A 3.4 magnitude earthquake which occurred on Tuesday morning off Cape Town’s west coast has reignited discussions around tremors in South Africa.
This isn’t the first seismic event in 2020 to leave South Africans shaken up but experts dismiss claims of a surge.
The Council for Geoscience in South Africa records minor earthquakes, also referred to as tremors, on a daily basis.
But rapid urbanisation, an increasing number of seismometers and the popularity of social media may explain why earthquakes appear to occur more frequently.
Shortly after midnight on Tuesday 17 November, a 3.4 magnitude earthquake was recorded 47km off South Africa’s West Coast at a depth of 5km. The nearby town of Saldanha was first to experience the seismic rumblings with Cape Town reporting tremors shortly thereafter.
While Tuesday’s early morning tremor received significant attention, the South African National Seismograph Network registered a 2.9 magnitude earthquake in the same vicinity less than 48-hours earlier at 14:43 on Sunday 15 November. In the last three months, the Seismograph Network, overseen by the Council for Geoscience (CGS), recorded no less than four seismic events – ranging between 2.5 to 3.2 magnitude – in the same 200km radius.
“We have recorded seismic events in these regions in the past, and as a rule of thumb in seismology we can expect similar sized earthquakes in the same regions in the future, we unfortunately just can’t say when,” says Michelle Grobbelaar of the GCS’ Seismology Unit in response to concerns around the frequency of seismic events. “We have not noticed an increase or ‘surge’ in the seismicity.”
Although South Africa’s most destructive earthquake on record, measuring 6.3 magnitude, occurred near Tulbagh in 1969, the Western Cape is not regarded as an epicentre of seismic activity in South Africa. Minor earthquakes are far more common in the northern parts of the country, with the North West province, Mpumalanga and Limpopo recording tremors on a daily basis.
On 1 October 2020, Johannesburg’s West Rand was shaken by a 2.8 magnitude earthquake which made headlines. Since then, the Seismograph Network has recorded at least 16 seismic events – ranging from 2.1 to 2.9 magnitude – all in the vicinity of Carletonville and Randfontein.
Data gathered by the Seismograph Network shows that 862 seismic events were recorded in South Africa from the beginning of the year to 17 November 2020. This equals an average of more than two tremors a day. In 2019, South Africa registered more than three tremors a day.
Alastair Sloan, of the University of Cape Town’s Geological Sciences Department, says that while cluster events, emanating from compounded stresses on fault lines, are not unheard of, an increase in frequency and intensity of tremors remained highly unlikely. “There is a very slightly elevated chance that there will be more similar events over the next days, weeks and months,” explains Sloan. “Larger damaging events are possible but very unlikely in the short term.”
Grobbelaar has offered an explanation about the perceived increase in frequency and why South Africans may think that earthquakes are becoming more common.
“What seismologists across the world have noticed is that due to the increased urbanisation, many more people experience the seismic events than in the past and people are more aware of earthquakes due to the media coverage and social media,” says Grobbelaar. “Additionally, seismologists are also installing more seismometers across the world, so we are able to locate more smaller earthquakes than in the past.”
The British Geological Survey (BGS) supports these theories, noting that earthquakes in populated places are far more noticeable than those which occur in remote areas. The BGS also reflects on earthquake clustering as described by Sloan, saying that while increases and decreases are a natural part of a ‘quasi-random phenomena’ such as seismic activity, people are more likely to notice a short-term rise in activity without factoring in previous decreases.
Social media also plays a role in the perceived increase in frequency, as instant communication allows for news of tremors to spread at a rapid rate. As noted by the BGS, this is particularly true for densely populated urban areas, where more people are likely to feel the tremor’s affects.
The advancement of seismometers, in both number and technology, means that smaller seismic events are being registered. As an example, the National Earthquake Information Centre, overseen by the United States Geological Survey, locates approximately 30,000 earthquakes around the world every year.
Similarly, the CGS is consistently upgrading the South African Seismograph Network to include more seismograph stations and finer detection of minor earthquakes. These improvements mean that more earthquakes are likely to be registered.