “For a Tsunami to occur a large portion of the ocean floor needs to move up or down, given the location of this event on a tectonic plate boundary of the mid-ocean ridge type, it is not very likely that a Tsunami would be generated. 

“Most of the motion at this type of plate boundary is accommodated by plates sliding horizontally past each other (not up or down motion). 

“Additionally, the USGS did not assign a tsunami flag to this event. The factors that control tsunami generation are magnitude of the event (generally above Mw 7), the type of faulting (up or down motion of the plates) and the depth (shallower higher risk).”