Largest earthquake to hit Africa for 11 years
On Monday 4 April 2017 at 17:40 UTC, a magnitude 6.5 earthquake struck in central Botswana, Africa. The rupture occurred at an approximate depth of 30 km (19 miles). Very strong shaking (Intensity 7) was felt close to the epicentre and weak shaking (Intensity 3) was reported 500 km away in Johannesburg, South Africa. This earthquake is the largest to have struck mainland South Africa for over eleven years.
Map showing earthquakes of magnitude 6.5 and greater that have occurred in Southern Africa since detailed records began (1900 onwards).
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The largest earthquakes in southern Africa are concentrated in the eastern countries of Tanzania, Malawi, and Mozambique, where the continent is slowly pulling apart (rifting) in an east-west orientation. This rifting forms the boundary between the Nubian and Somali plates (part of the larger African plate) and results in normal faulting earthquakes. For example, the 2006 magnitude 7.0 Mozambique earthquake, which caused several fatalities, was a normal faulting earthquake, which ruptured the southern end of the East African Rift.
The April 2017 Botswana event is also classified as a normal faulting earthquake, yet its focal mechanism, which shows the orientation of extension, is approximately perpendicular to the East African rift. Therefore, a different geological structure, which may somehow be broadly related to the East African rift, may have been responsible for this earthquake. Earthquakes in Botswana are extremely rare. Given that this event has occurred over 1000 km from the nearest tectonic plate boundary, we call these types of events 'intraplate earthquakes'. It is likely that the rupture occurred partly due to the gradual transfer of push and pull stresses from the East African Rift toward the more stable part of the continent. Occasionally, this stress is released along pre-existing weaknesses in Earth's crust as earthquakes. It is fundamentally the same reason why quakes occasionally occur in other stable regions such as the United Kingdom and the midwestern states of North America. |
Top: Focal mechanism ('beach-ball') for the Botswana earthquake indicating extension in anortheast-southwest direction. Bottom: schematic block diagram showing sense of crustal movement during normal faulting.
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Recorded earthquakes in Botswana from the USGS catalogue
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As with all earthquakes, there will be some aftershocks of this rupture. From the nearest real-time seismometer station located ~270 km away from the epicentre, smaller-sized aftershocks can be clearly seen occurring in the hours ensuing after the mainshock.
Two aftershocks have been recorded by Germany's GEOFON monitoring system. One of these aftershocks had a magnitude of 4.6; the other magnitude 4.1. Based on Bath's aftershock law, it is plausible to expect aftershocks as large as magnitude 5.4. |
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Possible complexity in the rupture
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Early estimates of the earthquake's depth suggest that it occurred deep within the continent (in the mid-lower crust). Therefore, there is likely to be very little data available from possible surface ruptures.
When a large global earthquake occurs, the GEOSCOPE seismic observatory, based at IPGP in Paris, automatically calculate rupture 'source-time functions'. These graphs show how the amount of energy released by the earthquake changes during the rupture.
The GEOSCOPE solution (right) shows at least two distinct peaks in energy release, possibly indicating that the earthquake was composed of multiple, distinct ruptures.
This complex source time function can also be seen from the rare seismic waveforms from distant stations.
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Multiple peaks in the rupture function indicate multiple fault ruptures. Source: Geoscope / IPGP (http://geoscope.ipgp.fr)
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Displacement seismic waveforms from a recording station in Armenia. Some complexity within the first 10 seconds of rupture (circled) can be seen.
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Stresses from Eastern Africa?
The central and western areas of Southern Africa are assumed to be tectonically very stable. The geology of the region is typically referred to as the 'Kaapvaal Craton'. Craton means a strong region of a continental tectonic plate that are typically stable for over a billion years. The Kaapvaal Craton contains rocks that are 3.6 to 2.5 billion years old. Large earthquakes in such stable tectonic environments occur very rarely.
A large quake in Botswana is puzzling because it is made of Proterozoic and Archean rocks, the oldest and generally most stable on Earth
— Caroline Beghein (@caro_aniso) April 4, 2017
Comparing the location and mechanism of the Botswana earthquake with a regional geological map reveals that the earthquake's epicentre may have occurred close to the boundary between Kaapvaal rocks and those of the Limpopo Belt - a distinct group of metamorphosed rocks with a slightly younger age of ~1.8 billion years old. This may simply just be a coincidence. Furthermore, given the lack of nearby seismic stations, this exact location may be uncertain. Nevertheless, it is worth considering what implications such a relationship may have.
2017 Botswana earthquake epicentre (red star) together with focal mechanism (blue beach ball) plotted on a regional geological map from Brown et al. (2008). The red arrows indicate the direction of horizontal extension during the earthquake.
Dr Eddie Dempsey is a geologist from the University of Durham; he specialises in analysing the structure of rocks that have been deformed deep within the crust. He agrees that there may be an influence on the stresses and resulting strains caused by rifting in East Africa. Stress may eventually be released along old but relatively weak tectonic suture zones to form occasional quakes like the Botswana event. Such old cratonic rocks, however, are typically very layered (high seismic anisotropy), which may complicate our analyses of this quake using seismic waves.
@EC_Kosters @SeismoSue @seismo_steve But this may also be purely coincidental. The lack of nearby seismic stations together complexity of the African crustal anisotropies make
— tectonictweets (@tectonictweets) April 4, 2017
@EC_Kosters @SeismoSue @seismo_steve The orientation of the focal mech is consistent with the regional fabric so there may be reactivation of ancient basement structures
— tectonictweets (@tectonictweets) April 4, 2017
Re-activation of an ancient tectonic boundary?
One research paper from the 1980s studied the nature of the geological contact between Kaapvaal and Limpopo further to the east in South Africa. Based on small changes in gravity across the two groups of rocks, the study interpreted that this geological contact demarcates an ancient (pre-Cambrian) plate boundary between two colliding plates.
Is it therefore possible that the inherent weakness of this ancient, billion-year-old plate boundary ruptured during the 2017 Botswana earthquake? Whilst the craton as a whole stands strong, discrete weak points could still be exploited by shifting tectonic stresses. Yet our first estimates of the earthquake's location are somewhat uncertain; therefore, much more research including on-the-ground field studies, as well as analysis of satellite and geophysical data will be needed in the next few months to better answer this question.
What we do know though is that earthquakes caused by intraplate deformation will always be surprising to us; they have the potential to produce very large and damaging earthquakes. Dr Susan Hough, a seismologist at the United States Geological Survey says that stable continental crust may occasionally produce earthquakes as large as magnitude 7.5.
@caro_aniso Magnitude 7 or maybe 7.5 is a typical Mmax estimated for "stable continental crust."
— Susan Hough (@SeismoSue) April 4, 2017
Everyone around the world, even those living in the most stable tectonic areas, should try to familiarise themselves with some simple procedures to follow if they experience shaking caused by an earthquake should the unexpected ever occur.
Update 1 (05/04): Interplay between tectonics and earthquake activity in southern Africa
I have made a quick map showing the distribution of ridges, rifts and micro-plates in Southern Africa and how these relate to earthquake activity in the region. Overall, the picture is very complicated but shows the main network of structures that may have contributed to the Botswana earthquake.
Update 2: Gravity sheds light on the possible fault(s) that ruptured
I have found a research paper from 2002 written by Ranganai et al. that maps small changes in gravity over south-eastern Botswana to interpret the location of ancient geological provinces and faults.
The April 2017 earthquake occurred in a highly deformed region separating the Kaapvaal Craton from the Zimbabwe Craton. The paper calls this region the Limpopo–Shashe Belt. This belt is composed of many northwest-southeast trending shear zones and reverse (thrust) faults that were formed as the Kaapvaal and Zimbabwe cratons slammed into each other billions of years ago (during the Archean eon). The authors speculate that this collision might have been similar to the same mountain building process that has resulted in the present-day Himalayan chain.
As the map shows below, the Mahalapye Shear Zone may be related to the fault that ruptured in April 2017. The orientation of the fault is parallel to the orientation of the focal mechanism. However, the rupture was extensional - not compressional like the mapped fault. Therefore, it is possible that the regional extensional rift tectonics in Southern Africa have reverse the sense of motion of these faults in a so-called stress inversion.
The April 2017 earthquake occurred in a highly deformed region separating the Kaapvaal Craton from the Zimbabwe Craton. The paper calls this region the Limpopo–Shashe Belt. This belt is composed of many northwest-southeast trending shear zones and reverse (thrust) faults that were formed as the Kaapvaal and Zimbabwe cratons slammed into each other billions of years ago (during the Archean eon). The authors speculate that this collision might have been similar to the same mountain building process that has resulted in the present-day Himalayan chain.
As the map shows below, the Mahalapye Shear Zone may be related to the fault that ruptured in April 2017. The orientation of the fault is parallel to the orientation of the focal mechanism. However, the rupture was extensional - not compressional like the mapped fault. Therefore, it is possible that the regional extensional rift tectonics in Southern Africa have reverse the sense of motion of these faults in a so-called stress inversion.
Gravity map of the southeastern Botswana region overlain with interpreted fault locations and the 2017 Botswana earthquake. Gravity map comes from Ranganai et al. (2002).
With the possible complexity of this earthquake that was suggested by the early data, it is possible that more than one of these closely-spaced faults ruptured during the magnitude 6.5 earthquake. More detailed analysis will be needed to look at this possible scenario.
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