The earthquake swarm along the Lesotho-South Africa border began in late December 2023 and persisted into 2025, with activity declining during the latter part of the sequence. Earthquake swarms are of growing interest as they do not exhibit the typical mainshock-aftershock decay described by the Omori-Utsu law; instead, they reflect complex and often externally driven processes. In intraplate regions such as southern Africa, where tectonic earthquakes are relatively infrequent, such sequences provide a rare opportunity to investigate subsurface processes within a stable continental setting. This study aims to constrain the dominant driving mechanisms of the swarm using migration analysis, velocity-duration scaling, and non-stationary Epidemic-Type Aftershock Sequence (ETAS) modelling. The swarm evolves from a radial cluster into a linear NE-SW trend, propagating at 1.43 ± 0.13 km/day with diffusivity values > 30 m2/s, suggesting that fluid diffusion alone is unlikely to explain the observed migration. The velocity-duration relationship places the sequence near the global Slow Slip-Driven (SSD) trend, indicating a transient aseismic slip phase early in the sequence. Non-stationary ETAS modelling was applied to separate background seismicity from earthquake triggering. Results show elevated background rates early in the sequence, followed by increased aftershock productivity, suggesting a transition from externally driven to internally triggered seismicity. Overall, the swarm was likely initiated by fluid circulation at depth that triggered a transient aseismic slip phase and was later sustained by earthquake interactions.

The city of Durban has previously experienced higher than expected ground motions from large distant earthquakes. It is potentially exposed to significant seismic hazard due to seismic site amplification, which needs to be estimated for effective mitigation efforts. Detailed stochastic one dimensional (1D) seismic site response analyses were performed at 90 sites in the city. Analytical models have demonstrated that they can simulate reasonably well the seismic ground motions amplification. The most widely used model is the equivalent linear approach. The approach computes the ground response of horizontally layered soil deposits subjected to transient and vertically propagating shear waves through a 1D soil column. Seven earthquake time histories together with developed sub-surface models were selected as input parameters to estimate the seismic site amplification at the 90 sites in the city. The used time histories were taken from the 2014 M5.5 Orkney earthquake with distance range (4.8-46.9 km). The uncertainties in ground motion input, variation in the shear wave velocity and variations in the shear modulus reduction and damping curves (i.e. variation of non-linear properties) were carefully modelled. Results obtained from this study were used to prepare maps of peak ground acceleration (PGA) at the surface and amplification factors. The minimum and maximum PGA at surface are estimated as 0.01 g and 0.30 g respectively. Based on the results of the analysis, the city may sustain amplification in the range of 0.7-4.7 at PGA with high values along the coast. The results indicate that the low shear wave velocity values, weak and soft material at shallow depths are responsible for the higher amplifications observed especially along the coast. Therefore, a site-specific design approach should be adopted for the seismic design of critical structures.

Ambient noise vibrations are widely used in the field of geotechnical earthquake engineering for site response analysis. Though generally weak, these vibrations are available anywhere and anytime and are an ideal source of energy for conducting seismic surveys. The use of horizontal to vertical spectral ratio (H/V) from ambient noise records, to characterise local site effects is now an established technique. This technique can be applied to estimate predominant frequencies, amplitude ratios and seismic vulnerability index of an area. Ninety sites were selected in Durban (KwaZulu-Natal Province) for ambient vibration measurements and H/V analysis. The analysis of recorded waveforms resulted in the determination of predominant frequencies as well as the amplitude ratios that can be assumed to be equivalent to amplification factors, allowing the city to be divided into zones of differing site effects. The results showed predominant frequencies ranging between 0.78 and 36.8 Hz and peak amplitude ratios in the interval 1.3 to 30.0. The predominant frequency and amplitude ratio results were then used to estimate a seismic vulnerability index, which was observed to vary from 0.2 to 87.3. The city was then successfully divided into three zones according to the obtained vulnerability index values. High vulnerability index values were found to correlate with coastal areas, thus indicating areas that are likely to suffer significant amplification of earthquake ground motion. These coastal areas are associated with thick sediments and alluvial plains, conditions which have been shown in many studies to have a significant impact on ground motion resulting in amplification. Building structures with natural frequencies close to the site predominant frequencies in these areas will experience large resonance effects. This study can be considered as a first step towards the microzonation of the city of Durban.

An earthquake (magnitude, ML = 3.8) occurred on 31 October 2019 at 11:19 hours Greenwich Mean Time (GMT) in the Sunduza area of the KwaZulu-Natal (KZN) Province in South Africa. The earthquake was located near the broad fracture zone of the Ntlakwe-Bongwan fault and was felt along the eastern coast of South Africa. The effects of the event were assessed by conducting macroseismic investigations through interviewing members of the public and completing questionnaires. Analysis of all the collected macroseismic data showed that maximum shaking with Modified Mercalli Intensity of IV-V and V were experienced near the epicentral area as well as along the coast. Similar intensity values were also experienced in some parts of Durban about 100 km northeast of the epicentre. Shaking at such long distances was attributed to ground motion amplification due to site effects. A fault plane solution of the event obtained using both the first motion polarities and amplitude ratios showed normal faulting along a fault oriented in a strike of 185.0°, dipping at 68.0° with a rake of -52.0°. The strike of the focal mechanism solution aligns with the strike of the Ntlakwe-Bongwan fault zone and nearby faults suggesting that the event might have ruptured along one of the fracture segments of this fault mapped to the north. The obtained dip is also in line with a high angle fault comparable to that observed along the Ntlakwe-Bongwan fault. The epicentral region is highly faulted although the activity is not known.

Macroseismic intensity information is an important instrument in the evaluation of historical seismicity. In an effort to improve the current South African earthquake catalogue, available macroseismic information of historical and early instrumental period earthquakes were used to determine source parameters (epicentre and magnitude of events). Forty-six earthquakes with more than eight Intensity Data Points (IDPs) each were analysed. Of these earthquakes, 16 had 20 or more IDPs and three had more than 80. The IDPs were dominated by relatively low intensity values, mostly determined from human perception of shaking, rather than structural damage, which is consistent with a stable continental region. The source parameters of these earthquakes were computed using a suite of state-of-the-art 3rd generation techniques. The quality of obtained solutions was greatly influenced by the spatial distribution of the IDPs, which was not uniform for all the events, with some events having clustered IDPs whilst others were linearly distributed. As a result, a quality ranking system of the results was developed to give an indication of the confidence in the results. Of the 46 earthquakes analysed, 36 had good quality results. Twelve events analysed in this study are new events that were not in previous catalogues. The results obtained help in improving the earthquake catalogue of the region, as there is an increase in the number of events recoded in the historical and early-instrument period. This kind of work and results obtained highlight the value of seeking out additional contemporary sources in order to revise the source parameters of earthquakes for which no or only very limited instrumental information is available.

Namibia is located in Southern Africa on a Stable Continental Region (SRC). Despite the fact that a number of earthquakes have been observed or recorded in Namibia since 1910, with the July 31, 2009 and the April 4, 2021 earthquakes both of moment magnitude (Mw) of 5.6, no up-to-date country-wide seismic hazard study has been conducted. As a result, the purpose of this research is to conduct a comprehensive Probabilistic Seismic Hazard Assessment (PSHA) for the entire country in order to aid in planning and seismic risk mitigation. The earthquake catalogue developed for this study spans the years 1910-2021 and includes both historical and instrumental events. Data from the International Seismological Centre (ISC), the Advanced Network of Seismological System (ANSS), and Namibia's National Seismological Network (NSN) were used to create the catalogue. The catalogue was then declustered to remove fore- and aftershocks as well as similar events; the resulting catalogue was used in conjunction with available geological evidence to identify, delineate, and characterise 11 seismic source zones as well as one fault source, the Hebron/Dreylingen fault. Four Ground Motion Prediction Equations (GMPEs) were carefully selected and used in the seismic hazard computations. The GMPEs were implemented in the calculations using a logic tree formalism with equal weights, which assisted in addressing the uncertainties associated with both the seismic sources and the ground motion models. Peak Ground Accelerations (PGA) obtained for a 10% chance of exceeding in 50 years ranged from 0.017 g to 0.149 g. The highest levels of hazard were observed in Namibia's north-western, north-central, central and southern regions. This study also produced seismic hazard maps that show the distribution of acceleration at different response periods (0s (PGA), 0.1s, 0.15s, 0.2. s, 0.3s, 0.5s and 1.0s) computed for a 10% probability of exceedance in 50 years. The findings from this Namibia's first large-scale seismic hazard study are expected to make a significant contribution for future land use planning in facility and infrastructural development in the country.

This work sought to establish the geophysical signature of platinum group elements in a mineralized sulphide host rock. It has been established that complex resistivity of rocks depends on frequency. The variation of resistivity magnitude and phase for the samples were taken for frequencies in the range 10-2 to 102 Hz. In the first part, the mathematical relationship between resistivity amplitude and phase with frequency was obtained based on the Cole-Cole model equation. A MATLAB Code based on the direct inversion of the apparent resistivity spectrum was used to extract the "Cole-Cole" parameters. In the second part an experimental procedure to obtain the data was designed and a discussion of the results obtained was made. Overally, the results confirm the model to be a good candidate for use in mapping the occurrence of platinum group elements in sulphide zones. Its frequency dependency parameter c is the critical anomaly detection parameter. Several efforts to solve this problem by other geophysical methods failed to yield the desired or useable results. The reason was the complex and inverse nature of the problem. The practical method chosen from a host of geophysical methods was complex resistivity measurements based on the "Cole-Cole model". The knowledge gap being bridged in this study involves the use of this method to detect the base of the main sulphide zone.

The Central Southern Africa is an area of moderate seismic activity generally caused by the presence of the East Africa Rift System. To improve the quantification of seismicity in this region, we propose a local magnitude scale ( M l ), based on the original Richter definition to be used by the Zimbabwe and Botswana national networks. The magnitude scale is developed using 621 seismic events that occurred between 1997–2000 and 2013–2018. These events were recorded by 61 broadband seismic stations located in Botswana, Zimbabwe, and South Africa. We evaluated 5306 traces of zero to peak maximum amplitude, recorded on the horizontal channel from simulated Wood-Anderson seismograms. All 5306 maximum amplitude measurements were inverted simultaneously to determine the attenuation constants. The resultant M l is defined as M_l = log _10A+0.80 ×log _10(R)+0.00086 × R-1.37 ± S in which A is ground displacement amplitude determined from instrument corrected synthetic Wood-Anderson seismograms in nanometres, R is the epicentral distance (km), and S is the station correction. Station corrections were determined for all stations during analysis resulting in values ranging between − 0.44 and + 0.31. The range in station corrections suggests a strong influence of local site effects on the amplitude of the seismic signal. Without station correction, the overall standard deviation of the magnitude residuals using our magnitude relation is 0.32, while using station corrections, the standard deviation 0.17. A comparison of our local magnitude relation with published m b l g relationships for Southern Africa shows that these two magnitude scales are almost equivalent. The M l equation for Central Southern Africa derived in this study has good correlation with m b values reported by the ISC and NEIC and attains systematically lower magnitude values than the South African relationship.

Recently, a swarm of seismic events occurred within the Kunene region (NW of Namibia) with magnitude values going up to M L 4.6. Some of the events alarmed the Anker community and have become of national interest. In order to understand the source of this seismic activity, the Geological Survey of Namibia (GSN) and the Council for Geoscience (CGS) of South Africa installed a temporary network of 10 seismic stations in the Anker area to monitor seismicity for a period of 3 months from 20 June to 24 September 2018. More than 1600 microseismic events were recorded and located within the boundaries of the array, which was roughly 50 km × 50 km. The stations were installed in a 50 km × 50 km radius to have an accurate overlap of the network coverage and also by the spatial distribution of the known seismicity in the study area. The magnitude of the events ranged from M L − 1.1 to 3.6 and occurred at depths of less than 20 km. To improve the location of earthquakes, a new velocity model was developed using high-quality seismic dataset recorded by the temporary network. The seismic data was used to determine a minimum 1-D velocity model, which was used for the relocation of the earthquakes using the double difference method. The relocated solutions provided information to explain the seismotectonics of the region and establish the source of the current seismic swarm. Lineaments were mapped using magnetic data to identify the sources of the seismicity. Focal mechanism analysis of some of the events indicates the area is highly faulted with predominantly normal faulting.

In this paper, we present the probabilistic seismic hazard analysis for Sudan. The seismic database used consists of a regional catalog compiled from the earthquake catalog for East and Southern Africa, NOAA, ISC and PDE. The catalog was checked for completeness with respect to time, and it was homogenized to a surface wave magnitude (Ms). Areal seismic zones were selected based on present day seismotectonic knowledge. The hazard calculations are done for rock sites, and a new ground motion predictive relation, SEA96, has been applied. In the results, regional hazard maps for a 50-year period, 100-year return periods and 10% probability of excess have been presented. High seismic hazard is found in Southern Sudan and slightly less values in the central part of the country. Furthermore, induced seismicity in some different parts of Sudan has been evaluated.

An earthquake of magnitude MW 6.5 occurred on the evening of the April 3, 2017 in Central Botswana, southern Africa. Although it is located in the continental interior of the African plate, the seismogenic area was previously considered as a stable region revealing a background seismicity associated with long-term deformation and faulting. The intraplate seismic activity and mainshock area are studied using the database of the Seismotectonic Map of Africa. The mainshock (25.156° E, 22.678° S, 28 km in depth) and aftershocks are located in a sparsely populated national park. The aftershock sequence, which is the largest event of MW 4.5 magnitude, followed the mainshock on the April 5, 2017. The analysis of Sentinel-1 interferogram shows 4–6 cm coseismic surface slip on a NW–SE elongated and ~30 km long surface deformation consistent with the mainshock and aftershock distribution, normal faulting mechanism, and source time function. Here, we study the faulting geometry and rupture characteristics using more than 1000 recorded aftershocks of magnitude ML ≥ 0.5. All seismic events are located at the eastern edge of the Central Kgalagadi Park near the mainshock location in two clear seismicity clusters. The NW–SE trending seismic clusters imply that a segmented fault is the source of these earthquakes, similar to the inferred direction from the normal faulting solution of the main shock. The earthquake rupture geometry results from an active normal fault that agrees with the aftershock locations at surface and at depth and confirms the thickness of ~28 km seismogenic layer.

The occurrence of the September 22, 2016 Manica-Zinave earthquake (Mw5.6) and its subsequent aftershocks provided data to study the physics of earthquakes along the southern end of the East African Rift System (EARS). The event occurred in the southern part of Mozambique, less than 40 km from the epicentre of the Mw7.0 Machaze earthquake, which occurred on February 22, 2006. About 320 aftershocks were recorded within a period of 6 months. The analysis of the Manica-Zinave event and its aftershocks was done to understand the tectonics of the region. The aftershocks were relocated using the double difference method. The focal mechanism for this event shows normal faulting, which is consistent with most events along the EARS. Statistical analysis of the aftershock sequence was performed for the calculation of the b-value and the aftershock decay rate p-value. The b-value and p-value obtained for this sequence are 1.02 and 0.86 respectively. These values are typical of aftershock sequences of heterogeneous stressed regions. Due to its proximity, the event might be an aftershock of the Machaze earthquake, which might have occurred due to stress readjustment within the area. The p-value obtained from the Omori law computation suggests that the region is highly stressed.

The 3 April 2017 MW 6.5, Moiyabana (Botswana) earthquake occurred in the continental interior of the African plate and in a seismogenic region previously considered as stable. We analyse the mainshock and aftershock sequence based on a local seismic network and local seismotectonic characteristics. The earthquake rupture geometry is constrained with more than 1,000 aftershocks recorded over a period of three months and from the InSAR analysis of Sentinel-1 images (ascending orbit). The mainshock (25.134 E, 22.565 S; depth 22 ± 3 km) was followed by more than 500 events of magnitude M ≥ 0.8 recorded in April 2017 including the largest aftershock (MW 4.6 on the 5 April 2017). Focal mechanism solutions of the mainshock and aftershocks display predominance of NW-SE trending and NE dipping normal faulting. Stress inversion of focal mechanisms obtained from the mainshock and aftershock database are compatible with a NE-SW extension under normal faulting regime. The InSAR study shows fringes with two lobes with 4 to 6 cm coseismic slip on a NW-SE elongated and 30-km-long surface deformation consistent with the mainshock location and normal faulting mechanism. The modelling of surface deformation provides the earthquake rupture dimension at depth with ~ 1 m maximum slip on a fault plane striking 315°, dipping 45°, -80° rake and with Mo 7.12 1018 Nm Although the seismic strain rate is of low level, the occurrence of the 2017 Moiyabana earthquake, followed by an aftershock sequence in the central Limpopo belt classifies the intraplate region as an active plate interior.

The 3 April 2017 MW 6.5, Moiyabana (Botswana) earthquake occurred in the continental interior of the African plate and in a seismogenic region previously considered as stable. We analyse the mainshock and aftershock sequence based on a local seismic network and local seismotectonic characteristics. The earthquake rupture geometry is constrained with more than 1,000 aftershocks recorded over a period of three months and from the InSAR analysis of Sentinel-1 images (ascending orbit). The mainshock (25.134 E, 22.565 S; depth 22 ± 3 km) was followed by more than 500 events of magnitude M ≥ 0.8 recorded in April 2017 including the largest aftershock (MW 4.6 on the 5 April 2017). Focal mechanism solutions of the mainshock and aftershocks display predominance of NW-SE trending and NE dipping normal faulting. Stress inversion of focal mechanisms obtained from the mainshock and aftershock database are compatible with a NE-SW extension under normal faulting regime. The InSAR study shows fringes with two lobes with 4 to 6 cm coseismic slip on a NW-SE elongated and 30-km-long surface deformation consistent with the mainshock location and normal faulting mechanism. The modelling of surface deformation provides the earthquake rupture dimension at depth with ~ 1 m maximum slip on a fault plane striking 315°, dipping 45°, -80° rake and with Mo 7.12 1018 Nm Although the seismic strain rate is of low level, the occurrence of the 2017 Moiyabana earthquake, followed by an aftershock sequence in the central Limpopo belt classifies the intraplate region as an active plate interior.

New MW–ML magnitude relations valid for the periods January 1971–March 1997, April 1997–September 2012 and October 2012–present are presented for South Africa. Three MW–ML relations were derived for the respective periods by applying a general orthogonal regression using 85 mining-related and tectonic events. Moment magnitudes (MW) were calculated using spectral analysis of the vertical component seismograms. A Brune model for source radiation was assumed. Attenuation functions previously derived from local seismic events were used. For each event, the final MW was computed by taking the average of the values determined at four or more stations. These MW values were used to compute new MW –ML regression equations for three previous ML relations that had been used to compile the South African earthquake catalogue. The new MW–ML relations were then compared with the MW determined from the Harvard Centroid Moment Tensor (CMT) project. The MW computed using the relation derived for the most recent period (October 2012–present) produces MW values that are similar to those reported by the Harvard CMT solutions. The relations can be used in the homogenization of the South African earthquake catalogue. Application of the new relations for the three periods will reduce uncertainty in the homogenized catalogues, which are important when calculating seismic hazard parameters (i.e. when estimating activity rates, b-values and Mmax).

Many years have passed since previous national seismic hazard maps were prepared for South Africa. In those maps, zone-less techniques were applied. The availability of more reliable seismicity and geological data has made it possible to update those maps using probabilistic seismic hazard analysis methodologies that take into consideration all available data. This paper presents a summary of the work conducted to produce the latest seismic hazard maps for South Africa. This involved the systematic compilation and homogenisation of an earthquake catalogue, which comprised both historical and instrumental events. The catalogue played a prominent role in the preparation and characterisation of the seismic source model. Two ground motion prediction equations were identified from available international models for regions that are tectonically similar to South Africa. These two models were then implemented in the hazard calculations, which were done using the OPENQUAKE software. Uncertainties associated with input parameters in both the seismic source and ground motion models were taken into account and implemented using the logic tree technique. Maps showing distribution of acceleration at three periods (0.0s, 0.15s and 2.0s) computed for 10% probability of exceedance in 50 years were produced.

Reliable local earthquake locations depend on many factors of which a major one is the velocity model. Currently the Council for Geoscience (CGS) has been using the global IASP91 velocity model for earthquake locations in the cluster networks. To continue improving the earthquake locations it is necessary that new velocity models are determined for each cluster region (Central and East Rand - CERAND, the Klerksdorp – Orkney – Stilfontein – Hartebeesfontein – KOSH and the Far West and West Rand - WRAND). The availability of good quality data recorded by the cluster networks since their inception in 2010 provides an opportunity to conduct this work. Thus data from the cluster networks database were selected according to set quality criteria to obtain parametric data for 130 earthquakes in the CERAND region, 404 in the KOSH region and 1024 in the WRAND region. These data were used to determine a minimum 1-D velocity model with associated station corrections for each of the regions using the VELEST software package. Comparison of epicentres obtained using the new velocity models to epicentres previously published by the CGS, showed improvement in the quality of the new locations. Thus, the new models will be implemented in the day-to-day analysis of data recorded in the three study regions by the cluster network of stations.