Central Botswana Research Articles

Human health risks from multiple exposure pathways of potentially toxic elements in nickel–copper mine tailings, Central Botswana

Human health risks from multiple exposure pathways of potentially toxic elements in nickel–copper mine tailings, Central Botswana

An ethnopharmacological survey of medicinal plants traditionally used by the BaKalanga people of the Tutume subdistrict in Central Botswana to manage HIV/AIDS, HIV-associated conditions, and other health conditions

Ethnopharmacological relevanceWhile access to antiretroviral therapy (ART) continues to improve worldwide, HIV infection and AIDS persist as serious health challenges, particularly in sub-Saharan Africa. Complementary and Alternative Medicines (CAM), as part of indigenous and pluralistic medical systems, are important contributors to primary health care worldwide. However, this knowledge remains relatively undocumented in many parts of sub-Saharan Africa such as the Tutume subdistrict of Central Botswana, where CAM is widely used including potentially for HIV/AIDS and HIV-associated conditions. Aim of the studyTo explore the extent to which CAM is used by the BaKalanga Peoples of the Tutume subdistrict, we performed an exploratory community-based project to record medicinal plant use from this relatively undocumented region, with a particular focus on species used for management of HIV/AIDS and HIV-associated conditions. Materials and methodsUsing the snowball sampling technique, we recruited 13 Traditional Health Practitioners (THPs) and conducted in-depth interviews to explore medicinal plant uses and treatment regimens. Plant specimens were collected and bio-authenticated. ResultsWe documented 83 plant species used as CAM to treat or manage a variety of conditions including HIV/AIDS, HIV-associated conditions, and other health conditions. Plants from the family Leguminosae were most frequently reported, comprising 21 species (25.3%), followed by 5 from both Euphorbiaceae and Combretaceae families (6.0%). Four plants (4.8%) were used specifically to manage HIV (Lannea edulis (Sond.) Engl. root, Aloe zebrina Baker root, Myrothamnus flabellifolia Welw. whole plant, and Harpagophytum procumbens var. subulobatum (Engl.) tuber), while an additional 7 (8.4%) were reported specifically for treating combinations of HIV-related symptoms. Notably, 25 (30.1%) have not been reported previously as CAM and/or lack reported bioactivity data. ConclusionsTo our knowledge, this is the first detailed ethnobotanical survey of CAM used by the BaKalanga Peoples of the Tutume subdistrict to manage HIV/AIDS and HIV-associated and other health conditions.

Characterisation of the Hydrogeological Properties of the Ntane Sandstone Aquifer Using Co-Seismic and Post-Seismic Groundwater Level Responses to the Mw 6.5 Moiyabana Earthquake, Central Botswana

The 3 April 2017 Mw 6.5 Moiyabana earthquake (Central Botswana) had a significant impact on groundwater levels; a gradual co-seismic increase and a stepwise decline in groundwater levels were observed in response to the earthquake at boreholes MH2 and Z12836, respectively. In this study, we investigated the response of groundwater levels to Earth tides by computing the amplitude and phase shift of the M2 tidal constituent to estimate the temporal variations of the storativity, transmissivity, and permeability of the Ntane sandstone aquifer (the main aquifer system) prior to and after the earthquake event. The storativity and permeability computed for borehole MH2 showed a decrease in magnitude of 3.17432 × 10−4 and 1.85 × 10−13 m2 respectively, indicating that strong ground shaking at borehole MH2 might have consolidated the aquifer material, thus resulting in decreased aquifer permeability. The aquifer coefficient of storativity decreased by 2.85 × 10−4 while permeability was enhanced by 0.047 × 10−13 m2 at borehole Z12836. Enhanced permeability might have resulted from increased/enhanced fracturing of the aquifer, fracture clearing and dynamic shaking.

Morupule fly ash as amendments in agricultural soil in Central Botswana

The potential of Morupule fly ash as a soil amendment was evaluated using spinach and rape vegetables planted successively in Palapye soils added with 0 fly ash (FA), 5% FA, 10% FA, and 15% FA (w/w) in combination with four nutrient sources as no fertilizer (-NF), compost (+COMP) at 20 tones ha −1 , inorganic NPK (+NPK) and a combination of 1 2 NPK and 1 2 compost (+NPK+COMP). Spinach was grown first for 40 days followed by rape as a second crop planted on the same pots and treatments after harvesting spinach. The application of FA increased the pH, EC, and the total concentration of As, Cu, and Pb of soil while compost increased the total Zn. The available concentration of Cr, Cu, Mn, Ni, and Pb were significantly reduced by FA. FA increased the yield of spinach and rape, especially in 10FA+NPK+COMP. The heavy metal concentration in the shoots of spinach and rape were below the maximum acceptable limits (MAL) for vegetables in 10FA+NPK and 10FA+NPK+COMP treatments. Overall, an application rate of 10% FA combined with compost and fertilization gave the highest yield and safe level of As, Cr, Cu, Mn, Ni, Pb, and Zn in the edible shoots. • First report on using Morupule fly ash as an amendment in agricultural soils in Botswana. • A combined with compost and inorganic fertilizer improved the growth and yield of spinach and rape grown successively. • Fly ash amendments reduced the available concentration of Cr, Cu, Mn, Ni, and Pb in soil. • Fly ash at 10% application combined with compost and inorganic fertilizer produced spinach and rape safe for human consumption.

Lacustrine geoarchaeology in the central Kalahari: Implications for Middle Stone Age behaviour and adaptation in dryland conditions

The Middle Stone Age (MSA) was a time of great human adaptation and innovation. In southern Africa, coastal locations have been viewed as key places for the development of human resource use and behaviour, with the dryness of the continental interior after c.130 ka regarded as both an obstacle to occupation and a limit on behaviour. Newly excavated MSA sites on the floor of the now-dry palaeolake Makgadikgadi basin, central Botswana, along with accompanying environmental data, have provided a significant opportunity to reassess the nature of MSA adaptation to, and behaviour under, dry conditions. Excavated sites dated to 80–72 ka and post 57 ka reveal purposeful early human use of an extensive 60,000 km2 lacustrine basin during dry, as opposed to lake-high, phases, as well as highlighting movement strategies for tool-making resource procurement. Findings have significant implications for theories of early human mobility and innovation, as well as for understanding the drivers, constraints and opportunities for the use of drylands. The deliberate selective movement of lithic raw materials within the basin for artefact manufacture evidences thoughtful adaptation to dry conditions within the lake basin. This research shows that open-air sites in the Kalahari drylands of central southern Africa can make important contributions to debates surrounding the development of human-environment relationships during the MSA, as well as challenging narratives of a hostile and largely empty landscape.

Rifting of the Kalahari Craton Through Botswana? New Seismic Evidence

AbstractBotswana is a country with relatively low seismic activity that experienced an unexpected Mw 6.5 earthquake on 3 April 2017. Using data from the first countrywide seismic network, we established a Botswana earthquake catalog for the period January 2014 to February 2018. Two areas of elevated seismic activity were detected. The first one is the Okavango Rift Zone in northern Botswana, an area that is known to be active. The other one is associated with the 2017 mainshock and its aftershocks in central Botswana; it follows the Paleoproterozoic suture between the Limpopo Belt and the Kaapvaal Craton. Double‐difference relocation of these events revealed a reactivated fault system with a northwesterly strike with the aftershocks occurring at shallower depths than the mainshock at 29 km. The focal mechanisms of the mainshock and selected aftershocks are of normal faulting type with strikes similar to the orientation of the fault system. The unidirectional rupture of the mainshock in the lower crust combined with the upward migration of the aftershocks along the Moiyabana Fault and a thin low velocity anomaly the uppermost mantle are consistent with the events being produced by eclogitization of a dry metastable granulite facies rock by fluid intrusion from the mantle in an extensional stress regime. The Okavango Rift Zone is generally interpreted as the terminus of the southwestern branch of the East African Rift System. We suggest that the recent earthquakes in central Botswana may be considered as the southern continuation of this branch.

Predictive mapping of soil copper for site-specific micronutrient management using GIS-based sequential Gaussian simulation

Given that soil properties including micronutrient contents vary in space and time, geospatial mapping is necessary for robust site-specific management planning. Kriging interpolation is often used in GIS environments for mapping, but it has inherent limitation of having a smooth effects. In this study, sequential Gaussian simulation (SGS) was used to map the spatial distribution of Cu concentration in and model the spatial uncertainties for an arable dryland in central Botswana. The field was divided into 30 parallel lines in an NE–SW orientation. Soil samples were collected at intervals of 25 m on each line; and 35 sampling points were obtained. A total of 1050 soils sampled at a depth of about 20 cm were air dried and analyzed using an Olympus Delta Sigma® portable X-ray fluorescence analyser. The average concentration of Cu (146 mg kg−1) in the Maibele Airstrip of Botswana is higher than the global average copper concentration (30 mg kg−1). Low copper contents were found dotted around the northern and southern edges of the study area, while high content zone is found in the interior. In comparison to kriging interpolation, SGS technique shows better performance. Kriging generally overestimated where lower values are probable and underestimated where higher values are probable. A large portion of the area has Cu content above the critical threshold of 125 mg kg−1. Since total and not plant available Cu were the measured parameter, we recommend that further study in this location should focus on confirming the plant available Cu in the high-risk areas.

92 Sustainable intervention for the improvement of goat health and socio-economic outcomes in rural communities in central Botswana

92 Sustainable intervention for the improvement of goat health and socio-economic outcomes in rural communities in central Botswana

Layered mound, inverted channels and polygonal fractures from the Makgadikgadi pan (Botswana): Possible analogues for Martian aqueous morphologies

Layered mounds and inverted channels with polygonal fractures from the Ntwetwe Pan in the Makgadikgadi Basin (central Botswana) have been herein investigated. These morphologies are from an evaporitic basin (the Makgadikgadi Basin) that is the remnant of an ancient Pleistocene lake and is currently part of the world’s largest evaporitic system.The mounds in the Ntwetwe Pan are characterized by a layered structure and low relief (max. 5 m above the pan floor) and can be in excess of 2 ​km wide. The mounds consist mainly of loose (non-lithified) sand and silt with high moisture contents, even during the dry season. Geophysical investigations have shown that groundwater processes, particularly those related to the capillary fringe that rises and conveys moisture through the mounds, are factors that make mound sediments resistant to wind erosion.The inverted channels, identified in the southern part of the Ntwetwe Pan, are characterized by gentle reliefs and depressions, which depend upon the distribution of calcretes and indurated sediments. Large scale (up to 100 ​m wide) polygonal fractures localized at the front of the channels, disappear at the transition with the present-day pan floor.We consider that these particular mounds, within the Ntwetwe Pan, are remnants of the strandline of the paleo-Makgadikgadi Lake, and that the inverted channels represent distributary channels of a relict fan delta, formed by an ephemeral river, most likely the paleo-Boteti River, during a Lake Paleo-Makgadikgadi highstand stage. We consider that large scale (up to 100 ​m wide) polygonal fractures, located on the channel-mouth lobes, represent large-scale desiccation cracks formed by rapid water evaporation from delta deposits.The results of this investigation highlight the importance of the paleo-drainage system and its interactions with the water table and wind-deflation as main geomorphological factors within salt pan environments. The mounds in the Makgadikgadi pans also show strong geomorphic similarities to spring mounds on the surface of Mars, localized in equatorial layered deposits (ELDs). These ELDs mounds are considered to result from cyclical groundwater upwelling, evaporation and wind deflation. The geological processes that resulted in the formation of mounds within the Makgadikgadi pans may, therefore, help to explain how similar layered deposits formed on Mars and confirm existing theories.

Characterizing coal seams hosted in Mmamabula Coalfield, central Botswana using pseudo-3D electrical resistivity imaging technique

We used 2D and 3D electrical resistivity imaging (ERI) data inversion to locate and characterize the coal seams in Mmamabula Coalfield. The 2D ERI data inversion usually assumes that subsurface geological units (e.g. coal seams) are infinitely continuous in the direction perpendicular to the profiles. However, this assumption might be violated due to the heterogeneous nature of fluvial/deltaic environments. The lateral variation of the subsurface material can be effectively determined in 3D ERI inversion models. In this study, 2D ERI data were acquired along ten parallel profiles trending north-south, then inverted to produce 2D and 3D models. From the 2D ERI inversion results, we observed that the shallower coal seam occurs around a consistent depth of 13.6 m whereas the second observable coal seam occurs at variable depths including at 50 m and 60 m along the two profiles. Discontinuities in coal seams were also identified. However, the information provided by 2D data inversion in such an environment is insufficient to determine the geometry of coal seams as well as to produce accurate and precise resource estimates for future exploitation. As an alternative, 3D data inversion gives better insight on the subsurface geology complexity and significantly resolves the geometry of coal within the study area. The presented 3D inversion results indicate that coal seams are intermittent and alternate well with other coexisting sedimentary units. We also deduced that they trend east-west and are split and adjoined in some areas along the north-south direction. Overall, 3D data inversion result resolves the elongated coal seams in Mmamabula Coalfield and reveals additional information that cannot be determined on 2D ERI inversion results.

Crustal and Upper Mantle Shear Wave Velocity Structure of Botswana: The 3 April 2017 Central Botswana Earthquake Linked to the East African Rift System

AbstractRayleigh wave group and phase velocity measurements obtained from ambient noise and earthquake data at 51 broadband stations were used to construct the first 3‐D crustal and upper mantle shear wave velocity model of Botswana. The model shows low crustal velocities associated with the Passarge and Nosop sedimentary basins, whereas the Kaapvaal, Zimbabwe, Maltahohe, and Congo Cratons are recognized by high mantle velocities. The lowest upper mantle shear wave velocity, beneath northeastern Botswana, is associated with the southwestern branch of the East African Rift System. This low‐velocity mantle anomaly appears to be linked to the crust of the Okavango Rift Zone and the location of the 3 April 2017 6.5 earthquake in central Botswana. We suggest that fluids or melt at the base of the crust from the southward continuation of the East African Rift Zone triggered the intraplate earthquake in an extensional tectonic setting.

Depositional style changes during the Permo‐Carboniferous–Early Jurassic evolution of the Central Kalahari Karoo Sub‐basin, Botswana

The Permo‐Carboniferous–Lower Jurassic Karoo succession in the Central Kalahari Karoo Sub‐basin of Botswana is largely confined to the subsurface, and contains coal, coalbed methane and groundwater resources, similar to other Karoo strata throughout southern Africa. Characterization of the stratigraphic architecture (i.e., spatiotemporal stacking character of sedimentary facies) is essential for various aspects of subsurface resource prediction (e.g., resource parameters; reservoir architecture; mining feasibility). This study uses sedimentary facies analysis on borehole cores from central Botswana and sequence stratigraphic principles to resolve large‐scale depositional style changes during the evolution of the greater Kalahari Karoo Basin in southwestern Gondwana. The results show that changes in the Karoo stratigraphic architecture define two, large‐scale overall fining‐upward depositional sequences (i.e., Lower and Upper Karoo) that are marked by regional subaerial unconformities. These regionally mappable depositional sequences contain pairs of high‐ to low‐amalgamation units, which are typified by stacked, multistorey sandstone bodies, succeeded by finer grained units that are sometimes rich in coal (e.g., the Dukwi‐Makoro‐Kamotaka vs. the Morupule‐Serowe‐Tlhabala formations in the Lower Karoo; the lower versus upper Mosolotsane‐Ntane formations in the Upper Karoo). The large‐scale depositional style changes, reflecting base‐level fluctuations and variation in the balance between accommodation and sediment supply, resulted from climatic changes (e.g., Permo‐Carboniferous deglaciation), tectonic events (e.g., regional pre‐Mosolotsane uplift event), and marine incursions. The latter is documented, for the first time, via the marine‐influenced ichnofauna in the uppermost Ecca Group. Further refining this initial sequence stratigraphic framework for the Botswanan Karoo will require acquisition of radioisotopic dates (e.g., U–Pb zircon ages) and modern seismic reflection data.

Structural equation modeling of the interactions between trace elements and soil organic matter in semiarid soils

The bioavailability of trace elements is strongly influenced by the physical and chemical properties of the soil system. This study sought to establish the relationships between the trace elements (barium, strontium and rubidium), soil organic matter and soil separates in semiarid soils from Central Botswana in a bid to improve our understanding of trace elements biogeochemistry. Trace element contents were obtained using portable X-ray fluorescence spectroscopy, while routine soil analysis methods were used for soil organic matter analyses and particle size distribution. Both multivariate statistical analysis and structural equation model were applied to the data. Results for correlation matrix and principal component analysis showed trace elements affinity for silt, clay and soil organic matter: barium and silt (r = 0.54, p < 0.01), rubidium with clay (r = 0.69, p < 0.001) as well as strontium with stronger affinity for clay (r = 0.83, p < 0.001) and soil organic matter (r = 0.88, p < 0.001). Principal component analysis showed that trace elements interactions with silt, clay and soil organic matter were observed more in profiles 3 and 7. Three structural equation models were obtained and in two of them, soil organic matter showed statistically highly significant direct effect on strontium which is further supported by all the multivariate statistical analysis results. Further research may be necessary to understand detailed aspects (e.g., complexations) between strontium and soil organic matter in semiarid soils.

Some Foreshocks and Aftershocks Associated with the Moiyabana Earthquake of 2017 in Botswana

On the 3rd of April 2017, an earthquake of moment magnitude 6.5 occurred near Moiyabana in central Botswana. This paper is aimed at studying the spatial distribution of the foreshock and aftershock sequences associated with the Moiyabana earthquake. The foreshocks and aftershocks data used were from the Botswana Geoscience Institute (BGI) and the Seisan software was used to analyze the foreshock and aftershock events. The analyses revealed nine epicenter locations of foreshocks which are spread out across the country and most of them are located in the central and southern parts of Botswana, while the aftershocks are clustered around the mainshock. Although five of the nine foreshocks occurred far from the boundaries of major tectonic units, the other four occurred near key features such as the Zoetfontein, Lecha and Chobe faults. The spatial distribution of aftershocks indicates that the stress released by the mainshock, re-activated the planes of weakness in the vicinity of the mainshock and farther away from the mainshock. Hence, this affected the Zoetfontein fault, the boundary between Passarge basin and Magondi belt and the boundary between the Kaapvaal craton and Limpopo mobile belt. The aftershocks also show a northwest-southeast trend, which probably indicates the rupture plane; and mainly lie within the Limpopo mobile belt that is sandwiched between the Kaapvaal craton to the south and Zimbabwe craton to the north. Furthermore, the aftershocks concentration to the south reveals a close relation in demarcating the boundary of the Kaapvaal craton and the Limpopo mobile belt.

Zirconium as a suitable reference element for estimating potentially toxic element enrichment in treated wastewater discharge vicinity.

The suitability of a reference element or normalizer used in assessing soil contamination levels using enrichment factor (EF) is important for soil quality assessment and monitoring. This study evaluated the results of using three reference elements Ti, Fe, and Zr for EF determination of Rb and Sr in soils within treated wastewater discharge vicinity, Central Botswana. The upper continental crust (UCC), world average values (WAV), and the local background values (LBV) were used in EF assessment of eight pedons. The elemental concentrations of the soils were determined with portable X-ray fluorescence (pXRF) analyzer. Relationships between the elements were strongly significant between Rb and Ti (r = 0.600, p < 0.01), Rb and Fe (r = 0.735, p < 0.01), Sr and Ti (r = 0.545, p < 0.01), and Sr and Fe (r = 0.841, p < 0.01). Second-level correlation analysis between contamination factor (CF) and EF levels showed Zr as the best reference element for Rb and Sr in the soils. Results from this study provide baseline knowledge necessary for contamination assessment and monitoring of soils with similar environmental conditions.

The nonglacial diamictite of Toutswemogala Hill (lower Karoo Supergroup, central Botswana): Implications on the extent of the late Paleozoic ice age in the Kalahari–Karoo Basin

ABSTRACTAlong the easternmost edge of the Karoo–Kalahari Basin (KKB) of Botswana, the Toutswemogala Hill succession exposes a 30–50-m-thick suite of siliciclastic deposits interpreted by some as glaciogenic in origin tied to the Permo-Carboniferous Late Paleozoic Ice Age (LPIA). Six facies associations (FA) were recognized in this succession, resting unconformably on a highly uneven Archean gneissic basement, and consisting from base to top of: 1) clast-supported breccia made up of angular cobbles and boulders ubiquitously derived from the underlying basement, 2) well-bedded siltstones sealing or locally interdigitated with the underlying breccia, and bearing abundant remnants of Glossopteris sp. leaves, 3) a chaotic to faintly laminated matrix-supported diamictite bearing angular and subrounded clasts and tree logs attributed to the genus Megaporoxylon, 4) cross-bedded conglomerate bearing well-rounded quartz and clasts, 5) planar-laminated to ripple-laminated, poorly sorted, muddy sandstones showcasing dispersed mud chips that grade upward into 6) poorly sorted, cross-bedded coarse-grained sandstones displaying convolute beds and abundant imprints of unidentifiable tree logs.No evidence of glaciogenic processes have been found in this succession, in the form of either pavement or clasts striations. The breccia and diamictite are interpreted as scree and mass-flow deposits, respectively. Along with the age of the deposits, inferred from the plant debris (upper Carboniferous to lower Permian), the stratigraphic position of this sedimentary succession resting on the Archean basement suggests that it corresponds to the Dukwi Formation, a stratigraphic equivalent of the Dwyka Group in the Main Karoo Basin. This would explain the resemblance of the facies to those recovered at the base of the central Kalahari–Karoo Basin and in the neighboring Tuli, Ellisras, and Tshipise basins. The absence of diagnostic criteria for glacial processes in the studied succession raises the question of the extent, in both time and space, of the LPIA-related ice masses over southern Africa and particularly in southeastern Botswana. It is suggested here that during this glacial epoch, spatially restricted ice masses were confined in bedrock valleys (valley glaciers) in an uplifted setting otherwise characterized by non-glaciogenic processes, further strengthening the scenario of fragmented ice masses over southern Gondwana.

Crustal Structure and Dynamics of Botswana

AbstractWe present the first nationwide crustal thickness and Vp/Vs maps for Botswana based on the analysis of new P wave receiver functions of NARS‐Botswana network integrated with previous receiver function results in Botswana. Using H‐K analysis, we found crustal thickness values ranging from 34 km for the Okavango Rift Zone to 49 km at the border between the Magondi Belt and the Zimbabwe Craton. For stations with significant sediment cover, a sediment correction was applied based on sequential H‐K stacking. We observed distinct differences for the Kaapvaal craton. The eastern part has a high Vp/Vs ratio typical of a predominantly mafic composition, suggesting lateral extension of the Bushveld mafic complex. On the other hand, the western part with a Vp/Vs ratio of 1.67 is felsic, probably as a result of delamination caused by Proterozoic rifting processes. Further to the west of the Kaapvaal Craton, we found a crustal thickness of 42 km and a Vp/Vs ratio of 1.76 for the Nosop Basin. These values are similar to other cratonic regions in Botswana, suggesting the presence of a buried craton as proposed by previous studies. We confirm a relatively thin crust (∼34–39 km), compared to the rest of Botswana, and high Vp/Vs ratio (∼1.84) underneath the Okavango Rift Zone found by previous receiver function studies. Notably, we also found a relatively thin crust (37 km) and high Vp/Vs ratio (1.84) in central Botswana underneath the Passarge Basin.

Source characteristics of the 2017 Mw6.4 Moijabana, Botswana earthquake, a rare lower-crustal event within an ancient zone of weakness

The 2017 Moijabana earthquake in central Botswana (Mw6.4) was a large and deep event for a continental interior and occurred in a region with little historical seismicity. Based on InSAR measurements of surface deformation spanning the event and teleseismic observations, we determine the ruptured fault plane and finite rupture model of the earthquake. Although this oblique normal-faulting earthquake is too deep to uniquely determine the rupture plane geometry from InSAR alone, the best-fitting fault plane constrained by the joint inversion of teleseismic waveforms and InSAR data has a southwest dip and a strike of 126°, roughly consistent with the geologically mapped strike of the Kaapvaal craton's northern edge. Our results indicate that the earthquake had a total duration of ∼10 s, characterized by two major asperities. The first asperity nucleated in the lower crust and then the rupture propagated up-dip. The lower crustal asperity shows a much shorter rise time compared with the shallower asperity, indicating that contrasts in stress or material properties may have played an important role in the rupture process. The earthquake appears to have occurred in the Limpopo belt, a Proterozoic orogenic belt that represents an ancient zone of weakness between the Archean Zimbabwe and Kaapvaal cratons. In the present day, this zone of weakness may be responding to the stress field imposed by the East African Rift System.

Geophysical evidence for crustal and mantle weak zones controlling intra-plate seismicity – the 2017 Botswana earthquake sequence

Large earthquakes away from plate boundaries pose a significant threat to human lives and infrastructure, but such events typically occur on previously unknown faults. Most cases of intra-plate seismicity result from compression related to far-field plate boundary stresses. The April 2017 MW 6.5 earthquake in central Botswana, and subsequent events, occurred in a region with no previously known large earthquakes, occurred away from major present day tectonic activity, and accommodate extension rather than compression. Here, we present results from an integrated geophysical study that suggests the recent earthquakes may be a sign of future activity, controlled by the collocation of a weak upper mantle and weak crustal structure, between otherwise strong Precambrian blocks. Magnetotelluric data highlights Proterozoic continent accretion structure within the region, and shows that recent extension and seismicity occurred along ancient thrust faults within the crust. Our seismic velocity and resistivity models suggest a weak zone in the uppermost mantle, that does not persist to greater depths, and is therefore unlikely to represent mantle upwelling. The Botswana events may therefore be indicative of top-down extension as a response to large scale extensional forces.

The 03 April 2017 Botswana M6.5 earthquake: Preliminary results

An earthquake of magnitude Mw 6.5 occurred on the evening of 3 April 2017 in Central Botswana, southern Africa. The event was well recorded by the regional seismic networks. The location by the Council for Geoscience (CGS) placed it near the Central Kgalagadi Game Reserve. Its effects were felt widely in southern Africa and were pronounced for residents of Gauteng and the North West Province in South Africa. In response to this event, the CGS, together with the Botswana Geoscience Institute (BGI), embarked on two scientific projects. The first project involved a macroseismic survey to study the extent and nature of the effects of the event in southern Africa. This involved CGS and BGI scientists soliciting information from members of the public through questionnaire surveys. More information was collected through questionnaires submitted online by the public. In total, 181 questionnaires were obtained through interviews and 151 online from South Africa, Zimbabwe and Namibia through collaboration between the CGS, the Meteorological Services Department of Zimbabwe and the Geological Survey of Namibia. All collected data were analysed to produce 79 intensity data points (IDPs) located all over the region, with maximum intensity values of VI (according to the Modified Mercalli Intensity scale) observed near the epicentre. This is quite a low value of maximum intensity for such a large event, but was expected given that the epicentral region is in a national park which is sparsely populated. The second scientific project involved the rapid installation of a temporary network of six seismograph stations in and around the location of the main event with the purpose of detecting and recording its aftershocks over a period of three months. Data recorded in the first month of April 2017 were collected and delivered to both the CGS and BGI for processing. More than 500 aftershock events of magnitude ML ≥ 0.8 were recorded and analysed for this period. All the events were located at the eastern edge of the Central Kgalagadi Park near the location of the main event in two clear clusters. The observed clusters imply that a segmented fault is the source of these earthquakes and is oriented in a NW-SE direction, similar to the direction inferred from the fault plane solution of the main event.