Stephen Hicks | Seismologist
  • Home
  • Research
    • Imaging fluids in the Caribbean subduction zone
    • Mantle flow in a subduction zone
    • Structure of a subduction megathrust
    • Earthquake sources
    • Full waveform earthquake simulations
  • Communicating science
    • Journal Articles
    • Conference Presentations
    • In the Media
    • International Seismology Day
  • Blog: Tectonic Debates
  • Earthquake Resources
    • Animations
    • Recent Earthquake Teachable Moments
    • UK Moment Tensor Solutions >
      • M4.3 Jersey, Channel Islands - 11 July 2014
      • M4.1Bristol Channel - 20 Feb 2014
      • M4.2 Ramsgate, Kent - 22 May 2015
    • Technical notes
  • CV
  • Get In Touch

Rupture along a billion-year-old plate boundary in Botswana?

4/4/2017

53 Comments

 

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.
Picture
Map showing earthquakes of magnitude 6.5 and greater that have occurred in Southern Africa since detailed records began (1900 onwards).
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.
Picture
Picture
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.
PictureRecorded earthquakes in Botswana from the USGS catalogue

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.

A quick interpretation of the Lobatse seismograph showing the #botswana #earthquake & its aftershocks @ALomaxNet @seismo_steve @LastQuake pic.twitter.com/qtCKRMjATk

— J H Gurney (@UKEQ_Bulletin) April 3, 2017

Possible complexity in the rupture

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.
Picture
Multiple peaks in the rupture function indicate multiple fault ruptures. Source: Geoscope / IPGP (http://geoscope.ipgp.fr)

@seismo_steve @UKEQ_Bulletin Here station GNI at 65deg - seems to agree with SCARDEC: some complexity in an ~10s rupture. https://t.co/ASQY70IflJ https://t.co/fYullzbKfG pic.twitter.com/todwide5eg

— Anthony Lomax □ (@ALomaxNet) April 3, 2017
Picture
Displacement seismic waveforms from a recording station in Armenia. Some complexity within the first 10 seconds of rupture (circled) can be seen.

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.
Picture
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.
Picture

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.
Picture
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.

Further information
  • Earthquake Report from Dr. Jay Patton
  • USGS earthquake page
  • EMSC earthquake page
  • UC Berkeley Seismo Blog
53 Comments
Januka
4/4/2017 15:51:49

Very nice summary Stephen!

Reply
Stephen Hicks
5/4/2017 08:06:40

Thank you for your comment!

Reply
Adele
4/4/2017 18:30:06

Excellent article. Thank you.

Reply
Stephen Hicks
5/4/2017 08:06:59

Thank you!

Reply
Ana Admira link
4/4/2017 19:20:35

that is an important information

Reply
Stephen Hicks
5/4/2017 08:11:56

Thank you for reading!

Reply
Flip Nothling
4/4/2017 22:58:37

Thank you. Interesting article.
Could the earthquake be related to the Limpopo-Okavango rift which developed and stopped, with the break up of Gondwana? The earthquake appears to be to the south of the dykes but the extension seems to have the correct direction. (I am not a geologist.)

Reply
Stephen Hicks
5/4/2017 08:16:27

Interesting points - thanks for commenting. It's possible that the earthquake might have been partly induced by Limpopo-Okavango. These will be part of the scientific questions that need to be asked. You certainly sound like a geologist!

Reply
Evan Proxie
5/4/2017 02:13:46

Please get deformation data. Maybe it will provide clues to what exactly happened. This is very puzzling indeed. Possibly a new fault line is being created or an entire tectonic plate is about to split. Hidden super volcano maybe I am still looking at data

Reply
Stephen Hicks
5/4/2017 08:14:05

Interesting points - thanks for reading!

Reply
Penny Rees
5/4/2017 06:39:09

Most informative, thank you. Has anyone considered the fact that evidence points to seismic activity resulting from fracking and that fracking is now being g undertaken in Botswana

Reply
Stephen Hicks
5/4/2017 08:14:50

Interesting, but I don't think fracking could directly produce such a large magnitude earthquake. Thanks for reading the post!

Reply
Paul Gray
5/4/2017 08:24:51

It is doubtful that fracking at this stage, has anything to do with this event, but it is most interesting.
There appears to be the geological faults, and additionally, areas of ancient magma outpouring, as well as extinct volcanoes and pipes. Some seem to relate to fault areas and others quite remote.

Reply
Dion van Zyl link
5/4/2017 07:02:24

Yes as mid-west USA and the UK have only experienced these recently since the advent of Fracking?...
Certainly not a geologist but 2 and 2 are bound to add up pretty close to 4,..
Overlaying maps could assist,. http://www.survivalinternational.org/news/9762
Thanks for the insights Steven!

Reply
Stephen Hicks
5/4/2017 08:17:08

Interesting point, but I don't think fracking could directly produce such a large magnitude earthquake. Thanks for reading my post.

Reply
Peter Bateman
6/4/2017 00:53:07

The fracking that was carried out in Botswana had been carried out at a depth of 300 - 400 meters a few years ago in sedimentary and therefore more plastic material. Fracture propagation is generally horizontal in this type of material following along material with a similar overburden pressure and similar material competency. This tectonic quake has happened at a suggested depth of 30,000 meters in a totally different material in a different location.

Penny Price
5/4/2017 08:19:05

Thank you for pulling together this very informative article so quickly. Appreciated.

Reply
Stephen Hicks
5/4/2017 19:54:56

Hi Penny, thanks for your comment.

Reply
Raf
5/4/2017 08:41:31

Very interesting and accurate summary. Bye from Italian geographers

Reply
Jerome Dyment
5/4/2017 08:45:49

The East African Rift does not separate the African and Indian plates, as mistakenly written, but the Nubian and Somali plates which together were earlier seen as a single African plate.

Reply
Stephen Hicks
5/4/2017 10:52:26

Hi Jerome

Many thanks for your comment and providing your expertise. Yes you were correct - I have now changed the text to reflect this.

Thanks again,
Stephen

Reply
Jérôme Dyment
7/4/2017 14:02:45

Great. The problem stll appears in the figure of update 1. Good job by the way, please continue!

David
5/4/2017 10:17:54

Great article. I can't clearly make out from the map if the 12 hours earlier South African earthquake was on the same ancient plate boundary.

Reply
Stephen Hicks
5/4/2017 10:55:31

Hi David

Thanks for your comment. I don't see any evidence that the Botswana quake was in any way related to the SA event. They are too far apart and the SA quake was much smaller.

Stephen

Reply
Bokamoso link
5/4/2017 15:25:45

I'm reading this from Botswana, very interesting. But in Moiyabana village Steve, where the quake initially started, it left a rapture in the ground. Does that therefore mean there is high possibility of yet another quake. Um saying this because the quake happened yet again in the early hours of the morning today

Reply
Stephen Hicks
6/4/2017 08:22:27

Dear Bokamoso

There will certainly be aftershocks, but unfortunately we cannot tell when and where the next one will occur. If you have a photo of the rupture, feel free to email it to me at s.hicks@soton.ac.uk

Thanks
Stephen

Reply
Desmond Peach
5/4/2017 16:32:36

Thank you for a splendid explanation of the situation in Botswana and surroundinding countries pertaining to the recent earth quake. The weanessesi in th Craton layer and the effect of gravity.on the stress plates was most interesting.

Reply
Stephen Hicks
5/4/2017 19:55:39

Desmond, thank you for commenting - I'm glad the post was understandable for you.

Reply
Abraham
5/4/2017 18:46:25

I thought the Tectonics are drifting towards North East in Africa. ..?Nice article and informative though.

Reply
Gaolatlhe
5/4/2017 19:25:57

Thanks Stephen for this eye opening article.

Reply
Gwandu
5/4/2017 19:28:17

Hi Steve thanks for the informative article. I am a geologists at Botswana Geoscience Institute. I suggest we should wait for data from all local seismic stations to accurately locate the epicentre. Then post earthquake high res satellite images can be used and compared with pre-earthquake images to check for any ground changes. Also both gravity and magnetic survey can follow ti check any differences in terms of subsurface structures.

Reply
Stephen Hicks
5/4/2017 19:57:19

Hi Gwandu.

Many thanks for taking the time to comment. This article was originally meant for a scientific audience but I'm glad it has stimulated some public interest. I will send you an email separately.

Many thanks,
Stephen

Reply
Mark Green
5/4/2017 21:20:51

Hi

I just wanted to say that this is the best science article I've read in ages. Obviously written by an expert in the field but also incredibly well explained to the lay reader.

You should start firing this post off to every popular scientific journal in the world because you're good at this.

Reply
Stephen Hicks
6/4/2017 08:20:49

Hi Mark,

Thank you very much indeed - that is very flattering to hear :).

Stephen

Reply
Christiaan Botha
7/4/2017 09:58:33

Hi Stephen, I think I was as surprised as Mark to learn that it was written for the scientific guys. If more scientific people write like this, we that have near no knowledge will read more of these articles as well. But, I'm sure some of your buddies appreciate the trend in which you did write it as well. Thereby I do not claim to understand everything, But thank you very much

Reply
Phyllis Elias
6/4/2017 08:15:01

Thank you for the clearest article I have ever had the pleasure of reading. I am sure that the children of Botswana will find it totally absorbing, since most of them will never have experienced a seismic event, but will be agog to know all they can about this one. I am passing the link to friends who are teachers in that wonderful country, as I know they will find it a huge help too.

Reply
Stephen Hicks
6/4/2017 08:20:14

Dear Phyllis

Many thanks for your kind comment - I hope you found it useful.

Stephen

Reply
Thabiso link
6/4/2017 10:27:38

This is the most informative article on this earthquake that i have come across so far. I have read Ranganai et al 2002 before from doing my research at ORI. Used to call it the genesis paper about the Okavango. So i think the overlaying of the maps maybe on to something. I love Gwandu's thoughts too. Hope you both get in touch.Keep us updated.

Reply
Abel link
6/4/2017 13:44:21

Good work and many thanks.
The actual epicenter is currently not accessible due to heavy sand and thick bushes. Based on the geological setting of that area is the faulting can either be strike slip fault or normal fault since is on extensional zones. At the moment is only speculations and assumptions. More reality will be revealed as team reaches the actual area. I was 170km when it happened.

Reply
Kenneth Toteng link
6/4/2017 18:28:28

Very interesting indeed. This quake as reported by neighbouring residents to the epicentre was more like a volcano that failed. They reported of a very loud explosion that occured there.

Reply
Fulvio
6/4/2017 21:12:49

Excellent summary. We've been in the field right after the earthquake and I can confirm that there is no rupture at the surface. There is a thick drape of quaternary sands in the area, at least 120/130 meters.
The epicenter is along a well known shear zone that runs across the country, it is surprising though that such an old structure can be reactivated by distant extensional tectonics. I don't believe in the direct link between fracking and shaking... but what about all the blasting carried in the area by the mines? As shown in your map in the mining areas of SA mine-related earthquakes are common. Am I wrong?

We might be back with some geophysical equipment to check if we can imagine the displacement.

Reply
Stephen Hicks
7/4/2017 06:42:48

Dear Fulvio

Thank you so much for your insightful comment. It would be great to collaborate on some studies on the earthquake. I am trying to apply for some funding in the UK to help support the effort. Feel free to send me an email at s.hicks@soton.ac.uk.

Thanks,
Stephen

Reply
Abel link
10/4/2017 10:28:10

Hi Fulvio
Just a question here regarding your comments.
1. Did you say you actually went to the epicenter and found no ground rapture?
I went to the area and we failed to reach the epicenter due to thick sand and thick bush cover.
I however been as a close as 50km from the area and there are visible cracks on the ground through the Kalahari sand cover. The Gope settlements which is also in close proximity to the epicenter has also suffered serious ground cracks. Surely the ground has suffered some 2 degree rapture for the quake.

Reply
Fulvio
10/4/2017 11:02:49

Hi Abel,
We drove along the CKGR fences and no signs of displacements or fractures. We were 5 km away from the epicenter. Unfortunately we've got there late in the afternoon and didn't have much time for exploring around.
We heard of the damages at Gope. Several houses have been damaged also in Moiyabana 120 km away from the epicenter.

Can you share the coordinates of the fractures you have found?

Stephen Hicks
10/4/2017 12:11:40

Dear Abel,

Thank you for noticing the possible rupture. If you took any photos, could you please send them to me at s.hicks@soton.ac.uk please? I can send post them on the blog for many people to see.

Thanks,
Steve

Paul Masilo
6/4/2017 23:11:25

Please update us after you get more data regarding the closely spaced fault rapture.
Usually these plate faults tend to collide and slide over each other and not reverse as it might be the case in Kaapvaal and Zimbabwe cratons. Also if the Mahalapye shear zone had a direct impact or relationship with 7.5 quake in April 2017

Reply
Trevor Thompson
9/4/2017 06:29:09

Most interesting discourse......

Reply
Ellie Martin
9/4/2017 19:27:51

Hi, I'm not a geologist or anything remotely related, I just find this sort of thing very interesting. I check the USGS quake site a couple times a day. I noticed the cluster of quakes in Botswana this morning, then went on Google to see what was up. Before I found your article, I learned about the Okavango Delta. Living in the Prairies in Canada, I found the idea that even geographically stable areas can have big quakes a tad unnerving. LOL I find this all quite fascinating.

Reply
Harold Van Zyl
19/4/2017 08:52:45

Hi Stephen,

Very informative article. Last year May when my wife and myself drove through the village of Hatsalatladi about 80 km south of the epicenter, there was a massive rupture which ran obliquely across the road and disappeared into the bush. This rupture could only have been cause by natural phenomenon. I followed the fracture for about 500 m. Took some photos of the structure, even one with my wife standing waist deep in one of the trenches. I can make these photos available to you, it does seem like we are dealing with a reactivation of some very old tectonic plates in the area. We don't really expect that kind of activity in this region. Most earthquakes here are associated with the deep gold mining in south Africa and some activity in the northeastern part of the country as a result of the southern most part of the African Rift Valley Structure.
Regards.

Harold.

Reply
M. Gabadirwe
12/8/2017 08:06:43

Good morning. Another significant tremor felt this morning at 4.37am of 4.8 magnitude near Sojwe-Botlhapatlou not very far from a major April 4th 2017 earthquake. Could these two be related in terms of what could be happening in this area? Isn't it high time for Botswana Geoscience Institute to put up a siesmic station around this area to monitor these geologically strange occurrences in Botswana.This frequency is worrying.... surely some geological phenomenon is going on and needs explanation.

Thank you.

Mohutsiwa

Reply
John Heldsinger
1/11/2017 18:17:25

Hi from Gaborone..very informative. We experienced another quake today 01/11/2107 at 1412..4.7 mag. This is now the 7th quake in 7 months. Any reason for the increased activity? And will it continue?

Reply
John Cruise
2/11/2017 07:28:36

Just a minor typo in your initial article. According to the US Geological Survey report, the earthquake happened Monday 3 April 2017, not 4 April which was a Tuesday.
As a South African mining engineer who has worked in South African gold mines and also was involved in the driving of prospect tunnels in the Gope pipe in 1995, I found your article most illuminating.

Reply
Cara Mengatasi Gemetaran (Tremor) Pada Tangan Dan Kaki link
7/11/2018 06:38:00

Good information, thank u and please visit back my article~!

Reply



Leave a Reply.

    Tectonic Debates


    About

    This blog focusses on a wide range of aspects in earthquake science including earth structure, induced seismicity and hazard communication.

    Archives

    April 2017
    March 2017
    September 2014
    April 2014
    February 2014

    Categories

    All
    Africa
    Botswana
    Cascadia
    Cascadia Initiative
    Doublet
    Earthquake
    Earthquake Early Warning
    Earthquakes
    Episodic Tremor And Slip
    ETS
    Japan
    Kaikoura
    Mexico
    New Zealand
    Ocean Bottom Seismometer
    Pacific Northwest
    Philippines
    Science Communication
    Seismic Hazard
    Slow Slip Events
    Tsunami

    RSS Feed

Powered by Create your own unique website with customizable templates.