After the mega-earthquakes and concomitant devastating tsunamis in Sumatra (2004) and Japan (2011), an investigation was launched into the potential risk of tsunami hazard to the coastal cities of South Africa. This paper presents the analysis of the seismic hazard of seismogenic sources that could potentially generate tsunamis, as well as the analysis of the tsunami hazard to coastal areas of South Africa.
While most of the world’s coastal areas bordering passive margins have no known records of large tsunamis (including South Africa), the 2004 Indian Ocean event has demonstrated that no coastline is immune to tsunami wave inundation.
2004 Indian Ocean earthquake and tsunami
Understanding Tsunamis and Their Causes
Tsunamis are caused by earthquakes or other seismic eruptions on the ground of the ocean and can cause massive tidal waves, which run with enormous force on land, causing great devastation. Great earthquakes such as the 2004 Indian Ocean earthquake are generally associated with megathrust events in subduction zones. Their seismic moments can account for a significant fraction of the global seismic moment across century-scale periods.
On 26 December 2004, at 07:58:53 local time (UTC+7), a 9.2-9.3 earthquake struck with an epicenter off the west coast of Aceh in northern Sumatra, Indonesia. The earthquake caused a massive tsunami with waves up to 30 m (100 ft) high, known as the Boxing Day Tsunami after the Boxing Day holiday, or as the Asian Tsunami, which devastated communities along the surrounding coasts of the Indian Ocean, killing an estimated 227,898 people in 14 countries, especially in Aceh (Indonesia), Sri Lanka, Tamil Nadu (India), and Khao Lak (Thailand).
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The 2004 Indian Ocean earthquake was unusually large in geographical and geological extent. An estimated 1,600 km (1,000 mi) of fault surface slipped (or ruptured) about 15 m (50 ft) along the subduction zone where the Indian plate slides under (or subducts) the overriding Burma plate.
As well as the sideways movement between the plates, the 2004 Indian Ocean earthquake resulted in a rise of the seafloor by several metres, displacing an estimated 30 km3 (7.2 cu mi) of water and triggering devastating tsunami waves. The waves radiated outwards along the entire 1,600 km (1,000 mi) length of the rupture (acting as a line source). This greatly increased the geographical area over which the waves were observed, reaching as far as Mexico, Chile, and the Arctic.
The sudden vertical rise of the seabed by several metres during the earthquake displaced massive volumes of water, resulting in a tsunami that struck the coasts of the Indian Ocean. The tsunami, like all the others, behaved differently in deep water than in shallow water.
In deep ocean water, tsunami waves form only a low, broad hump, barely noticeable and harmless, which generally travels at the high speed of 500 to 1,000 km/h (310 to 620 mph); in shallow water near coastlines, a tsunami slows down to only tens of kilometres per hour but, in doing so, forms large destructive waves.
How to Prepare in Case of a Tsunami | Disasters
Tsunami Events and Impact on South Africa
A total of 3 tidal waves classified as a tsunami since 1969 have killed 3 people in South Africa. Tsunamis therefore occur only rarely here. The strongest tidal wave registered in South Africa so far reached a height of 2.9 meters. On 08/27/1969, no losses of human lifes have been registered by this tsunami.
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The biggest impact in terms of lifes, injuries, destroyed homes and the economy had been a tsunami on 12/26/2004. A tidal wave of up to 1.5 meters killed 3 humans and destroyed vast areas.
The 2004 Indian Ocean tsunami was noticed as far as Struisbaai in South Africa, about 8,500 km (5,300 mi) away, where a 1.5-metre-high (5 ft) tide surged on shore about 16 hours after the earthquake.
The northern regions of the Indonesian island of Sumatra were hit quickly, while Sri Lanka and the east coast of India were hit roughly 90 minutes to two hours later. Because of the distances involved, the tsunami took anywhere from fifteen minutes to seven hours to reach the coastlines.
The subduction zones of Makran, South Sandwich Island, Sumatra, and the Andaman Islands were identified as possible sources of mega-earthquakes and tsunamis that could affect the African coast. Numerical tsunami simulations were used to investigate the realistic and worst-case scenarios that could be generated by these subduction zones.
The simulated tsunami amplitudes and run-up heights calculated for the coastal cities of Cape Town, Durban, and Port Elizabeth are relatively small and therefore pose no real risk to the South African coast.
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Although the South African coast is not known to have been affected by large tsunami events, it is exposed to local submarine landslides and tsunamigenic subduction zones of the Indonesian and South Sandwich Islands (Kijko et al., 2018).
Tsunami travel time map for the April 1, 1946 Pacific-wide tsunami caused by an earthquake south of Unimak Island, Alaska.
Risk Assessment and Mitigation
In the area you have selected (South Africa) tsunami hazard is classified as medium according to the information that is currently available. This means that there is more than a 10% chance of a potentially-damaging tsunami occurring in the next 50 years.
Based on this information, the impact of tsunami should be considered in different phases of the project for any activities located near the coast. Project planning decisions, project design, and construction methods should take into account the level tsunami hazard. The areas at risk of tsunami will increase as global mean sea level rises.
Consider the impact of tsunami inundation on the availability and function of: transport, communications, water, sanitation and energy infrastructure, and public health for continued operation of the project. If the project is involves the development of critical infrastructure (e.g., a hospital, fire station, or power transmission line), investigate the cascading effect of vulnerable network dependencies of the project (e.g. power supply and computer and communication networks) that may impact the project, even if the project itself is not inundated.
Early warning may be required to enable successful evacuation and to enable temporary shut-down of the project if this could reduce consequential and compounding damage. Consider purchasing insurance to cover potential losses to the project.
Tsunami Animation
The Need for Early Warning Systems
While it is understood that coastal hazards such as tsunamis and storms are unavoidable natural hazards, there is a need for a more focused approach towards establishing a real-time tsunami early-warning system for South Africa. However, the deployment and maintenance of these instruments is a considerable challenge that will require dedicated effort. A workshop to discuss these options is planned for 2021.
The first two seismic events along the South African coast on 26 September 2020 sparked fear and curiosity among residents and coastal managers. This was verified by sea-level data recorded in the UNESCO sea-level station monitoring facility located at Marion Island and the South African Weather Service (SAWS).
Within the last twenty years, two destructive and deadly tsunami events have taken place. The 2004 Indian Ocean tsunami was recorded 12 hours after the initial rupture at the Gqeberha (Port Elizabeth, South Africa) tide-gauge with an anomalous wave of 2.9 m above mean sea level. These events and other extreme marine waves such as storm surges have increased awareness and interest among scientists and coastal managers in these unavoidable and often unpredictable natural coastal hazards.
However, more recently there has been renewed concern with the occurrence of at least four earthquakes within a space of five months along the South African coast. This was followed by a magnitude 2.5 earthquake in Cape Town at least 90 minutes after the first earthquake. Seismic tremors were reportedly felt up to 100 km from the epicentre, but no damages were reported.
The magnitude 6.2 and 5.1 earthquakes occurred on divergent plate boundaries separating the African and Antarctic plates to the west of South Africa’s Prince Edward Islands in the Southern Ocean.
We anticipate that findings from this PhD research and establishment of a tsunami early-warning monitoring system have the potential to assist in the development of extreme coastal hazards mitigation plans for the ever-growing coastal population of almost eight million people living along the South African coast.
The table below shows the largest tsunamis in South Africa since 1969:
| Date | Height (meters) | Fatalities |
| August 27, 1969 | 2.9 | 0 |
| December 26, 2004 | 1.5 | 3 |
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