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Seismic hazard assessment of the Kivu rift segment based on a new seismotectonic zonation model (western branch, East African Rift system)
Abstract
In the frame of the Belgian GeoRisCA multi-risk assessment project focusing on the Kivu and northern Tanganyika rift region in Central Africa, a new probabilistic seismic hazard assessment has been performed for the Kivu rift segment in the central part of the western branch of the East African rift system. As the geological and tectonic setting of this region is incompletely known, especially the part lying in the Democratic Republic of the Congo, we compiled homogeneous cross-border tectonic and neotectonic maps.
... The area is Proceeding; Geothermal Association of Kenya Congress, Safari Park Hotel, Nairobi, 16-18 July 2024 dominated by Paleoproterozoic Rusizian formations (an extension of the Ubendian Belt known to the DRC side) and the Mesoproterozoic Karagwe-Ankolean belt (formerly the Kibaran belt) lithology, with some alkaline and basalts associated with the Virunga volcanic and South Kivu Volcanic province. Fissure eruptions of continental nephelinites, which occurred north of Idjiwi Island, marked the beginning of rifting in the North Tanganyika region during the mid-Miocene (20-21 Ma) (Delvaux et al., 2017). This volcanism indicates a deep source as result of the neotectonic fissure eruptions throughout the basement geology. ...
... Financial assistance is still being considered as of right now. (Delvaux et al., 2017) 2. RESULTS (Wong and Herzen, 1974) suggests that there is a deep fissure to the lithosphere's upper mantle that closes south of the Kivu rift. Heat flow beneath the thermal springs is therefore not a necessary condition for geothermal projects. ...
... Characteristics of the new seismic catalogue compiled for the Kivu rift region(Delvaux et al., 2017) Homogeneous cross-border synthetic geological map of the study region (background) with overprinted neotectonic features (neotectonic faults, late Quaternary volcanic centers, thermal springs, and fossil hydrothermal travertine). Hill-shading from the SRTM DEM (90 m resolution) ...
Potential geothermal resources exist in the Western Branch of the Eastern African Rift System, which might be more widely distributed for utilization. The neighboring countries of Democratic Republic of the Congo (DRC), Uganda, Rwanda, Burundi, and Malawi, which share this segment of the African rift and some geothermal manifestations. The exploration of geothermal energy is advancing, with distinct plans for each country without cross border collaboration. There is a need for a thorough geothermal survey because recent drilling tests in the Virunga Volcanic Province (Karisimbi) and Rukwa Volcanic Province (Ngozi) have produced unsatisfactory results. The development of geothermal resources in the Western Branch of the Eastern African Rift System (WBEARS) must focus on determining what the heat sources for the geothermal prospects are. The DRC's geothermal committee is currently looking for a pilot geothermal area to conduct geophysical and geochemical studies. Even though all thermal springs have been identified, the DRC has created a criteria of evaluating the geothermal prospects based temperature, flow rate, topography, geology, accessibility, and safety.. The DRC's thermal springs range in temperature from 20 to 100 degrees Celsius on surface. Geophysics studies have been recommended for locating 3D structures to define the geothermal reservoirs. Countries that share borders on the southern Kivu region should collaborate to explore the geothermal potential of the highest potential area. Nyangezi is regarded as a possible pilot field for the DRC and the CPGL (Collaborative Programme for Geothermal Energy in the Great Lakes Region) because of its location offers the best target for exploration and development of the geothermal resources. A few geothermal manifestations occur less than 20 kilometers from the eruptive centres. These sources, however, are regarded as less reliable because they might be impacted by heat from volcanic eruptions rather than actual geothermal reservoirs. There is a need to evaluate the utilisation the magma-related heat source for development of geothermal energy.
... West of Lake Tanganyika, the lithostratigraphy consists of gneisses, micaschists, and granites of Paleoproterozoic age (Laghmouch et al., 2018). West of Lake Kivu, there is a synclinorium of slightly metamorphosed glacial deposits of Neoproterozoic age (Villeneuve, 1976;Delvaux et al., 2017). The lithostratigraphical units of Neoproterozoic, Mesoproterozoic, and Paleoproterozoic age can be deeply weathered (up to tens of meters) and have threshold angles of slope stability of ca. ...
... Additionally, we visualize the border between the rejuvenated landscapes (inside the Rift shoulders) and the surrounding relict landscapes (black dashed line) and the active and inactive faults that drove the Rift formation (red and green lines). The blue lines represent reactivations of pre-rift faults (Delvaux et al., 2017). Locations i and ii correspond to Fig. 6a and b, respectively. ...
... Yet, large earthquakes are known to occur sporadically in the NTK Rift. Delvaux et al. (2017), for example, predict that earthquakes of M w 7 occur every 275 to 852 years. Such earthquakes could potentially mobilize total landslide volumes of up to 108 m 3 (Marc et al., 2016), which would increase our observed mobilization rate by a factor of ca. ...
... The study is conducted in the rift flank west of Lake Kivu in the DR Congo (Fig. 1a). It is one of the most seismic regions of the African continent, crossed by active faults and composed of six main rock types of varying age ( Fig. 1b) (Delvaux et al., 2017;Laghmouch et al., 2018). A significant portion of the study area is made of lithologies from the Archean, the Mesoproterozoic and the Neoproterozoic, with various degrees of chemical weathering and fracturing (Kampunzu et al., 1998). ...
... None of the dated landslide events were triggered by earthquakes (Dewitte et al., 2021). This does not discard the role of earthquakes in triggering landslides in the region but instead reminds us that the return period of earthquakes with a magnitude large enough to trigger slope instabilities can be much longer than a few decades (Delvaux et al., 2017). Their potential impact, rather localised compared to that of climatic drivers, can be inexistent during a narrow time window of observation (Dewitte et al., 2021;Depicker et al., 2021b). ...
... Vanmaercke et al., 2017). Using the fault pattern is the most appropriate option to tackle the seismic zonation context since the most detailed seismic hazard assessment for this part of the continent is at a spatial resolution of 2.2 km, i.e. at a resolution that is too coarse for our study (Delvaux et al., 2017). ...
Tropical mountainous regions are often identified as landslide hotspots with growing population pressure. Anthropogenic factors are assumed to play a role in the occurrence of landslides in these densely populated regions, yet the relative importance of these human-induced factors remains poorly documented. In this work, we aim to explore the impact of forest cover dynamics, roads and mining activities on the characteristics and causes of landslides in the rift flank west of Lake Kivu in the Democratic Republic of the Congo (DR Congo). To do so, we compile a comprehensive multi-temporal inventory of 2730 landslides. The landslides are of different types and are grouped into five categories that are adapted to study the impact of human activities on slope stability: old (pre-1950s) and recent (post-1950s) deep-seated landslides, shallow landslides, landslides associated with mining and landslides associated with road construction. We analyse the landslides according to this classification protocol via frequency–area statistics, frequency ratio distribution and logistic regression susceptibility assessment. We find that natural factors contributing to the cause of recent and old deep-seated landslides were either different or changed over time. Under similar topographic conditions, shallow landslides are more frequent, but of a smaller size, in areas where deforestation has occurred since the 1950s. We attribute this size reduction to the decrease in regolith cohesion due to forest loss, which allows for a smaller minimum critical area for landsliding. In areas that were already deforested in the 1950s, shallow landslides are less frequent, larger and occur on less steep slopes. This suggests a combined role between regolith availability and soil management practices that influence erosion and water infiltration. Mining activities increase the odds of landsliding. Landslides associated with mining and roads are larger than shallow landslides but smaller than the recent deep-seated instabilities, and they are controlled by environmental factors that are not present under natural conditions. Our analysis demonstrates the role of human activities on the occurrence of landslides in the Lake Kivu region. Overall, it highlights the need to consider this context when studying hillslope instability characteristics and distribution patterns in regions under anthropogenic pressure. Our work also highlights the importance of using landslide classification criteria adapted to the context of the Anthropocene.
... Jackson & Blenkinsop, 1993;Mavonga, 2007;Nyblade & Langston, 1995). Furthermore, except for a handful of local studies (Cohen et al., 2013;Delvaux et al., 2017;Kervyn et al., 2006;Shillington et al., 2020;Vittori et al., 1997), little chronostratigraphic data exist in the EAR Western Branch to constrain the paleoseismic history of its active faults. ...
... Except for the Kivu Rift (Delvaux et al., 2017;Wood et al., 2017), onshore-offshore active fault databases have not been developed within the Western Branch. The strategies employed to map faults in the MAFD are therefore relevant elsewhere along the rift system and in other regions with onshore and offshore active faults. ...
... With regard to active fault databases in the Western Branch of the EAR, the above discussion indicates that although there are limited chronostratigraphic data, this is not necessarily a barrier to defining and mapping "active" faults (Delvaux et al., 2017;Meghraoui et al., 2016;. In the MAFD, we therefore first define faults as active if they have accommodated displacement in the current tectonic regime. ...
We present the Malawi Active Fault Database (MAFD), an open‐access (https://doi.org/10.5281/zenodo.5507190) geospatial database of 113 fault traces in Malawi and neighboring Tanzania and Mozambique. Malawi is located within the East African Rift's (EAR) Western Branch where active fault identification is challenging because chronostratigraphic data are rare, and/or faults are buried and so do not have a surface expression. The MAFD therefore includes any fault that has evidence for displacement during Cenozoic East African rifting or is buried beneath the rift valley and is favorably oriented to the regional stresses. To identify such faults, we consider a multidisciplinary data set: high‐resolution digital elevation models, previous geological mapping, field observations, seismic reflection surveys from offshore Lake Malawi, and aeromagnetic and gravity data. The MAFD includes faults throughout Malawi, where seismic risk is increasing because of population growth and its seismically vulnerable building stock. We also investigate the database as a sample of the normal fault population in an incipient continental rift. We cannot reject the null hypothesis that the distribution of fault lengths in the MAFD is described by a power law, which is consistent with Malawi's relatively thick seismogenic layer (30–40 km), low (<8%) regional extensional strain, and regional deformation localization (50%–75%) across relatively long hard‐linked border faults. Cumulatively, we highlight the importance of integrating onshore and offshore geological and geophysical data to develop active fault databases along the EAR and similar continental settings both to understand the regional seismic hazard and tectonic evolution.
... The study is conducted in the Rift flank west of Lake Kivu in the DR Congo (Fig. 1a). It is one of the most seismic regions of the African continent, crossed by active faults and composed of six main rock types of varying age ( Fig. 1b) (Delvaux et al., 2017;Laghmouch et al., 2018). The presence of mineral resources (gold and 3T minerals -tin, 90 tantalum and tungsten) favours the proliferation of, often illegal, artisanal and small-scale mining and quarrying (Van Acker, 2005;Bashwira et al., 2014). ...
... None of the dated landslide events were triggered by earthquakes (Dewitte et al., 2021). This do not discard the role of earthquakes in triggering 135 landslides in the region, but instead this reminds us that the return period of earthquakes with a magnitude large enough to trigger slope instabilities can be much longer than a few decades (Delvaux et al., 2017). Their potential impact, rather localized compared to that of climatic drivers, can be inexistent during a narrow time window of observation (Delvaux et al., 2017;Dewitte et al., 2021;Depicker et al., 2021). ...
... This do not discard the role of earthquakes in triggering 135 landslides in the region, but instead this reminds us that the return period of earthquakes with a magnitude large enough to trigger slope instabilities can be much longer than a few decades (Delvaux et al., 2017). Their potential impact, rather localized compared to that of climatic drivers, can be inexistent during a narrow time window of observation (Delvaux et al., 2017;Dewitte et al., 2021;Depicker et al., 2021). ...
Tropical mountainous regions are often identified as landslide hotspots with particularly vulnerable populations. Anthropogenic factors are assumed to play a role in the occurrence of landslides in these populated regions, yet the relative importance of these human-induced factors remains poorly documented. In this work, we aim to explore the impact of forest cover dynamics, roads and mining activities on the occurrence of landslides in the Rift flank west of Lake Kivu in the DR Congo. To do so, we compile an inventory of 2730 landslides using © Google Earth imagery, high resolution topographic data, historical aerial photographs from the 1950’s and extensive field surveys. We identify old and recent (post 1950’s) landslides, making a distinction between deep-seated and shallow landslides, road landslides and mining landslides. We find that susceptibility patterns and area distributions are different between old and recent deep-seated landslides, which shows that natural factors contributing to their occurrence were either different or changed over time. Observed shallow landslides are recent processes that all occurred in the past two decades. The analysis of their susceptibility indicates that forest dynamics and the presence of roads play a key role in their regional distribution pattern. Under similar topographic conditions, shallow landslides are more frequent, but of smaller size, in areas where deforestation has occurred since the 1950’s as compared to shallow landslides in forest areas, i.e. in natural environments. We attribute this size reduction to the decrease of regolith cohesion due to forest loss, which allows for a smaller minimum critical area for landsliding. In areas that were already deforested in 1950’s, shallow landslides are less frequent, larger, and occur on less steep slopes. This suggests a combined role between regolith availability and soil management practices that influence erosion and water infiltration. Mining activities increase the odds of landsliding. Mining and road landslides are larger than shallow landslides but smaller than the recent deep-seated instabilities. The susceptibility models calibrated for shallow and deep-seated landslides do not predict them well, highlighting that they are controlled by environmental factors that are not present under natural conditions. Our analysis demonstrates the role of human activities on the occurrence of landslides in the Lake Kivu region. Overall, it highlights the need to consider this context when studying hillslope instability characteristics and distribution patterns in regions under anthropogenic pressure. Our work also highlights the importance of considering the timing of landslides over a multi-decadal period of observation.
... The NTK Rift, in which Bujumbura is located, is situated in the central section of the western branch of the East African Rift (Figure 1a). In this region, tectonic uplift, accompanied with seismic activity and faulting, has initiated landscape rejuvenation through knickpoint retreat, enforcing topographic steepening [16,[36][37][38]. This tectonic setting, combined with a tropical climate that favors the occurrence of intense rainfall events and deep weathering, makes the region a landslide hotspot in Africa [6,16]. ...
... During that same period, none of the observed landslides in the region were triggered by earthquakes [16]. While we cannot exclude that earthquakes can trigger landslides (or play a role in their occurrence), their return period can be long and their impacts as triggering factors can be unnoticed over such a short window of observation [16,37]. Moreover, deep-seated landslides in the region can occur without any apparent trigger, due to the long-term evolution of preconditioning drivers alone, such as rock weathering and regolith formation [40]. ...
... Rift sediments are thick, while gneiss and granites are usually highly weathered. The region is crossed by active faults related to the rifting dynamics [36,37]. Typically of the NTK Rift, the climate of Bujumbura is tropical, with an average annual rainfall of 1400 mm (the southern catchment of Bujumbura [45], with most of the precipitation occurring during the October-May rainy season [46]. ...
Accurate and detailed multitemporal inventories of landslides and their process characterization are crucial for the evaluation of landslide hazards and the implementation of disaster risk reduction strategies in densely-populated mountainous regions. Such investigations are, however, rare in many regions of the tropical African highlands, where landslide research is often in its infancy and not adapted to the local needs. Here, we have produced a comprehensive multitemporal investigation of the landslide processes in the hillslopes of Bujumbura, situated in the landslide-prone East African Rift. We inventoried more than 1200 landslides by combining careful field investigation and visual analysis of satellite images, very-high-resolution topographic data, and historical aerial photographs. More than 20% of the hillslopes of the city are affected by landslides. Recent landslides (post-1950s) are mostly shallow, triggered by rainfall, and located on the steepest slopes. The presence of roads and river quarrying can also control their occurrence. Deep-seated landslides typically concentrate in landscapes that have been rejuvenated through knickpoint retreat. The difference in size distributions between old and recent deep-seated landslides suggests the long-term influence of potentially changing slope-failure drivers. Of the deep-seated landslides, 66% are currently active, those being mostly earthflows connected to the river system. Gully systems causing landslides are commonly associated with the urbanization of the hillslopes. Our results provide a much more accurate record of landslide processes and their impacts in the region than was previously available. These insights will be useful for land management and disaster risk reduction strategies.
... Active continental rifting in our study area is driven by the divergence of the Victoria and Nubia plates that started at ca. 11 Ma and currently continues at a rate of ca. 2 mm/yr (Saria et al., 2014;Pouclet et al., 2016). Due to this setting, there is widespread seismic activity, active volcanism, and uplift, initiating landscape rejuvenation through knickpoint retreat (Smets et al., 2015;Delvaux et al., 2017;Dewitte et al., 2021). Adding to the geological complexity of the NTK rift is the wide variability in age and strength of rock formations. ...
... The southwest is largely covered by either weathering-resistant quartzites or weathering-prone gneiss and micaschists of Paleoproterozoic age (2500-1600 Ma). Within the rift shoulders, the same pattern of Meso-and Paleoproterozoic rocks is observed, save for the occurrence of much younger lithologies such as the river and lake sediments in the Ruzizi floodplain and the volcanic deposits (12 Ma-present) found around Bukavu and north of Goma (Delvaux et al., 2017;Laghmouch et al., 2018). ...
... Seismic activity is a first factor that could explain why slope has a different impact on landslide erosion in rejuvenated and relict landscapes. Generally, there is more and stronger seismic activity within the rejuvenated landscapes (Delvaux et al., 2017). We hypothesize that the higher seismic activity would result in elevated landslide erosion rates on longer timescales due to the occurrence of major landslide events triggered by large earthquakes (Delvaux and Barth, 2010;Marc et al., 2015). ...
Deforestation is associated with a decrease in slope stability through the alteration of hydrological and geotechnical conditions. As such, deforestation increases landslide activity over short, decadal timescales. However, over longer timescales (0.1–10 Myr) the location and timing of landsliding is controlled by the interaction between uplift and fluvial incision. Yet, the interaction between (human-induced) deforestation and landscape evolution has hitherto not been explicitly considered. We address this issue in the North Tanganyika–Kivu rift region (East African Rift). In recent decades, the regional population has grown exponentially, and the associated expansion of cultivated and urban land has resulted in widespread deforestation. In the past 11 Myr, active continental rifting and tectonic processes have forged two parallel mountainous rift shoulders that are continuously rejuvenated (i.e., actively incised) through knickpoint retreat, enforcing topographic steepening. In order to link deforestation and rejuvenation to landslide erosion, we compiled an inventory of nearly 8000 recent shallow landslides in © Google Earth imagery from 2000–2019. To accurately calculate landslide erosion rates, we developed a new methodology to remediate inventory biases linked to the spatial and temporal inconsistency of this satellite imagery. Moreover, to account for the impact of rock strength on both landslide occurrence and knickpoint retreat, we limit our analysis to rock types with threshold angles of 24–28∘. Rejuvenated landscapes were defined as the areas draining towards Lake Kivu or Lake Tanganyika and downstream of retreating knickpoints. We find that shallow landslide erosion rates in these rejuvenated landscapes are roughly 40 % higher than in the surrounding relict landscapes. In contrast, we find that slope exerts a stronger control on landslide erosion in relict landscapes. These two results are reconciled by the observation that landslide erosion generally increases with slope gradient and that the relief is on average steeper in rejuvenated landscapes. The weaker effect of slope steepness on landslide erosion rates in the rejuvenated landscapes could be the result of three factors: the absence of earthquake-induced landslide events in our landslide inventory, a thinner regolith mantle, and a drier climate. More frequent extreme rainfall events in the relict landscapes, and the presence of a thicker regolith, may explain a stronger landslide response to deforestation compared to rejuvenated landscapes. Overall, deforestation initiates a landslide peak that lasts approximately 15 years and increases landslide erosion by a factor 2 to 8. Eventually, landslide erosion in deforested land falls back to a level similar to that observed under forest conditions, most likely due to the depletion of the most unstable regolith. Landslides are not only more abundant in rejuvenated landscapes but are also smaller in size, which may again be a consequence of a thinner regolith mantle and/or seismic activity that fractures the bedrock and reduces the minimal critical area for slope failure. With this paper, we highlight the importance of considering the geomorphological context when studying the impact of recent land use changes on landslide activity.
... The long rainy season during nine months per year explains a high density of the hydrographic network with active rivers which crossing the steep topography and then incising vertically and generating river landslides and bank erosion (Moeyersons and Trefois 2012). For geomorphological setting, low slope angles are founded in the West followed by very steep topography associated to recent tectonic uplift with North-Southern oriented active faults system (Smets et al. 2016;Delvaux et al. 2017) and the Eastern with moderate slopes. The lithology is dominated by complex rocks of Precambrian age. ...
... We collected all active faults available in the area and reclassi ed into four classes (Fig. 5f). Delvaux et al. (2017) showed that active faults are associated with the African rifting. In the region of Bujumbura, these faults are oriented to North-South direction. ...
... The positive correlation in slope between 35-40°w ould be related to gravity control (Sidle and Bogaard 2016). The faults show a good correlation for the highest density, which is consistent with the literature (e.g., Kubwimana et al. 2018), and then support a tectonic control of landslides occurred in the region (Delvaux et al. 2017). The lithology is positive in the gneiss units consistent with Jacobs et al. (2017) at a regional scale and Kubwimana et al. (2018) locally. ...
The aim of this research is the modelling of landslide susceptibility in the hillslopes of Bujumbura using the Weights-of-Evidence model, a probabilistic data modelling approach relevant for predicting future landslides at a regional scale. Initially, characteristics and spatial mapping of different landslides type were identified (fall, flow, slide, complex) by thorough interpretation of high-resolution remote sensing data (mountainous areas with difficult access) and intensive fieldwork. Subsequently, the main landslides controlling factors were selected (lithology, fault density, land use, drainage density, slope aspect, curvature, slope angle, and elevation) using in-depth field knowledge and relevant literature. A landslide inventory map with a total of 569 landslide sites was constructed using the data from various sources. Out of those 569 landslide sites, 285 (50.1%) of the data taken before the 2000s was used for training and the remaining 284 (49.9%) sites (post-2000 events) were used for the accuracy assessment purpose. Thereafter, a prediction map of future landslides was generated with an accuracy of 73.7%. The main geo-environmental landslides factors retained are the high density of drainage networks, the lithology often made with weathered gneiss, the high fault density, the steep topography and the convex slope curvature. The landslide susceptibility map validated was reclassified into very high, high, moderate, low and very low zones. The established susceptibility map will allow with the interaction of the real terrain to locate roads, dwellings, urban extension areas, dams located in high landslides risk zones. These infrastructures will require intervention to address their vulnerability with new facilities, slope stabilization, creation of bypass roads, etc. The susceptibility map produced will be a powerful decision-making tool for drawing up appropriate development plans. Such an approach will make it possible to mitigate the socio-economic impacts due to slope instabilities.
... Limited volcanic activity also manifests occasionally. Based on fault orientations and stress patterns from the inversion of focal mechanism data (Delvaux et al. 2016 ), this group can be divided into five area sources: the NE-SW normal faulting regime of the Albertine Rift (containing the Albertine Graben, Semliki Basin and Rwenzori Mountains), the normal faulting regime of the Lake Kivu Basin, including the Virunga volcanic area, the ESE-WNW normal faulting regime of Lake Tanganyika, and the right lateral fault regime of the souther n par t of Lake Tanganyika, which belongs to the Tanganyika-Rukwa Malawi rift segment (including Lake Tanganyika, the Mbeya triple junction, and the Rungwe v olcanic province; Mav onga 2007 ; Mavonga & Durrheim 2009 ). ...
... The Central African group encompasses the region west of the Western Rift, extending to parts of the Democratic Republic of Congo, Zambia, Zimbabwe and Botswana. The nor ther n area includes the eastern Congo Basin, where seismicity decreases, moving away from the Western Rift (Delvaux et al. 2016 ). The southern zones are characterized by a series of NE-SW trending rifts (Upemba, Mweru and Kariba Rifts), striking roughly perpendicular to the Western Rift. ...
The Epidemic-Type Aftershock Sequence (ETAS) model is currently the most powerful statistical seismicity model that reproduces the general characteristics of earthquake clustering in space and time. However, its application can be hampered by biased parameter estimations related to earthquake catalogue deficiencies, particularly in regions where the spatial coverage of local recording networks is relatively poor. Here, we systematically investigate the possible influences of the effect introduced by data truncation through the choice of the cut-off magnitude ( and missing events due to heterogeneity of the seismic network on ETAS parameter estimates along the East African Rift System (EARS). After dividing the region into six source zones based on rheological and mechanical behaviours, the ETAS model is fitted to the earthquakes within each zone using the Davidon–Fletcher–Powell optimization algorithm. The fits and variations in parameter estimates are compared for each zone to the others and the seismological implications are discussed. We found that some parameters vary as a function of primarily driven by changes in catalogue size. Additionally, a systematic regional dependency of ETAS parameters is found across source zones. Furthermore, a median heat flow value for each analysed source zone in the EARS is calculated. In contrast to previous findings in other tectonic settings, the results reveal no significant correlations between the crustal heat flows and the ETAS parameters describing earthquake productivity () and the relative efficiency of an earthquake with magnitude M to produce aftershocks (). Our findings have significant implications for understanding the mechanisms of earthquake interaction and, therefore, provide tight constraints on the model's parameters that may serve as a testbed for existing earthquake forecasting models in this region where the vulnerability of local buildings and structures exacerbate seismic risk.
... On the other hand, the other consequence of this tectonics is the fracture of the rocks on the regional level (Chorowicz et al., 1988;Clark, 1989;Ebinger, 1989;Rumvegeri, 1991;Salpeteur et al., 1992;Francois, 1995;Laerdal & Talbot, 2002;Lezzar et al., 2002;Delvaux & Barth, 2010;Simon et al., 2014;Dabo & Aïfa, 2013;Delvaux et al., 2017;Yantambwe & Cailteux, 2019). These different breaks followed a common preferential direction which contributed to the formation of the East African Rift (Villeneuve, 1987;Ebinger, 1989;Rumvegeri, 1991;Laerdal & Talbot, 2002;Lezzar et al., 2002;Delvaux & Barth, 2010;Simon et al., 2014;Dewaele et al., 2016;Delvaux et al., 2017). ...
... On the other hand, the other consequence of this tectonics is the fracture of the rocks on the regional level (Chorowicz et al., 1988;Clark, 1989;Ebinger, 1989;Rumvegeri, 1991;Salpeteur et al., 1992;Francois, 1995;Laerdal & Talbot, 2002;Lezzar et al., 2002;Delvaux & Barth, 2010;Simon et al., 2014;Dabo & Aïfa, 2013;Delvaux et al., 2017;Yantambwe & Cailteux, 2019). These different breaks followed a common preferential direction which contributed to the formation of the East African Rift (Villeneuve, 1987;Ebinger, 1989;Rumvegeri, 1991;Laerdal & Talbot, 2002;Lezzar et al., 2002;Delvaux & Barth, 2010;Simon et al., 2014;Dewaele et al., 2016;Delvaux et al., 2017). But most of these breaks have been filled by hydrothermal solutions during geological time forming veins (Salpeteur et al., 1992;Bibentyo et al., 2015;Dewaele et al., 2016). ...
The locality of Mabuku is located in the eastern DRC, in a landscape where tectonic effects are visible. The main cause of this environmental disturbance is known as tectonics. This study aims to determine the influence of regional tectonics (like Kibalian, Ruzizian and Kibarian tectonics) on rocks of the locality of Mabuku, and their types. After the fieldwork and the data processing, we got the following results: firstly, none filled faults were due to constraints of the compressive type, which are also similar to those affecting the formations of the group of Ruzizi. Secondly, the schistosity of these rocks was due to extensive type stresses and its direction is similar to Kibarian tectonics. Moreover, the veins were generated by a constraint of the compressive type, bounding them to Kibalian tectonics. In conclusion, in the locality of Mabuku there's a combination of the Ruzizian, Kibalian, and Kibarian tectonics.
... The steep slopes created are conducive to land movement and increased erosion. The current topography in Bukavu has therefore been established thanks to several main factors: tectonics, to which must be added volcanic activities, erosions, mainly regressive erosion, the earthquake, and the anthropogenic action currently observed by Delvaux et al. [20]. Edelman [21] reported such cities to face more immediate problems than those in the developed world and have fewer resources to deal with them. ...
... The seismicity catalog compiled for the Lake Kivu region by Delvaux et al. [20] contains 1054 earthquakes between 1931 and 2015 and of equivalent magnitude Mw up to 6.3. It also includes all available aftershocks and calculated focal mechanisms of selected earthquakes. ...
... All these belts were formed in the late Neoproterozoic to early Paleozoic at ca. 600-450 Ma (Fig. 1a) [2,5,13]. The Kibaran Belt (Mesoproterozoic) separates the Congo Craton (Archean-Paleoproterozoic) from the Bangweulu-Tanzania Block (Archean-Paleoproterozoic) (Fig. 1a). ...
... The Lemera granite pegmatites are slightly richer in LREE (light rare earth element) and depleted 13 in HREE (heavy rare earth element) (Fig. 8). It is to be noticed that the majority of the Lemera granite pegmatite samples displayed a positive Eu anomaly (only a few samples presented a slight negative Eu anomaly), and this characteristic was also displayed by the leucogranite rocks of the KAB. ...
This paper reports the results from an investigation on petrogenesis and geochemistry of the Lemera granite pegmatites and the related tin ore deposits. Seven granite pegmatite samples were collected from outcrops and analyzed for bulk-rock geochemistry using ICP-MS and XRF spectrometry. The analyzed samples show a relative abundance of Al2O3 over Fe2O3 (T) and MgO (12.95, 0.96, and 0.16, all in percentage, on average and respectively). The alumina saturation index ranges from 1.33 to 2.05. The abundances of some important rare metals, as well as some related key parameters, are as follows: Sn (1-138 ppm), Ta (0.20-0.60 ppm), Nb (1-8 ppm), Cs (1.20-3.20 ppm), Rb (3-223 ppm), Zr (7-201 ppm), K/Rb (150-272), K/Cs (9.06-23.3), Nb/Ta (3.33-14). The results suggest that the Lemera granite pegmatites crystallized from a fertile, peraluminous, and S-type granitic magma through a fractional crystallization process in a late to post-collisional setting. Mineralization of Sn and Zr are noticed.
... The detailed methodology used for the seismic hazard evaluation is explained in Delvaux et al. (2017), and we will in this section only summarize the most important points. To evaluate the seismic hazard for the Kivu rift region, we compiled a regional seismic catalogue based on the ISC reviewed earthquake catalogue, completed by data from BUL (until 1990), EAF (since 1991), ENT, NAI, PRE, LSZ, CGS, NEIC, and GSHAP catalogues, spanning 126 years, with one thousand and sixty eight (1068) events. ...
... The region of the Kivu rift was subdivided into seven seismic source areas based on the regional geological structure, neotectonic fault systems, basin architecture and distribution of thermal springs and earthquake epicenters (Fig. 3). The seismic hazard map (Fig. 3) was computed using the Gutenberg-Richter seismic hazard parameters determined by the maximum likelihood method and existing attenuation laws with the Crisis 2012 software (details in Delvaux et al. 2017). The PGA values obtained (475 years return period) are higher than previous estimates (Mavonga and Durrheim 2009). ...
The Kivu rift, in the middle of the western branch of the East African Rift system, has a particular setting within the African continent. It represents the most recent (late Cenozoic) evolution of the Mesoproterozoic Karagwe-Ankole Belt of the Great Lakes Region in Central Africa. Its architecture and evolution have been profoundly influenced by the tectonic framework inherited from the Kibaran and Pan-African orogenic events. In order to build a new detailed seismic hazard map, we have compiled regional geological and neotectonic maps, re-examined the tectonic evolution, investigated the brittle structures, and determined the paleo-stress field evolution. The Kivu rift appears heterogeneous and complex. It probably started as an isolated segment that progressively linked with the adjacent segments of the Western Rift Branch. Its architecture and structural inheritance are reflected in the seismic activity and the current stress field. This results in a marked lateral variability of the Gutenberg–Richter parameters and seismic hazard estimates.
... This small land-locked country of eastern-equatorial Africa, also known as the "Land of the Thousand Hills" for its hilly to mountainous landscape, is located along the geodynamic structure of the East African Rift System (Figure 1a). To the West, the Congo-Nile ridge (or Kivu Ridge) bordering Lake Kivu results from the extensional tectonic activity along the eastern edge of the Kivu graben that was initiated during the late Miocene [4,5]. The altitude of the Kivu Ridge ranges between 1500 m and 3000 m, and the slopes locally reach up to 60 • . ...
Rwanda, in eastern tropical Africa, is a small, densely populated country where climatic disasters are often the cause of considerable damage and deaths. Landslides are among the most frequent hazards, linked to the country’s peculiar configuration including high relief with steep slopes, humid tropical climate with heavy rainfall, intense deforestation over the past 60 years, and extensive use of the soil for agriculture. The Karongi region, in the west-central part of the country, was affected by an exceptional cluster of more than 700 landslides during a single night (6–7 May 2018) over an area of 100 km2. We analyse the causes of this spectacular event based on field geological and geomorphology investigation and CHIRPS and ERA5-Land climate data. We demonstrate that (1) the notably steep slopes favoured soil instability; (2) the layered soil and especially the gravelly, porous C horizon allowed water storage and served as a detachment level for the landslides; (3) relatively low intensity, almost continuous rainfall over the previous two months lead to soil water-logging; and (4) acoustic waves from thunder or mechanical shaking by strong wind destabilized the water-logged soil through thixotropy triggering the landslides. This analysis should serve as a guide for forecasting landslide-triggering conditions in Rwanda.
... The activities of Nyiragongo and Nyamulagira are probably directly related to the opening of the Western Rift Valley (Kasahara et al. 1991;Wauthier et al. 2015). The 2002 eruption of the Nyiragongo volcano, Delvaux et al. 2017) for example, was associated with regional rifting events (Komorowski et al. 2002(Komorowski et al. /2003Tedesco et al. 2007a, b). This devastating eruption left more than 120, 000 people homeless (Tedesco et al. 2007a). ...
On 26th May 2021, an earthquake with a moment magnitude Mw 5.1 hit the densely populated cities of Gisenyi (Rwanda) and Goma (D.R. Congo) which sit on the active East African Rift System. It was one of the largest earthquakes associated with the 2021 Mount Nyiragongo eruption. Although of moderate magnitude, the earthquake substantially damaged manmade structures. This paper presents field observations on the geotechnical impact, building damage, and factors contributing to the heightened destruction caused by this moderate earthquake. The damage pattern observed in the field indicates that masonry structures with inadequate seismic detailing were the most damaged buildings. In addition, the statistical analysis of the damaged buildings indicates most of the damaged structures were located in plains covered by volcanic soil. The intensity of the waves was estimated using the building damage data based on the European Macroseismic Scale (EMS-98). An intensity distribution map was generated for the surveyed area, suggesting EMS-98 intensity of VIII or IX along the eastern basin boundary fault and VII around the cities of Goma and Gisenyi where the land is composed of black cotton soil of volcanic origin. The higher intensity values along the eastern basin-bounding fault indicate that a reevaluation of the seismic hazard for the region is necessary. Since this is the first-ever such damage survey for the region, the developed intensity map can be used to understand the correlation between the intensity of the ground motion and damage severity which contributed to the seismic hazard assessment of the study area.
... This has led to a lack of ground-motion studies and very few ground-motion models (GMMs) are available for the area (e.g. Delvaux et al., 2017;Mavonga, 2007). ...
The southern East African Rift System (EARS) is an early-stage continental rift with a deep seismogenic zone. It is associated with a low-to-moderate seismic hazard, but due to its short and sparse instrumental record, there is a lack of ground-motion studies in the region. Instead, seismic hazard assessments have commonly relied on a combination of active crustal and stable continental ground-motion models (GMMs) from other regions without accounting for the unusual geological setting of this region and evaluating their suitability. Here, we use a newly compiled southern EARS ground-motion database to compare six active crustal GMMs and four stable continental GMMs. We find that the active crustal GMMs tend to underestimate the ground-motion intensities observed, while the stable continental GMMs overestimate them. This is particularly pronounced in the high-frequency intensity measures (>5 Hz). We also use the referenced empirical approach and develop a new region-specific GMM for southern EARS. Both the ranked GMMs and our new GMM result in large residual variabilities, highlighting the need for local geotechnical information to better constrain site conditions.
... Nonetheless, the study predicted that Lake Tanganyika rift was prone to the highest level of seismic hazard, in which a maximum PGA of 0.16 g was expected for a 10% POE in 50 years. Delvaux et al. (2017) performed a classical PSHA based on a novel seismo-tectonic zonation which integrates neotectonic fault systems, seismology, geology, location of thermal springs and basin architecture of the Kivu rift region. One major drawback in this study is that completeness magnitudes were estimated visually from the change in slope of Frequency-Magnitude-Distribution (FMD), yet other statistical approaches appraised by Woessner and Wiemer (2005) would effectively give more realistic estimates. ...
Uganda lies between the eastern and western arms of the East African Rift System, the largest seismically active rift above sea level. With increasing population, urbanisation and rapid construction, seismic risk in the country is escalating fast and is compounded by the high vulnerability of the building stock and inadequate disaster prevention and mitigation strategies. Hence, there is an urgent need to assess Uganda’s resilience against seismic risks. This paper presents a Monte-Carlo based probabilistic seismic hazard model for Uganda, as the first step towards the development of a seismic risk and resilience assessment framework for the country. In addition to fault segment data, earthquake catalogues are compiled for the period between 1900 and 2022 to estimate recurrence parameters for source zones in the area of interest. Area source zones incorporating focal mechanisms are used to stochastically model a national hazard framework for Uganda. A logic tree approach is applied to implement four ground motion prediction equations for both stable continental and active shallow crust geologies. Mean hazard curves, uniform hazard spectra, earthquake disaggregation and spectral pseudo-accelerations for major Ugandan cities are derived in addition to hazard maps for the country. The findings are largely consistent with previous regional studies and confirm that western Uganda is exposed to the highest level of seismicity. The model presented herein can be used to kick-start the update and continuous improvement of Uganda Seismic Design Code and the National Policy for Disaster Preparedness and Management.
... We cannot ignore that the seismicity in the region has the potential to be at the origin of large landslides (Delvaux et al., 2017;Maki Mateso et al., 2023), although over the last decades no seismically-triggered landslides were observed . As said in Section 5.1, the cluster of large landslides in the Ruzizi Gorge is unique, and if earthquakes played a role in their occurrence, it is likely to be minor; the triggering conditions being highly facilitated by the rapidly formed threshold hillslopes associated with the landscape rejuvenation (e.g., Yenes et al., 2015). ...
... The Pan-African Orogeny refers to the following belts: the Damara Belt in Namibia; the Arabo-Nubian Shield in the northeast of Africa; and the Mozambique Fold Belt in the southeast of Africa. All these belts were formed in the late Neoproterozoic to early Paleozoic at (600−450) Ma ( Fig. 1(a)) [2,5,13]. Kibaran Belt (Mesoproterozoic) separates the Congo Craton (Archean-Paleoproterozoic) from the Bangweulu-Tanzania Block (Archean-Paleoproterozoic) ( Fig. 1(a)). ...
... Thermal springs often occur along tectonically active faults and are closely associated with the intensity and scale of modern activities of fault zones (Li et al., 2021;Delvaux et al., 2017). The association of the Isuria fault with the Majimoto hot spring (Fig. 3a) on the hanging wall therefore provides further evidence of current activity along the fault. ...
Tectonic activity impacts the environment; therefore, identifying the influence of active faulting on environmental factors, such as soil development and vegetation growth patterns, is valuable in better understanding ecosystem functions. Here, we illustrate how tectonic activity and the lithology of bedrock influence temporal and spatial patterns of vegetation and soil parameters in a fault-controlled river basin.
The Mara River basin lies in a region of previously unrecognised active normal faulting, dominated by the Utimbara and Isuria faults, resulting in areas of relative uplift, subsidence and tilting. Faulting leads to spatially variable erosion and soil formation rates as well as disruption and modification of drainage systems. On a small scale, steep escarpments cast shade and provide shelter. All of these factors might be expected to exert controls on ecosystem dynamics on a range of lengths and timescales. Here, we investigate tectonic controls on ecological processes in the Mara River basin using TanDEM-X and Sentinel-2 data. We use high-resolution digital elevation models (DEMs) to map the Utimbara and Isuria faults and to measure the height of the escarpments (up to 400 m) along the length of the faults. Total fault offset can be estimated by correlating Neogene phonolite lavas (thought to be 3.5–4.5 Myr old) on either side of the faults. If the age is correct, slip rates can be estimated to be on the order of 0.1 mm yr-1. Analysis of DEMs also reveals the presence of recent earthquake scarps in the hanging-wall sediments of the main faults and extensive alluvial fan formation on the hanging wall. Low mountain front sinuosity values and the presence of steep escarpments also suggest recent activity. Drainage is displaced across the fault traces, and, in one area, it is possible to map the lateral channel migration of the Mara River due to hanging-wall tilting.
We used a 5-year normalised difference vegetation index (NDVI) time series, the clay mineral ratio (CMR) and a moisture stress index (MSI) to investigate spatiotemporal vegetation patterns and soil formation. Whilst lithology does exert some control, as expected, we observed that the downthrown hanging wall of the faults, especially directly adjacent to the escarpment, is consistently associated with a higher degree of vegetation, wetland formation and clay distribution. Analysis of spectral indices shows that the overall spatial pattern of vegetation cover is seasonally low in the flat plains and perennially high in the vicinity of more complex, tectonically influenced structures. The NDVI highlights several locations with permanently healthy vegetation along the escarpment which extend downslope for several kilometres. Our study shows that in the Mara River basin, active normal faulting is an important stabiliser of vegetation growth patterns, likely caused by favourable hydrological and pedological conditions along the escarpments; tectonic activity has a direct beneficial influence on ecological processes in this climatically sensitive region.
... These are notably amphiboloschists of sedimentary origin, dolerites, diabases, hornblende gabbros, granites, syenites and alaskites (Villeneuve, 1980;François, 1995;Buyse et al., 2020;Odhipio et al., 2020;Villeneuve et al., 2022); 3) Recent formations of the Tertiary, are the sediments with age varying from upper Pliocene, Pleistocene and Holocene. They include often considerable and very varied deposits: fluvial and lacustrine alluvium, terrestrial deposits, volcanic edifices and flows, ashes, tuffs, and finally, travertines and peats (Lepersonne, 1949;Cahen, 1952;Cahen et al., 1966;BRGM, 1976;Delvaux et al., 2017;Ilombe et al., 2017). ...
The geology of the DRC has already been studied on regional plans, on the scale of a province or territory. But these studies are too synthetic and they do not consider the variations of petrographic facies that can occur at the scale of a locality or a village. Thus, to overcome this possibility, the present study aims to make a petrographic characterization of the rocks of the locality of Mabuku in the territory of Beni. To achieve this, we proceeded by a microscopic analysis of 10 field samples (selected on the basis of the variability of the petrographic characteristics of the rocks). Analyzes of these samples at the petrographic laboratory of the University of Nairobi in Kenya at the LPA and LPNA revealed that the lithology of the Mabuku locality consists of orthogneiss, schists, dolerite and diorite intrusions, quartzites , laterite and sandstones. However, there are four dominant formations. These are the orthogneiss found to the north; schist, dominant in the eastern part; doleritic intrusions in the northwest part; and the sandstones which are distributed along a SW-NE diagonal. Nevertheless, important gaps remain to be completed from the geochemical and metallogenic point of view in the locality of Mabuku.
... It includes seven earthquakes between M w 4.7 and M w 5.9 with an epicentral distance ranging from 21 to 52 km (refs. 74 78 . Note that IMERG-GPM data generally underestimate actual rainfall amounts 78 . ...
The movement of large, slow-moving, deep-seated landslides is regulated principally by changes in pore-water pressure in the slope. In urban areas, drastic reorganization of the surface and subsurface hydrology—for example, associated with roads, housings or storm drainage—may alter the subsurface hydrology and ultimately the slope stability. Yet our understanding of the influence of slope urbanization on the dynamics of landslides remains elusive. Here we combined satellite and (historical) aerial images to quantify how 70 years of hillslope urbanization changed the seasonal, annual and multi-decadal dynamics of a large, slow-moving landslide located in the tropical environment of the city of Bukavu, Democratic Republic of the Congo. Analysis of week-to-week landslide motion over the past 4.5 years reveals that it is closely tied to pore-water pressure changes, pointing to interacting influences from climate, weathering, tectonics and urban development on the landslide dynamics. Over decadal timescales, we find that the sprawl of urbanized areas led to the acceleration of a large section of the landslide, which was probably driven by self-reinforcing feedbacks involving slope movement, rerouting of surface water flows and pipe ruptures. As hillslopes in many tropical cities are being urbanized at an accelerating pace, better understanding how anthropogenic activity influences surface processes will be vital to effective risk planning and mitigation.
... Lateral northward channel migration of the Mara River due to tilt related with subsidence along the Utimbara fault, red arrows indicate direction of tilt on Google Earth imagery © Google Earth 2022.320Thermal springs often occur along tectonically active faults and are closely associated with the intensity and scale of modern activities of fault zones(Li et al., 2021;Delvaux et al., 2017). The association of the Isuria fault with the Majimoto hot spring(Fig. ...
Tectonic activity impacts the environment, therefore, identifying the influence of active faulting on environmental factors, such as soil development and vegetation growth patterns, is valuable in better understanding ecosystem functions. Here, we illustrate how tectonic activity and lithology of bedrock influence temporal and spatial patterns of vegetation and soil parameters in a fault-controlled river basin. 10 The Mara River Basin lies in a region of previously unrecognised active normal faulting, dominated by the Utimbara and Isuria faults, resulting in areas of relative uplift, subsidence and tilting. Faulting leads to spatially variable erosion and soil formation rates as well as disruption and modification of drainage systems. On a small scale, steep escarpments cast shade and provide shelter. All of these factors might be expected to exert controls on ecosystem dynamics on a range of lengths and timescales. Here, we investigate tectonic controls on ecological processes in the Mara River Basin using TanDEM-X and 15 Sentinel-2 data. We use high-resolution digital elevation models (DEMs) to map the Utimbara and Isuria faults and to measure the height of the escarpments (up to 400 m) along the length of the faults. Total fault offset can be estimated by correlating Neogene phonolite lavas (thought to be 3.5-4.5 Ma old) on either side of the faults. If the age is correct, slip rates can be estimated to be on the order of 0.1mm yr-1. Analysis of DEMs also reveals the presence of recent earthquake scarps in the hanging wall sediments of the main faults and extensive alluvial fan formation on the hanging wall. Low mountain front 20 sinuosity values and the presence of steep escarpments also suggest recent activity. Drainage is displaced across the fault traces, and, in one area, it is possible to map the lateral channel migration of the Mara River due to hanging wall tilting. We used a 5-year Normalised Difference Vegetation Index (NDVI) time-series, Clay Mineral Ratio (CMR) and Moisture Stress Index (MSI) to investigate spatiotemporal vegetation patterns and soil formation. Whilst lithology does exert some control, as expected, we observed that the downthrown hanging wall of the faults, especially directly adjacent to the 25 escarpment, is consistently associated with a higher degree of vegetation, wetland formation and clay distribution. Analysis of spectral indices shows that the overall spatial pattern of vegetation cover is seasonally low in the flat plains and perennially high in the vicinity of more complex, tectonically influenced structures. The NDVI highlights several locations with permanently healthy vegetation along the escarpment which extend downslope for several kilometres. Our study shows that in the Mara River Basin, active normal faulting is an important stabiliser of vegetation growth patterns, likely caused by 30 favourable hydrological and pedological conditions along the escarpments; tectonic activity has a direct beneficial influence on ecological processes in this climatically sensitive region.
... [7][8], ID 810-1 (2022) v.2.3r20220912 *2022.9.14#c481b2cb distribution of earthquakes in Rwanda with notable increase of seismicity in the west of the country in the proximity of the Albertine Rift and Kivu Lake. Such phenomena correlated with the topography of the country have thereby been visualised using the advanced cartographic methods, as described in existing publications on seismicity in Rwanda and Lake Kivu [11][12][13][14][15]. ...
This paper presents a case of the script-based cartographic data processing by the Generic Mapping Tools (GMT) and R language for geophysical mapping. The study area is located in Rwanda, Africa, which is notable for complex geological setting due to its specific location within the East African Rift. The active continental zone of the East African Rift extending in western Rwanda has importance on seismicity of the country. It affects gravity anomaly and influences the distribution and shape of the geomorphological landforms, as reflected in the topography of Rwanda. The aim of this study is to apply the cartographic command-line techniques for mapping. The geophysical goal is to highlight the correlations between the distribution , depth and magnitudes of earthquakes, topography and geophysics. Visualizing the geophysical anomalies and topography for the morphometric setting of Rwanda's terrain supports geological monitoring. The data include the General Bathymetric Chart of the Oceans (GEBCO), Earth Gravitational Model of 2008 (EGM2008), satellite derived gravity and the Shuttle Radar Topography Mission (SRTM). Nine new thematic maps on Rwanda are presented and evaluated to demonstrate the influence of the East African Ridge on regional distribution of the earthquakes and volcanos in Rwanda. The maps demonstrated accurate and effective mapping achieved due to the functionality of R and GMT scripts. A regional analysis shows that the distribution of the earthquakes and volcanoes is the highest in the western part of Rwanda and the lowest in the east proving the correlation of the active seismicity with the tectonic setting and geology of the country. The console-based scripts of R and GMT are presented with provided link to the author's GitHub repository with a full access to the codes.
... Because the sediment load of rivers increases non-linearly with flow rate (Mulder & Syvitski, 1995), during extreme flooding events rivers might have high enough sediment loads to trigger plunging flows. Lake Kivu also sits in an active rift valley so is prone to earthquakes (Delvaux et al., 2017). Earthquakes are well known triggers for slope failures that lead to turbidity currents, with prominent examples including the Grand Banks earthquake of 1929 that triggered a massive turbidity current that travelled 250 km beneath the North Atlantic, covering an area of 1000 km 2 with 0.5 to 2.0 m of sediment (Piper & Aksu, 1987). ...
When a sediment laden river flows into a stratified water body, the water mass can either intrude as an overflow, interflow or underflow, depending upon the density contrast. Different modes of sediment driven convection occur in each case. For the case of overflows, convective sedimentation occurs beneath the plume, whereby sediment rich plumes rapidly transport fine materials to depth. If underflow of dense sediment laden waters initially occurs, then after sediment has been deposited, the light interstitial material can subsequently loft and potentially mixes the entire water column. For an interflow, both lofting and sediment driven convection can occur above and below the pycnocline. All of these different regimes can be described in terms of two dimensionless parameters: namely RS = DrS / DrC and RA = DrA /DrC, where DrA is the density contrast between the upper layer and the river inflow (due to just salinity or temperature differences), DrC is the density contrast due to sediment between river and upper‐layer, and DrS is the density contrast between upper and lower layers (due to just salinity or temperature differences). Laboratory experiments were used to describe the vigour of convection in terms of these dimensionless parameters, which then allows behaviour of various inflows to be predicted. In most cases the convective velocities observed were an order of magnitude faster than Stokes settling velocities. These observations are also applied to predict how a turbidity current could lead to lofting and possible overturn of the stratification of Lake Kivu, a large meromictic lake between Rwanda and the Democratic Republic of the Congo.
... Southwards of the MER, the EARS has the western and eastern branches (Fig. 1). The western branch is seismically very active (Delvaux et al., 2012) and the active faults are providing good information in understanding of the tectonics activities of the region (e.g., Nyblade and Langston, 1995;Shudofsky, 1985;Delvaux et al., 2017). The eastern branch is magmatically very active (Albaric et al., 2010). ...
Space-time distribution of earthquakes in the East African Rift System (EARS) has been investigated for the years 2013–2016. The spatial earthquakes distribution in the time window considered in this study has revealed that the Main Ethiopian Rift (MER) is structurally linked to the eastern branch of the EARS. A relatively high level of seismicity has been obtained in this study compared to the ISC (International Seismological Centre) catalog for the same time period which implies the region is seismically more active than we thought. In addition to the rift floor, the rift margins and the surrounding plateaus are found to be seismically active. The western and eastern branches of the EARS are found to be more active than the other rift segments for the time period considered. The frequency-magnitude distribution (FMD) of events during this study period provided an average b- and a -values of ~1.01 and ~6.5, respectively. The b-value result is implying relatively high stress with tectonic origin earthquakes occurrence dominating the region. On the other hand, threshold magnitude completeness of Mc 3.0 is obtained which is the minimum value that has never been observed previously for such large area. The seismic energy map of the region has been systematically investigated and peaks energy-release is observed at Afar depression, eastern and western branches. The energy mapping shows that the MER is structurally connected to the eastern branch of the EARS which is consistent with seismicity distribution. The overall depth of occurrence of events in the eastern and the western branches of the EARS are relatively deeper than the one found for Afar and MER. This implies that the seismogenic thickness increases in EARS when we go from north to south which is in agreement with rift age, magmatism and influence of the African Super plume on the crust and upper mantle deformation in EARS.
... (2) Faults: This kind of geological structure has a prominent effect on the stability of rock mass [51,52]. In the study area there is a spectacular thrust nappe structure characterized by strong faulting activity. ...
Landslides are one of the major geohazards threatening human society. The objective of this study was to conduct a landslide hazard susceptibility assessment for Ruijin, Jiangxi, China, and to provide technical support to the local government for implementing disaster reduction and prevention measures. Machine learning approaches, e.g., random forests (RFs) and support vector machines (SVMs) were employed and multiple geo-environmental factors such as land cover, NDVI, landform, rainfall, lithology, and proximity to faults, roads, and rivers, etc., were utilized to achieve our purposes. For categorical factors, three processing approaches were proposed: simple numerical labeling (SNL), weight assignment (WA)-based and frequency ratio (FR)-based. Then 19 geo-environmental factors were respectively converted into raster to constitute three 19-band datasets, i.e., DS1, DS2, and DS3 from three different processes. Then, 155 observed landslides that occurred in the past decades were vectorized, among which 70% were randomly selected to compose a training set (TS1) and the remaining 30% to form a validation set (VS1). A number of non-landslide (no-risk) samples distributed in the whole study area were identified in low slope (
... The low topography is located in the far West followed by steep topography associated with recent tectonic uplift with active faults system oriented to North-South [15]. The Eastern is moderate slopes of a relict landscape. ...
As in other hilly and mountainous regions of the world, the hillslopes of Bujumbura are prone to landslides. In this area, landslides impact human lives and infrastructures. Despite the high landslide-induced damages, slope instabilities are less investigated. The aim of this research is to assess the landslide susceptibility using a probabilistic/statistical data modeling approach for predicting the initiation of future landslides. A spatial landslide inventory with their physical characteristics through interpretation of high-resolution optic imageries/aerial photos and intensive fieldwork are carried out. Base on in-depth field knowledge and green literature, let’s select potential landslide conditioning factors. A landslide inventory map with 568 landslides is produced. Out of the total of 568 landslide sites, 50 % of the data taken before the 2000s is used for training and the remaining 50 % (post-2000 events) were used for validation purposes. A landslide susceptibility map with an efficiency of 76 % to predict future slope failures is generated. The main landslides controlling factors in ascendant order are the density of drainage networks, the land use/cover, the lithology, the fault density, the slope angle, the curvature, the elevation, and the slope aspect. The causes of landslides support former regional studies which state that in the region, landslides are related to the geology with the high rapid weathering process in tropical environments, topography, and geodynamics. The susceptibility map will be a powerful decision-making tool for drawing up appropriate development plans in the hillslopes of Bujumbura with high demographic exposure. Such an approach will make it possible to mitigate the socio-economic impacts due to these land instabilities
... We also examined the earthquake record and selected events significant enough for the potential triggering of hillslope instability over Ikoma based on the Keefer's (Keefer, 2002) relationship between maximum epicentral distances to landslides and earthquake magnitude. For the period 2010-2020, we found six earthquakes with a magnitude (Mw) between 4.8 and 5.8 and an epicentral distance ranging from 9 to 48 km (Delvaux et al., 2017;Fig. 3. Schematic workflow for 2D correlation and image inversion. ...
Slow-moving landslides exhibit persistent but non-uniform motion at low rates which makes them exceptional natural laboratories to study the mechanisms that control the dynamics of unstable hillslopes. Here we leverage 4.5+ years of satellite-based radar and optical remote sensing data to quantify the kinematics of a slow-moving landslide in the tropical rural environment of the Kivu Rift, with unprecedented high spatial and temporal resolution. We measure landslide motion using sub-pixel image correlation methods and invert these data into dense time series that capture weekly to multi-year changes in landslide kinematics. We cross-validate and compare our satellite-based results with very-high-resolution Unoccupied Aircraft System topographic datasets, and explore how rainfall, simulated pore-water pressure, and nearby earthquakes control the overall landslide behaviour. The landslide exhibited seasonal and multi-year velocity variations that varied across the landslide kinematic units. While rainfall-induced changes in pore-water pressure exerts a primary control on the landslide motion, these alone cannot explain the observed variability in landslide behaviour. We suggest instead that the observed landslide kinematics result from internal landslide dynamics, such as extension, compression, material redistribution, and interactions within and between kinematic units. Our study provides, a rare, detailed overview of the deformation pattern of a landslide located in a tropical environment. In addition, our work highlights the viability of sub-pixel image correlation with long time series of radar-amplitude data to quantify surface deformation in tropical environments where optical data is limited by persistent cloud cover and emphasize the importance of exploiting synergies between multiple types of data to capture the complex kinematic pattern of landslides.
Tectonic fragmentation of continents is commonly accommodated by continental-scale networks of rift basins and microplates along evolving divergent plate boundaries. Yet, little is known of the geometrical controls of the microplate initiation. We explore the East African Western Rift, where the Nubian-Victoria plate boundary is collocated with a ~200-km wide region of seismicity, low wave speed lithosphere, and hot springs that extend outboard of the previously proposed active rift axis and westward into the Congo Craton, across the poly-deformed intersection zone of Paleoproterozoic NW-trending Ruzizian-Ubendianand Mesoproterozoic NE-trending Kibaran orogenic belts. To unravel the mechanisms by which the continental lithosphere is accommodating the regional tectonic strain, we explore a network of two contiguous poorly-studied rift basins in eastern DRC: the NW-striking Luama Rift, commonly characterized as a ‘failed’ Mesozoic rift, and its adjacent NE-striking Kamituga Rift located craton-ward. In the two rift basins, we unveil and analyze previously unknown widespread systems of 2 - 170-km long active faults with 10 - 130-m high scarps, and resolve their contemporary stress states, and length-scale attributes. The results reveal: 1) failure-optimal orientation of faults in the contemporary EARS stress field, 2) power-law fault-length distribution in the border-faulted Luama Rift compatible with rejuvenation of a failed rift, and ‘conditional’ power-law distribution in the Kamituga Rift, compatible with incipient extension along a southwestward continuation of the Kivu Rift, and 3) a previously unknown microplate, the Itombwe Microplate. We propose that the seemingly broad active deformation along the Western rift is accommodated by a nucleating microplate and incipient rifts.
Earthquake likelihoods have occupied humankind ever since, and the estimation of potential magnitudes is crucial for a multitude of aspects of safety. In this work, we present a probabilistic analysis of extreme magnitudes in 16 regions across the globe characterized by different seismicity to invert the traditional question of «what probability is associated with certain magnitudes». We combine the Gutenberg-Richter Law and Rank-Ordering-Statistics in a methodological approach to estimate what magnitude ranges can be almost certainly (i.e., with 95%) expected, and what magnitudes become reasonably unlikely beyond those ranges. This approach allows for estimating probabilities for maximal magnitudes per region and comparing thereto the maximal magnitudes (mr) that appeared in reality. The method explores the maximal magnitudes that could occur or be exceeded with a probability of 95%, if the respective mr are equal to or greater than these 95%-predictions, and how probable it is, that also these mr could be reproduced or exceeded. We suspect a lack of great magnitudes in the Alps and a surplus across the Atlantic Ocean from these statistical considerations.
This paper aims to comprehensively update seismic hazard assessments for Sudan and South Sudan using the Probabilistic Seismic Hazard Analysis (PSHA) method. To achieve this goal, a new and up-to-date earthquake database was developed, which includes a newly unified and updated declustered catalog, earthquake sources, and focal mechanism database. Different magnitudes were homogenized to the moment magnitude (Mw) using region-specific conversion relationships based on orthogonal regression. In addition, two types of seismotectonic idealization were considered in the seismicity assessment: the area source model and the linear (fault) source model. Focal mechanisms were used to refine the stress regime for seismicity sources derived from formal inversion analysis. To handle uncertainty, the logic-tree framework is employed, with three different Ground Motion Prediction Equations (GMPEs). The results are obtained in terms of the PGA and for the first time the spectral accelerations at two vibration periods of 0.1 s and 1 s for 475 and 975 years return periods, respectively. Hazard curves and Uniform Hazard Spectra were obtained for three considerably vulnerable cities, and PSHA disaggregation was performed in the highest risk regions with nearby seismic sources. The highest PGA values were 0.195 g and 0.285 g, with a 10% and 2% chance of exceeding these values in 50 years, respectively, in South Sudan along the western branch of the East African Rift System (EARS). In view of the increase in population and infrastructure development in the region, these results will be invaluable for seismic safety and design.
The Red Sea is a plate boundary separating the Arabian Plate and the African Plate. It extends from the Bab Al-Mandab Strait in the south to the Sinai triple junction in the north. Earthquake activity occurring in the Red Sea could affect several countries such as Ethiopia, Sudan, Egypt, Yemen, and the Kingdom of Saudi Arabia. A plan with a huge budget has been drawn up to develop the northern part of the Red Sea, especially the NEOM project. This plan also includes the construction of the King Salman Bridge. Therefore, the assessment of the seismic hazard (SH) in this seismically active region is necessary for the sustainability of these mega projects. This study presents a probabilistic seismic hazard assessment (PSHA) along the Red Sea coasts. An earthquake catalogue including the available historical records and instrumental seismicity is compiled. It is processed by converting various magnitude types to the moment magnitude (Mw). Repetitions within the compiled catalogue are removed, and then, the catalogue is de-clustered. A completeness test is applied to the compiled catalogue. In addition to the three areal source models, a recently developed fault model is utilized to represent the seismicity along and around the Red Sea. Four ground-motion prediction equations (GMPEs) are used to estimate the ground motion. The SH inputs are implemented through a logic-tree framework, and the resulting Peak Ground Acceleration (PGA) maps and spectral acceleration (SA) maps at 0.2 and 1.0 second for return periods of 475 and 2,475 years illustrate that the maximum acceleration values are concentrated around the East African Rift, the Harrat Lunayyir volcanic field, and the Gulf of Aqaba. Seismic hazard curves for some important cities located on the Red Sea coast are produced and de-aggregated to obtain the magnitude and distance of the earthquake source dominating each seismic hazard curve.
Le bassin versant topographiue du lac Kivu se tend sur deux pays, la RDC et le Rwanda. Il a à son centr la plus grande ile lacustre du Continent (Idjwi).
dans cette Présentation, nous avons porté une intervation sur la pétrographie du Flanc Ouest sur deux territoires, Masisi (Nord Kivu) et Kalehe (Sid Kivu).
Les intrusions basiues logent les bords du lac, celles acides au sommet de Mutumb. le terrain est fragmenté ci et là en micro grabens par la tectonogénèse
L’Afrique évolue tectoniquement avec finalité la fragmentation en microplaques. La trace est l’activité du rift en graben servant du futur plancher océanique ; les rides océaniques étant les points chauds, cas de Virunga. Le SREA a deux branches (Rift valley, albertin) toutes actives à volcans. La branche Ouest, dont fait le Rift Kivucien est une dépression NE-SW à NW-SE. Le lac Kivu à l’aplomb de l’axe sert de point de virgation. Dans cette partie rift, une série des failles E-W jouant périodiquement comme failles transformantes avec agitation des compartiments, glissement du profil pédogènique.
La faille E-W dans la vallée de Renga, au Nord-Kivu, serait borgne sous les alluvions. L’écartement cisaillant est périodiquement la cause de rupture de l’axe routier. Selon l’histoire et dire des riverains, au Sud de Minova, à la confluence Lulunga et Mubimbi la haute tension connaissait une casse des files, ce qui traduit un cisaillement des lèvres jointives. Notre lever y dénichant une brèche tectonique longue sur des Kilomètre.
Une faille au Sud-Kivu, est un exemple illustratif. Bordée de la cité stannifère de Nyabibwe, la rupture routiere perpétuelle et basculement des ponts au NE de la cité, la fissuration des murs de l’école primaire Nyabibwe centre cité, l’affaissement de la zone Sud de la cité est une attestation de la faille. Les arbres formant un arc dont la concavité tourne vers le sommet est aussi une matérialisation du jet des masses. Il s’agit d’une faille courbe dont l’activité évolue vers le Nord avec trop de glissements et éboulements.
Earthquake hazard assessment is the first step towards implementing prevention, preparedness, and response or faster recovery actions to reduce the risk of seismic disasters. In this paper, we present a comprehensive study on present-day seismicity in terms of the estimated waiting time and conditional probability in Africa by 2022 – 2072 using four competing distribution models (Brownian passage time (BPT), gamma, lognormal, and Weibull). We also investigate how much Coulomb stress change () induced by previous earthquakes that occurred in neighboring active sources should revise the probability of occurrence at the location of the next events. We analyze large earthquakes with moment magnitude, , collating data from the Global Centroid Moment Tensor and from several published literature that list fault plane solutions of large African earthquakes since 1900. We assume that the dataset is stationary and consists of independent events. First, the model's parameters are estimated and the results of the statistical analysis of the interevent times show clear evidence of quasi-periodic recurrence behavior for large earthquakes 6.0 in different seismotectonic regions in Africa. Next, a comparison among the distribution models is performed with the aim of selecting the most suitable one. The results in terms of the maximum likelihood criterion and its extension (Akaike Information Criterion) indicate that, in general, the BPT and Weibull models had similar fits to each other, and performed slightly better than gamma and lognormal models. Then, we use Gaussian random distributions to treat parameter uncertainties (e.g., aperiodicity, maximum expected magnitude, slip rate, and mean recurrence time) of the distribution models associated with each seismotectonic region. From repeated Monte Carlo draws, we assess uncertainties of the 50-year conditional probability values for the next earthquake obtained from two distribution models (BPT and BPT + ) related to the 50th percentile. The results of the BPT distribution indicate very high chances of future earthquakes in the study region where the conditional probability of a large earthquake reaches 99.5%, 95.6%, 83.1%, and 82.2% for the western branch of the East African Rift System (EARS), northwest Africa, the Afar region, and the eastern branch of EARS, respectively. Taking into account the effect of stress change from interacting sources (BPT + ), these probabilities are slightly modified to 99.8%, 98.4%, 89.9%, and 87.3% for the western branch of EARS, northwest Africa, the Afar region, and the eastern branch of EARS, respectively. These marginal increases suggest that the estimated effect of the earthquake interaction introduced by the coseismic slip of previous earthquakes on neighboring active sources is minor if compared with the uncertainties affecting the renewal models used for the basic time-dependent conditional probability assessment.
Historical and instrumental earthquake catalogues in low strain rate regions are not necessarily indicative of the long-term spatio-temporal distribution of seismicity. This implies that probabilistic seismic hazard analysis (PSHA) should also consider geologic and geodetic data through fault-based seismogenic sources. However, it is not always clear how on-fault magnitude-frequency distributions should be described and, if the seismogenic layer is especially thick, how fault sources should be extrapolated down-dip. We explore these issues in the context of a new PSHA for Malawi, where regional extensional rates are 0.5-2 mm/yr, the seismogenic layer is 30-40 km thick, the instrumental catalog is ∼60 years long, and fault-based sources were recently collated in the Malawi Seismogenic Source Model. Furthermore, Malawi is one of several countries along the East African Rift where exposure to seismic hazard is growing, but PSHA does not typically consider fault sources. We use stochastic event catalogs to explore different fault source down-dip extents and magnitude-frequency distributions. Our PSHA indicates that hazard levels are highest for a Gutenberg-Richter on-fault magnitude-frequency distribution, even at low probabilities of exceedance (2% in 50 years), whilst seismic hazard levels are also sensitive to how relatively short (<50 km) fault sources are extrapolated down-dip. For sites close to fault sources (<40 km), seismic hazard levels are doubled compared to previous instrumental-seismicity based PSHA in Malawi. Cumulatively, these results highlight the need for careful fault source modelling in PSHA of low strain rate regions, and the need for new fault-based PSHA elsewhere in the East Africa Rift.
Active fault data are commonly used in seismic hazard assessments, but there are challenges in deriving the slip rate, geometry, and frequency of earthquakes along active faults. Herein, we present the open-access geospatial Malawi Seismogenic Source Model (MSSM; 10.5281/zenodo.5599616), which describes the seismogenic properties of faults that formed during ongoing east African rifting in Malawi. We first use empirically derived constraints to geometrically classify active faults into section, fault, and multifault seismogenic sources. For sources in the North Basin of Lake Malawi, slip rates can be derived from the vertical offset of a seismic reflector that dated lake cores indicate is 75 ka. Elsewhere, slip rates are constrained from advancing a systems-based approach that partitions geodetically derived rift extension rates in Malawi between seismogenic sources using a priori constraints on a regional strain distribution and a hanging wall flexural extension in magma-poor continental rifts. Slip rates are then combined with source geometry and empirical scaling relationships to estimate earthquake magnitudes and recurrence intervals, and their uncertainty is described from the variability in logic tree outcomes used in these calculations. Sources in the MSSM are 5–269 km long, which implies that large-magnitude (Mw 7–8) earthquakes may occur in Malawi. However, low slip rates (0.05–2 mm yr-1) mean that the frequency of such events will be low (recurrence intervals of ∼103–104 years). We also find that, for 9 out of 11 faults in Lake Malawi's North Basin, differences in the slip rates, when estimated independently from the geodetic data and the offset seismic reflector, are not statistically significant. The MSSM represents an important resource for investigating Malawi's increasing seismic risk and provides a framework for incorporating active fault data into seismic hazard assessment elsewhere in the East African Rift and other tectonically active regions.
This study was conducted to contribute to improvement and sustainable development of Congolese cities, particularly Bukavu city. GIS and Remote Sensing technics were used to estimate the built-up suitable areas in Bukavu taking into account the slopes, rivers, exclusion zones, protected zones, lakes, roads, identified landslides areas, erosion and flooding. These exclusion areas were subtracted from the total city area to determine the suitable areas. 2030 and 2050 population projections were made based on 2016 estimations. The results obtained show that only 68.6% (∼29.7 Km²) are suitable for built-up. Based on 2016 estimation, the population will double by 2030 and triple by 2050. While the areas that can be developed for settlements can accommodate only ∼186 000 people, the surplus will be relocated in two selected satellite cities, namely Nyatende and Miti-Murhesa centres, having ∼38 and ∼125 km² of suitable areas respectively. Both vertical and horizontal development with an average plot size of 350 m² would reduce the negative impacts and ensure a sustainable city environment. To maintain the sustainability of Bukavu and its former name of “green city”, both horizontal and mixed extensions will be considered while the two selected zones have to be well planned for new satellites cities and new Bukavu city development.
The DRC encompasses both intra-plate and active tectonic areas associated with the Congo Craton and the western branch of the East African Rift System, respectively. The seismic hazard assessment is based on the new Sub-Saharan-Global Earthquake Model earthquake catalogue (SSA–GEM) with homogeneous magnitude representation. The final declustered catalogue spans 55 years (from 1960 to 2015) with 398 events and a magnitude of completeness of about 4.5. The maximum credible magnitude of earthquakes was determined using the entire catalogue from 1900 to 2015. The seismotectonic zonation into 15 seismic source zones was done on the basis of the regional geological structure, neotectonic fault systems, distribution of thermal springs, regional strain rate model developed for the East African Rift by Saria and earthquake epicenters. The Gutenberg Richter parameters were determined using Aki’s maximum likelihood method Aki (Bulletin of the Earthquake Research Institute, Tokyo University, 43, 237-239, 1965) and Weichert’s method Weichert (Bulletin of the Seismological Society of America, 70, 1337-1346, 1980), which were compared with extension of the Aki-Utsu b value estimator for incomplete catalogues by Kijko and Smit. Hazard computations were performed using the Open Quake engine (version 2.7.0-1). The peak Ground Acceleration and spectral acceleration at periods of 0.05, 0.1, 0.2, 0.5, 1 and 2s were calculated using 4 Ground Motion Prediction Equations: two for active shallow crust and two for stable continental conditions for soil sites corresponding to Vs30 = 600, 760 and 1500 m/s at 11 cities of the DRC and surrounding areas. The results are consistent with those obtained using the regional frequency-independent attenuation law using Crisis 2012 software.
In low strain rate regions, extrapolation of incomplete instrumental earthquake records is a limitation for probabilistic seismic hazard analysis (PSHA). This limitation can be addressed by incorporating geologic and geodetic data into fault-based seismogenic sources, although it is not always clear how on-fault magnitude-frequency distributions should be described and, if the seismogenic layer is especially thick, how these sources should be extrapolated down-dip. We explore these issues in the context of a new PSHA for Malawi, where regional extensional rates are 0.5-2 mm/yr, the seismogenic layer is 30-40 km thick, the instrumental catalog is ~60 years long, and fault-based sources were recently collated in the Malawi Seismogenic Source Database. Furthermore, Malawi is one of several countries along the East African Rift where exposure to seismic hazard is growing, but PSHA typically considers instrumental records alone. We use stochastic event catalogs to explore different fault-source down-dip extents and magnitude-frequency distributions. These indicate that hazard levels are highest for a Gutenberg-Richter on-fault magnitude-frequency distribution, even at low probabilities of exceedance (2% in 50 years), whilst seismic hazard levels are also sensitive to how relatively short (<50 km) fault sources are extrapolated down-dip. For sites close to fault sources (<40 km), seismic hazard levels are doubled compared to previous instrumental-seismicity based PSHA in Malawi. Cumulatively, these results highlight the need for careful fault source modelling in PSHA of low strain rate regions, and the need for new fault-based PSHA elsewhere in East Africa.
This paper presents the first comprehensive analysis of present‐day stress from boreholes near an active continental rifting zone in the Moatize Basin, Mozambique. The state of present‐day stress in this area that is located ~100 km away from the Eastern African Rift System (EARS) is poorly understood, and most of our knowledge is from earthquake focal mechanisms that provide stress information from the deeper part of the lithosphere, and to a lesser extent from surface geological features in the Malawi region. Considering the limited reliability of earthquake‐derived stress orientations near plate boundaries, poor coverage of low to moderate magnitude earthquakes in eastern Africa, and ambiguity about the latest activity of geological structure; other well‐established methods are required to shed light on the active tectonics of EARS. In this study, we analyse stress orientation using log data from 95 vertical boreholes in a mine site to investigate the neotectonic stress pattern of the region. Analysis of 17.9 km of televiewer logs resulted in interpretation of 1,188 stress‐related borehole failures. The results indicate a mean regional trend of 045º ± 31º for the maximum horizontal stress (SHmax). Our investigation reveals that the regional state of stress in the study area is controlled by superposition of stress sources that act at very different spatial scales. The consistency between our results and predictions by plate‐scale geodynamic models of stress orientations confirm that the regional pattern of stress in this area is mainly controlled by first (>500 km) and second (distances between 500‐100km) order stress sources (i.e., large tectonic forces and lateral density variations). However, high‐resolution data used in this study reveals that third (between 100‐1 km) and fourth (<1 km) order stress sources from stiffness contrasts, rock fabric and geological structures have a great impact on the stress perturbations at smaller and local scales.
La forêt claire de type miombo, le « miombo woodland » des auteurs anglo-saxons, est un type de végétation largement distribué en Afrique zambézienne où il fournit des produits forestiers, ligneux ou non, à des millions d'habitants. En RD Congo, il couvre près de 23 % de la surface forestière totale et reste le type de forêt le plus dominant (>50 %) dans l'ex-province du Katanga (Kabulu et al., 2008; Potapov et al., 2012). Autour de la ville de Lubumbashi, les causes de la régression de sa couverture, soutenues par la croissance démographique rapide, sont principalement : (i) le développement agricole, (ii) la production de charbon de bois, (iii) l'expansion de la ville et (iv) les activités minières. Les perturbations engendrées par cette déforestation seraient responsables du raccourcissement de la durée des pluies (Sanga-Ngoie & Fukuyama, 1996; Assani, 1999), d'une malnutrition persistante dans le milieu rural (Malaisse, 1997) et de la perte de biodiversité (Barima et al., 2011; Vranken et al., 2011). L'ampleur inquiétante de ces conséquences a conduit plusieurs chercheurs à quantifier la déforestation autour de Lubumbashi à travers le concept de «rayon de déforestation». Nous présentons une méta-analyse des études ayant circonscrit le rayon de déforestation autour de Lubumbashi. Ce rayon, utilisé à la fois pour exprimer la superficie (zone circulaire), l’intensité et l'ampleur (distance à la ville) de la déforestation, a été déterminé à travers les observations de la production de charbon de bois in situ et la télédétection. Les observations effectuées dans les villages des producteurs de charbon de bois expriment le rayon de déforestation à travers la distance qui les sépare de la ville, ce qui reflète plutôt l'ampleur de la déforestation. Ces estimations de distances, qui n'augmentent pas nécessairement avec le temps comme attendu par ailleurs, varient selon les auteurs, les années d'observation et les distances des villages visités par rapport à la ville; en plus, souvent elles ne considèrent pas les taches de miombo peu accessibles situées entre les villages à proximité de Lubumbashi. Les études in situ ignorent les taches de miombo proches de la ville, et semblent donc surévaluer l'ampleur de la déforestation. A partir de cette approche, des projections de la suppression complète du miombo ont été réalisées (Assani, 1999). Force est de constater que les difficultés d'accès et la privatisation de certaines concessions font que des taches de miombo subsistent sur des courtes distances à la ville et le seront jusqu'à l'horizon 2050 (Vranken et al., 2011). Par contre, les études basées sur la télédétection surestiment parfois ce rayon, mais aussi la résistance des taches de miombo, en ignorant leur taux de dégradation, sur des courtes distances à la ville. Malaisse & Binzangi (1985) ont considéré que les taches de miombo qui subsistent sur des courtes distances à la ville, et identifiées par télédétection comme telles, correspondent plutôt aux savanes secondaires que Kabulu et al. (2008) ont identifié comme des complexes de forêt claire et savanes boisées. Il en résulte que ces deux approches ne sont pas cohérentes ou compatibles dans l'étude de l'importance de la déforestation autour de la ville de Lubumbashi en raison de la variabilité des protocoles méthodologiques au sein de chaque approche, mais aussi entre les approches. Ces observations empêchent le développement d'une politique appropriée de conservation et d'exploitation durable de l'écosystème en question. Par conséquent, une harmonisation des approches utilisées en termes méthodologique et conceptuel s'impose. Elle pourrait former le point départ d'une relecture rétrospective critique des estimations historiques de la déforestation et de la dégradation du miombo, afin d'interpréter correctement les dynamiques spatio-temporelles de cet écosystème unique et crucial pour la population katangaise.
he Albertine Rift, which is part of the East African Rift, is a region prone to repetitive slope instability due to its intense rainfall, high weathering rate, and consistent seismic activities. Although this has led to several publications about landslides in the past. Unfortunately, not many attempts at gathering systematic data on landslide research in the western Albertine rift have been made so far. This study analyses the research carried out in landslides in the Albertine rift during 2003-2019 on several parameters including the title, authors, institutions, publication year, keywords, the names of journals publishing the articles, and language. The majority of papers were written in French (62%). Geo-Eco-Trop journal has published the greatest number of papers (46%). The majority of authors (67%) produced only one paper. Moreover, Moeyersons had the highest co-authorship connections (15), followed by Dille, Trefois, and Nobil with 14, 10, and 7 connections respectively. 13 institutions from 6 countries contributed to the researches. The most producing institutions were Royal Museum for Central Africa, followed by the “Université Officielle de Bukavu”. 49 Keywords were used, among which, 43 (88%) appeared only once and 6 (12%) appeared twice. The most frequently occurring words are landslides and the tropical environment.
Active fault data are commonly used in seismic hazard assessments, but there are challenges in deriving the slip rate, geometry, and frequency of earthquakes along active faults. Herein, we present the open-access geospatial Malawi Seismogenic Source Database (MSSD), which describes the seismogenic properties of faults that have formed during East African rifting in Malawi. We first use empirical observations to geometrically classify active faults into section, fault, and multi-fault seismogenic sources. For sources in the North Basin of Lake Malawi, slip rates can be derived from the vertical offset of a seismic reflector that is estimated to be 75 ka based on dated core. Elsewhere, slip rates are constrained from advancing a ‘systems-based’ approach that partitions geodetically-derived rift extension rates in Malawi between seismogenic sources using a priori constraints on regional strain distribution in magma-poor continental rifts. Slip rates are then combined with source geometry and empirical scaling relationships to estimate earthquake magnitudes and recurrence intervals, and their uncertainty is described from the variability of outcomes from a logic tree used in these calculations. We find that for sources in the Lake Malawi’s North Basin, where slip rates can be derived from both the geodetic data and the offset seismic reflector, the slip rate estimates are within error of each other, although those from the offset reflector are higher. Sources in the MSSD are 5–200 km long, which implies that large magnitude (MW 7–8) earthquakes may occur in Malawi. Low slip rates (0.05–2 mm/yr), however, mean that the frequency of such events will be low (recurrence intervals ~103–104 years). The MSSD represents an important resource for investigating Malawi’s increasing seismic risks and provides a framework for incorporating active fault data into seismic hazard assessment in other tectonically active regions.
Human activity influences both the occurrence and impact of landslides in mountainous environments. Population pressure and the associated land-use changes are assumed to exacerbate landslide risk, yet there is a lack of statistical evidence to support this claim, especially in the Global South where historical records are scarce. In this work, we explore the interactions between population, deforestation and landslide risk in the Kivu Rift in Africa. To do so, we develop a holistic landslide risk model that evaluates 58 years of population and forest-cover trends. We show that the current landslide risk in the eastern Democratic Republic of the Congo (DRC) is twice as high as in neighbouring Rwanda and Burundi. Congolese households, on average, populate more hazardous terrain, probably as a result of conflicts and economic pull factors such as mining. Moreover, the recent large-scale deforestation of primary rainforest in the DRC has considerably exacerbated the landslide risk. Our analysis demonstrates how the legacy of deforestation, conflicts and population dynamics is reflected in the landslide risk in the Kivu Rift.
We present the Malawi Active Fault Database (MAFD), a geospatial database of 114 active fault traces in Malawi, and in neighboring Tanzania and Mozambique. The MAFD has been developed from a multidisciplinary dataset: high resolution digital elevation models, field observations, aeromagnetic and gravity data, and seismic reflection surveys from offshore Lake Malawi. Active faults longer than 50 km are found throughout Malawi, where seismic risk is increasing due to its rapidly growing population and its seismically vulnerable building stock. The MAFD also provides an opportunity to investigate the population of normal faults in an incipient continental rift. We find that the null hypothesis that the distribution of fault lengths in the MAFD is described by a power law cannot be rejected. Furthermore, a power-law distribution of faults in Malawi is consistent with its thick seismogenic crust (~35 km), and low (<8%) regional extensional strain that is predominantly (50-75%) accommodated across relatively long hard-linked border faults. Cumulatively, the data and inferences drawn from the MAFD highlight the importance of integrating onshore and offshore geological and geophysical data to develop active fault databases along the East African Rift and similar continental settings, both to understand the regional seismic hazard and tectonic evolution.
Seismic hazard is commonly characterised using instrumental seismic records. However, these records are short relative to earthquake repeat times, and extrapolating to estimate seismic hazard can misrepresent the probable location, magnitude, and frequency of future large earthquakes. Although paleoseismology can address this challenge, this approach requires certain geomorphic setting, is resource intensive, and can carry large inherent uncertainties. Here, we outline how fault slip rates and recurrence intervals can be estimated by combining fault geometry, earthquake-scaling relationships, geodetically derived regional strain rates, and geological constraints of regional strain distribution. We apply this approach to southern Malawi, near the southern end of the East African Rift, and where, although no on-fault slip rate measurements exist, there are constraints on strain partitioning between border and intra-basin faults. This has led to the development of the South Malawi Active Fault Database (SMAFD), a geographical database of 23 active fault traces, and the South Malawi Seismogenic Source Database (SMSSD), in which we apply our systems-based approach to estimate earthquake magnitudes and recurrence intervals for the faults compiled in the SMAFD. We estimate earthquake magnitudes of MW 5.4–7.2 for individual fault sections in the SMSSD and MW 5.6–7.8 for whole-fault ruptures. However, low fault slip rates (intermediate estimates ∼ 0.05–0.8 mm/yr) imply long recurrence intervals between events: 102–105 years for border faults and 103–106 years for intra-basin faults. Sensitivity analysis indicates that the large range of these estimates can best be reduced with improved geodetic constraints in southern Malawi. The SMAFD and SMSSD provide a framework for using geological and geodetic information to characterise seismic hazard in regions with few on-fault slip rate measurements, and they could be adapted for use elsewhere in the East African Rift and globally.
We present the Seismotectonic Map of Africa based on a geological, geophysical and geodetic database including instrumental seismicity and re-appraisal of large historical events, and harmonization and homogenization of earthquake parameters in the catalogues. Although establishing the seismotectonic framework of the African continent is a difficult task, several previous and ongoing projects provide a wealth of data and outstanding results. The database of large and moderate earthquakes in different geological domains includes the coseismic and Quaternary faulting that reveals the complex nature of the active tectonics in Africa. The map benefits from previous works on local and regional seismotectonic maps that needed to be integrated with the lithospheric and upper mantle structures, seismic anisotropy tomography and gravity anomaly, into a continental framework. The synthesis of earthquake and volcanic studies obtained from the analysis of late Quaternary faulting and geodetic data will serve as a basis for hazard calculations and the reduction of seismic risks. The map will be useful for the seismic hazard assessment and earthquake risk mitigation for significant infrastructures and their socio-economic implications in Africa. The constant population increase and infrastructure growth in the continent that exacerbate earthquake risk justify the necessity of continuously updating this map. The database and related map are prepared in the framework of the IGC Project-601 "Seismotectonics and Seismic Hazards in Africa" of UNESCO-IUGS, funded by the Swedish International Development Agency and UNESCO-Nairobi for a period of 4 years (2011 - 2014, now extended to 2016).
On 24 October 2002, Mw 6.2 earthquake occurred in the central part of the Lake Kivu basin, Western Branch of the East African Rift. This is the largest event recorded in the Lake Kivu area since 1900. An integrated analysis of radar interferometry (InSAR), seismic and geological data, demonstrates that the earthquake occurred due to normal-slip motion on a major preexisting east-dipping rift border fault. A Coulomb stress analysis suggests that diking events, such as the January 2002 dike intrusion, could promote faulting on the western border faults of the rift in the central part of Lake Kivu. We thus interpret that dike-induced stress changes can cause moderate to large-magnitude earthquakes on major border faults during continental rifting. Continental extension processes appear complex in the Lake Kivu basin, requiring the use of a hybrid model of strain accommodation and partitioning in the East African Rift. Key Points:: Mw 6.2 earthquake due to normal slip on a major preexisting east-dipping rift border fault Diking could promote faulting on border faults of the rift in the central part of Lake Kivu Diking plays a major role in accommodating upper crustal extension in a considered magma-poor rift © 2015. The Authors. Geochemistry, Geophysics, Geosystems published by Wiley Periodicals, Inc. on behalf of American Geophysical Union.
A summit and upper flank eruption occurred at Nyamulagira volcano, Democratic Republic of Congo, from 2–27 January 2010. Eruptions at Nyamulagira during 1996–2010 occurred from eruptive fissures on the upper flanks or within the summit caldera and were distributed along the ~N155E rift zone, whereas the 2011–2012 eruption occurred ~12 km ENE of the summit. 3D numerical modeling of Interferometric Synthetic Aperture Radar (InSAR) geodetic measurements of the co-eruptive deformation in 2010 reveals that magma stored in a shallow (~3.5 km below the summit) reservoir intruded as two subvertical dikes beneath the summit and southeastern flank of the volcano. The northern dike is connected to an ~N45E-trending intra-caldera eruptive fissure, extending to an ~2.5 km maximum depth. The southern dike is connected to an ~N175E-trending flank fissure extending to the depth of the inferred reservoir at ~3.5 km. The inferred reservoir location is coincident with the reservoir that was active during previous eruptions in 1938–1940 and 2006. The volumetric ratio of total emitted magma (intruded in dikes + erupted) to the contraction of the reservoir (rv) is 9.3, consistent with pressure recovery by gas exsolution in the small, shallow modeled magma reservoir. We derive a modified analytical expression for rv, accounting for changes in reservoir volume induced by gas exsolution, as well as eruptive volume. By using the precise magma composition, we estimate a magma compressibility of 1.9–3.2 × 109 Pa−1 and rv of 6.5–10.1. From a normal-stress change analysis, we infer that intrusions in 2010 could have encouraged the ascent of magma from a deeper reservoir along an ~N45E orientation, corresponding to the strike of the rift transfer zone structures and possibly resulting in the 2011–2012 intrusion. The intrusion of magma to greater distances from the summit may be enhanced along the N45E orientation, as it is more favorable to the regional rift extension (compared to the local volcanic rift zone, trending N155E). Repeated dike intrusions beneath Nyamulagira’s SSE flank may encourage intrusions beneath the nearby Nyiragongo volcano.
The geological record at rifts and margins worldwide often reveals considerable along-strike variations in volumes of extruded and intruded igneous rocks. These variations may be the result of asthenospheric heterogeneity, variations in rate and timing of extension; alternatively, pre-existing plate architecture and/or the evolving kinematics of extension during breakup may exert first order control on magmatism. The Main Ethiopian Rift (MER) in East Africa provides an excellent opportunity to address this dichotomy: it exposes, along-strike, several sectors of asynchronous rift development from continental rifting in the south to incipient oceanic spreading in the north. Here we perform studies of volcanic cone density and rift obliquity along strike in the MER. By synthesizing these new data in light of existing geophysical, geochemical and petrological constraints on magma generation and emplacement, we are able to discriminate between tectonic and mantle geodynamic controls on the geological record of a newly forming magmatic rifted margin. The timing of rift sector development, the three-dimensional focusing of melt, and the ponding of plume material where the rift dramatically narrows, each influence igneous intrusion and volcanism along the MER. However, rifting obliquity plays an important role in localizing intrusion into the crust beneath en-echelon volcanic segments. Along-strike variations in volumes and types of igneous rocks found at rifted margins thus likely carry information about the development of strain during rifting, as well as the physical state of the convecting mantle at the time of breakup.
In regions with large, mature fault systems, a characteristic earthquake model may be more appropriate for modelling earthquake occurrence than extrapolating from a short history of small, instrumentally observed earthquakes using the Gutenberg–Richter scaling law. We illustrate how the geomorphology and geodesy of the Malawi Rift, a region with large seismogenic thicknesses, long fault scarps, and slow strain rates, can be used to assess hazard probability levels for large infrequent earthquakes. We estimate potential earthquake size using fault length and recurrence intervals from plate motion velocities and generate a synthetic catalogue of events. Since it is not possible to determine from the geomorphological information if a future rupture will be continuous (7.4 ≤ M
W ≤ 8.3 with recurrence intervals of 1,000–4,300 years) or segmented (6.7 ≤ M
W ≤ 7.7 with 300–1,900 years), we consider both alternatives separately and also produce a mixed catalogue. We carry out a probabilistic seismic hazard assessment to produce regional- and site-specific hazard estimates. At all return periods and vibration periods, inclusion of fault-derived parameters increases the predicted spectral acceleration above the level predicted from the instrumental catalogue alone, with the most significant changes being in close proximity to the fault systems and the effect being more significant at longer vibration periods. Importantly, the results indicate that standard probabilistic seismic hazard analysis (PSHA) methods using short instrumental records alone tend to underestimate the seismic hazard, especially for the most damaging, extreme magnitude events. For many developing countries in Africa and elsewhere, which are experiencing rapid economic growth and urbanisation, seismic hazard assessments incorporating characteristic earthquake models are critical.
The East African Rift (EAR) is a type-locale for investigating the processes that drive continental rifting and breakup. The current kinematics of this ~5000-km long divergent plate boundary between the Nubia and Somalia plates is starting to be unraveled thanks to a recent augmentation of space geodetic data in Africa. Here, we use a new data set combining episodic GPS measurements with continuous measurements on the Nubian, Somalian, and Antarctic plates, together with earthquake slip vector directions and geologic indicators along the Southwest Indian Ridge to update the present-day kinematics of the EAR. We use geological and seismological data to determine the main rift faults and solve for rigid block rotations while accounting for elastic strain accumulation on locked active faults. We find that the data is best fit with a model that includes three microplates embedded within the EAR, between Nubia and Somalia (Victoria, Rovuma, and Lwandle), consistent with previous findings but with slower extension rates. We find that earthquake slip vectors provide information that is consistent with the GPS velocities and helps to significantly reduce uncertainties of plate angular velocity estimates. We also find that 3.16 My MORVEL average spreading rates along the Southwest Indian Ridge are systematically faster than prediction from GPS data alone. This likely indicates that outward displacement along the SWIR is larger than the default value used in the MORVEL plate motion model.
The improvement of the geodetic coverage within the African Plate over
the last decade together with an extended GPS position time-series
allows improved accuracy in determining the velocity field than prior
geodetic studies. Using this new velocity field of the whole African
continent, the best model proposed here remains consistent with previous
studies including the existence of two small plates along the East
African Rift System (EARS, Victoria and Rovuma). We focus specifically
on the velocities along this plate boundary by estimating both the
geodetic and the seismic moment rate. Whereas we use a scalar form of
the Kostrov relation to calculate the geodetic moment rate, the seismic
moment rate is obtained by integrating the cumulative truncated
Gutenberg-Richter earthquake distribution of local events in the
39-yr-long worldwide catalogue, using a maximum likelihood method. This
statistical method allows us to take into account the probable
incompleteness of the existing catalogue and to assume the seismic
moment rate calculated from this short catalogue to be representative of
the long-term seismic deformation. The comparison of geodetic and
seismic energy release sheds light on the variations of mechanical
behaviour related to intracontinental extension along the EARS. The
southward increase, observed along the rift, of the proportion of
geodetic moment seismically accommodated suggests a significant control
of the thermal structure associated with different states of rifting
evolution.
The shear wave velocity structure of the upper mantle beneath the East
African plateau has been investigated using teleseismic surface waves
recorded on new broadband seismic stations deployed in Uganda and
Tanzania, as well as on previously deployed stations in Tanzania and
Kenya. Rayleigh wave phase velocities at periods between 20 and 182 s,
measured with a two-plane wave method, have been used to create phase
velocity maps, and dispersion curves extracted from the maps have been
inverted to obtain a quasi-3-D shear wave velocity model of the upper
mantle. We find that phase velocities beneath the Tanzania Craton and
areas directly north and west of the craton are faster, at all periods,
than those beneath the Western and Eastern branches of the East African
Rift System. At periods <50 s, the western branch is slower than the
Eastern Branch, but at periods greater than 50 s, this relationship is
reversed. Anisotropy is found at all periods, with a generally
north-south fast polarization direction. The shear wave velocity model
shows a seismically fast lithosphere (lid) beneath the Tanzania Craton
to depths between 150 and 200 km. The fast velocities in this depth
range extend to the north beneath the Uganda Basement Complex and to the
east beneath the northern Tanzania divergence zone, indicating that
these regions together form a rigid block around which rifting has
occurred within weaker mobile belt lithosphere. The Eastern and Western
branches are slower than the craton at lithospheric mantle depths, and
both branches show variable structure in the upper 200 km of the mantle,
with the lowest velocities found beneath areas of Cenozoic volcanism. At
depths greater than ˜225 km, a low velocity anomaly is present
beneath the entire East African plateau that may extend into the mantle
transition zone. Velocities in the low velocity region are reduced by
≥10 per cent relative to lid velocities, and if attributed only to
temperature variations, would represent an unrealistic thermal
perturbation of >400 K. Consequently, it is likely that the velocity
reduction reflects a combination of thermal and compositional changes,
and also possibly the presence of partial melt. The width and thickness
of the low velocity anomaly is greater than typically expected for a
plume head and is more easily attributed to an upward continuation of
the lower mantle African superplume structure into the upper mantle.
Seismic hazard assessment and mitigation of catastrophes are primarily
based on the identification and characterization of seismically active
zones. These tasks still rely heavily on the existing knowledge of the
seismic activity over the longest possible time period. The first
seismic network in Equatorial Africa (IRSAC network) was operated from
the Lwiro scientific base on the western shores of Lake Kivu between
1953 and 1963. Before this installation, the historical record of
seismic activity in Central Africa is sparse. Even for the relatively
short period concerned, spanning only 50-60 years, the historical record
is far from being complete. A first attempt has been made by Herrinckx
(1959) who compiled a list 960 felt seisms recorded at the
meteorological stations between 1915 and 1954 in Congo, Rwanda and
Burundi. They were used to draw a density map of felt seisms per square
degree. We completed this data base by exploiting the meteorological
archives and any available historical report to enlarge the database
which now reaches 1513 entries between 1900 and 1959. These entries
have been exanimate in order to identify possible historical seismic
events. Those are defined by 3 or more quasi-simultaneous records
observed over a relatively short distance (a few degrees of
latitude/longitude) within a short time difference (few hours). A
preliminary list of 115 possible historical seisms has been obtained,
identified by 3 to 15 different stations. The proposed location is taken
as the average latitude and longitude of the stations where the felt
seisms were recorded. Some of the most important ones are associated to
aftershocks that have been felt at some stations after the main shocks.
The most recent felt seisms have been also recorded instrumentally,
which helps to validate the procedure followed. The main difficulties
are the magnitude estimation and the possible spatial incompleteness of
the recording of felt seism evidence at the margin of the observation
network. The distribution of these historical felt seisms mach the
distribution of the instrumental epicenters. The results obtained may be
used to complete the existing catalogues of historical seismicity.
Herrinckx, P. (1959). Séismicité du Congo belge.
Compilation des seismes observés aux stations climatologiques
entre 1909 et 1954. Académie royale des Sciences coloniales.
Classe des Sciences naturelles et médicales. Mémoire
in8°. Nouvelle série, 11(5), 1-55
The Aki (1965) maximum likelihood estimate of the Gutenberg–Richter b-value is extended for use in the case of multiple catalogs with different levels of completeness. The most striking feature of this newly derived estimator is its simplicity—it is more manageable than the well-known and already easy to use Weichert (1980) solution to the analogs problem. In addition, confidence intervals for the newly derived estimator are provided.
The maximum likelihood estimation of earthquake hazard parameters (maxi-mum regional magnitude, m .... earthquake activity rate k, and b parameter in the Gutenberg-Richter equation) is extended to the case of mixed data containing large historical events and recent complete observations. The method accepts variable quality of complete data in different parts of a catalog with different threshold magnitude values. As an illustration, the procedure is applied for the estimation of seismicity parameters in the area of Calabria and eastern Sicily.
Geophysical, geochemical and biological data are integrated to unravel the origin and evolution of an unusual rift lake. The northern basin of Lake Kivu contains about 0.5 km of sediments which overlie a basement believed to be crystalline rocks of Precambrian age. Volcanic rocks at the northern end of the lake have created large magnetic anomalies of up to 300γ. Heat flow varies from 0.4 to 4 hfu. The extreme variability may be due in part to sedimentation or recent changes in the temperature of the bottom water. Sharp boundaries in the vertical temperature and salinity structure of the water across the lake can best be explained as separate convecting layers. Such convecting cells are the result of the increase in both temperature and salinity with depth.
Earthquake aftershock sequences have been found to approximately satisfy three empirical scaling relations: i) the Gutenberg-Richter frequency-magnitude scaling, ii) Båth's law for the difference in the magnitude of a mainshock and its largest aftershock, and iii) the modified Omori's law for the temporal decay of aftershock rates. The three laws are incorporated to give a generalized Omori's law for aftershock decay rates that depend on several parameters specific for each given seismogenic region. It is shown that the characteristic time c, first introduced in the modified Omori's law, is no longer a constant but scales with a lower magnitude cutoff and a mainshock magnitude. The generalized Omori's law is tested against earthquake catalogs for the aftershock sequences of the Landers, Northridge, Hector Mine, and San Simeon earthquakes.
This volcanism, in the west branch of the African rift, includes three cycles of different tectonic significance. The tholeiitic basalts of the Mesozoic-Cainozoic traps (first cycle) underline the pre-continental rift stage. The sodic alkaline lavas of Tertiary and Quaternary age (second cycle) characterize the rift and its evolution. The volcanic activity migrates with time, from the edge to the centre of the rift; the undersaturation of the basic products decreases in the same way. Quaternary transitional magmas (third cycle) occur in the axial part of the rift and characterize the crustal thinning indicated by the geophysical data. Mean chemical compositions are tabulated for 51 rocks from the three cycles. A model is proposed for magmatic evolution from a continental extensive stage (Kivu type) to a proto-oceanic regime (Afar type).-R.A.H.
The Biega massif, in the eastern part of Zaire, is a plutono-volcanic ring complex which intrudes the Kibaro-Burundian metasedimentary rocks. It is formed by silica-saturated or oversaturated alkaline rocks (some of them with peralkaline affinities). Crystallization conditions of the corresponding magmas have been established by comparison with experimental systems. Magma origin is discussed in terms of direct melting of the lower crust or upper mantle. Geochronological data indicate that the Biega complex and other alkaline or peralkaline complexes of the Interlacustrean area (Central Africa), have Panafrican age and might be considered to represent a change of tectonic regime which began at the end of Kibaran in this part of the continent.-English summary
The study of rift basin’s morphology can provide good insights into geological features influencing the development of rift valleys and the distribution of volcanism. The Kivu rift segment represents the central section of the western branch of the East African Rift and displays morphological characteristics contrasting with other rift segments. Differences and contradictions between several structural maps of the Kivu rift make it difficult to interpret the local geodynamic setting. In the present work, we use topographic and bathymetric data to map active fault networks and study the geomorphology of the Kivu basin. This relief-based fault lineament mapping appears as a good complement for field mapping or mapping using seismic reflection profiles. Results suggest that rifting reactivated NE-SW oriented structures probably related to the Precambrian basement, creating transfer zones and influencing the location and distribution of volcanism. Both volcanic provinces, north and south of the Kivu basin, extend into Lake Kivu and are connected to each other with a series of eruptive vents along the western rift escarpment. The complex morphology of this rift basin, characterized by a double synthetic half-graben structure, might result from the combined action of normal faulting, magmatic underplating, volcanism and erosion processes.
The Precambrian formations of Katana are constituted od sandstones, conglomerates, shales in the lower level and of graphitic black shales or reddish-ferrugineous shales on top of the formation (upper level). These formations split up into several members are organized in symetric and gentle folds, the directions of which is mainly N-S with an axial plane shistosity (foliation- or fracture-type according to the material). Subsequent faults, mostly E-W trending have shifted folded structures through a disconnection on the left. Stratigraphical and tectonic comparisons with adjacent areas lead us to locate these formations towards the top of lower Burundian (lower series of Rwanda).
The Kivu rift is part of the western branch of the East African Rift system. From Lake Tanganyika to Lake Albert, the Kivu rift is set in a succession of Precambrian zones of weakness trending NW-SE, NNE-SSW and NE-SW. At the NW to NNE turn of the rift direction in the Lake Kivu area, the inherited faults are crosscut by newly born N-S fractures which developed during the late Cenozoic rifting and controlled the volcanic activity. From Lake Kivu to Lake Edward, the N-S faults show a right-lateral en echelon pattern. Development of tension gashes in the Virunga area indicates a clockwise rotation of the constraint linked to dextral oblique motion of crustal blocks. The extensional direction was W-E in the Mio-Pliocene and ENE-WSW in the Pleistocene to present time.
The volcanic rocks are assigned to three groups: (1) tholeiites and sodic alkali basalts in the South-Kivu, (2) sodic basalts and nephelinites in the northern Lake Kivu and western Virunga, and (3) potassic basanites and potassic nephelinites in the Virunga area. South-Kivu magmas were generated by melting of spinel + garnet lherzolite from two sources: an enriched lithospheric source and a less enriched mixed lithospheric and asthenospheric source. The latter source was implied in the genesis of the tholeiitic lavas at the beginning of the South-Kivu tectono-volcanic activity, in relationships with asthenosphere upwelling. The ensuing outpouring of alkaline basaltic lavas from the lithospheric source attests for the abortion of the asthenospheric contribution and a change of the rifting process. The sodic nephelinites of the northern Lake Kivu originated from low partial melting of garnet peridotite of the sub-continental mantle due to pressure release during swell initiation. The Virunga potassic magmas resulted from the melting of garnet peridotite with an increasing degree of melting from nephelinite to basanite. They originated from a lithospheric source enriched in both K and Rb, suggesting the presence of phlogopite and the local existence of a metasomatized mantle. A carbonatite contribution is evidenced in the Nyiragongo lavas.
New K-Ar ages date around 21 Ma the earliest volcanic activity made of nephelinites. A sodic alkaline volcanism took place between 13 and 9 Ma at the western side of the Virunga during the doming stage of the rift and before the formation of the rift valley. In the South-Kivu area, the first lavas were tholeiitic and dated at 11 Ma. The rift valley subsidence began around 8–7 Ma. The tholeiitic lavas were progressively replaced by alkali basaltic lavas until to 2.6 Ma. Renewal of the basaltic volcanism happened at ca. 1.7 Ma on a western step of the rift. In the Virunga area, the potassic volcanism appeared ca. 2.6 Ma along a NE-SW fault zone and then migrated both to the east and west, in jumping to oblique tension gashes.
The uncommon magmatic evolution and the high diversity of volcanic rocks of the Kivu rift are explained by varying transtensional constraints during the rift history.
The maximum expected (theoretical) earthquake intensities with 75% probability of not being exceeded in 50, 100, 200 and 400 years are 6.0-6.2, 6.5-6.7, 6.9-7.1 and 7.3-7.5, respectively for Budapest.
Intensity, strain release, and first‐motion studies of earthquakes along the Western Rift Valley of Africa, using data from the seismograph network of the Institut pour la Recherche Scientifique en Afrique Centrale in the eastern Belgian Congo, during the period from May 1953 to April 1956 are presented. Regions of activity shifted during the period of observation, an increase in activity west of Lake Albert in July 1956 being especially noticeable. Intensity maps for three earthquakes show an elongation of isointensity regions parallel to the Rift. The rate of strain release in the area increased with the commencement of the Lake Albert shocks, changing from 35 × 10 ⁸ ergs ½ to 155 × 10 ⁸ ergs ½ per 100 days. Strain released during several aftershock sequences was proportional to the logarithm of the time after the initial shock.
First‐motions from Rift Valley earthquakes to the stations of the Institut pour la Recherche Scientifique en Afrique Centrale (IRSAC) do not give a unique determination of the predominant type of faulting. The data are consistent with dip slip faulting along steeply clipping faults parallel to known faults in the region with motion such that the elevated areas are being further elevated and the depressed areas further depressed. They are also consistent with strike slip faulting along near‐vertical faults with the eastern sides moving north. In agreement with the near station results, first‐motion data from distant stations for two shocks west of Lake Albert indicate some combination of reverse dip‐slip and sinistral strike‐slip motion along a fault plane dipping toward the Rift Valley.
The seismically and volcanically active Kivu Rift, in the western branch of the East African RiftSystem, is a type locale for studies of high-elevation, humid-climate rift basins, as well as magmatic basin development. Interpretations of offshore multi-channel seismic (MCS) reflection data, terrestrial radar topography, lake bathymetry, and seismicity data recorded on a temporary array provide new insights into the structure, stratigraphy and evolution of the Kivu rift. The Kivu rift is an asymmetric graben controlled on its west side by a ~110 km-long, N-S striking border fault. The southern basins of the lake and the upper Rusizi river basin are an accommodation zone effectively linking 1470 m-high Lake Kivu to770 m-high Lake Tanganyika. MCS data in the eastern Kivu lake basin reveal a west-dipping half graben with at least 1.5 km of sedimentary section; most of the ~ 2 km of extension in this sub-basin is accommodated by the east-dipping Iwawa normal fault, which bounds an intrabasinal horst. Lake Kivu experienced at least three periods of near desiccation. The two most recent of these approximately correlate to the African Megadrought and Last Glacial Maximum. There was a rapid lake level transgression of at least 400 m in the early Holocene. The line load of the Virunga volcanic chain enhances the fault-controlled basin subsidence; simple elastic plate models suggest that the line load of the Virunga volcanic chain depresses the basin by more than 1 km, reduces flank uplift locally, and broadens the depocentre. Not only do the voluminous magmatism and degassing to the lake pose a hazard to the riparian population, but our studies demonstrate that magmatism has important implications for short-term processes such as lake levels, inflow and outlets, as well as long term modification of classic half-graben basin morphology.This article is protected by copyright. All rights reserved.
This paper presents a record of information on the seismicity of the Sudan. In addition to events for which only felt information exists, there are a number of early teleseismic locations of earthquakes which have been reexamined using information from contemporary station bulletins. Most activity is close to the southern borders of Sudan, near the African rift system, but the northeast is affected by earthquakes in the Red Sea, and the central intraplate region has also experienced earthquakes capable of producing damage.
Magma movement and fault slip alter the magnitude and orientation of the stress in the surrounding crust. Observations of a sequence of events clustered in space and time provide information about the triggering mechanism and stress interactions between magma intrusion, earthquakes and eruptions. We investigate the syn- and post-intrusion stress changes associated with the 2007 Gelei dyke intrusion episode and subsequent eruption of nearby Oldonyo Lengai. Previous studies produced a kinematic model of the 2007 June-August sequence involving similar to 1 m slip on a normal fault followed by the intrusion of the 7-10-km long Gelei dyke, collapse of a shallow graben and the deflation of the Gelei magma chamber. Immediately following this, the volcano Oldoinyo Lengai (< 10 km away) experienced a new phase of explosive activity lasting for several months. Here, we present new geodetic observations covering Gelei and Oldoinyo Lengai in 2008 September-2010. We show continued slip on graben-bounding faults above the Gelei dyke. The eruption of Oldoinyo Lengai was accompanied by the intrusion of a 4 km-long E-W-trending dyke followed by deflation of a shallow source directly below the summit of the volcano. Next, we use stress calculations to investigate a number of hypotheses linking these events. (1) Before the onset of surface deformation, a dyke sufficiently deep and narrow to be geodetically undetectable could still have produced sufficient stress changes to trigger slip on the normal fault (i.e. the sequence could have been magmatically driven). (2) Stresses at the dyke tip would have been sufficient to overcome the effect of continued slip on the normal fault, allowing the dyke to propagate upwards into a region of clamping. (3) The Gelei sequence would have produced a significant stress change on the chamber beneath Oldoinyo Lengai. These static stress calculations allow us to discuss the roles played by dynamic stress, deeper magmatic changes and background stresses throughout the sequence with implications for the stress triggering of both seismic and volcanic hazards.
The earthquake generating stress fields of the Western Rift Valley of Africa are discussed in detail from the data obtained by the seismological network of IRS, Zaire, and by the seismological stations in Africa. The present analysis is based on the method of composite mechanism solution using the intial motions of P-waves from earthquakes which occurred during the period from 1958 to 1970. The stress fields are analyzed for ten sub-regions divided on the ground of the characteristics of seismicity. Quite stable solutions are obtained for the sub-regions of Lake Kivu and its vicinities, which are located in the central part of the Western Rift Valley, and shows that the stress field are of the normal faulting type. The direction of tension axis of each solution is generally perpendicular to the rift system or to the local faults. The perpendicularity is consistent with the results reported by several authors studying focal mechanisms of earthquakes occurring in the African Rifts and the ocean ridges. In contrast with this general conclusion, the strike-slip type of focal mechanism prevails in sub-regions of the southern part of the Western Rift Valley, and a heteogeneous stress field is suggested in the sub-region of the active volcanoes of Nyiragongo and Nyamuragira. -Authors
The history of Lake Kivu is strongly linked to the activity of the Virunga volcanoes. Subaerial and subaquatic volcanoes, in addition to lake-level changes, shape the subaquatic morphologic and structural features in Lake Kivu's Main Basin. Previous studies revealed that volcanic eruptions blocked the former outlet of the lake to the north in the late Pleistocene, leading to a substantial rise in the lake level and subsequently the present-day thermohaline stratification. Additional studies have speculated that volcanic and seismic activities threaten to trigger a catastrophic release of the large amount of gases dissolved in the lake. The current study presents a bathymetric mapping and seismic profiling survey that covers the volcanically active area of the Main Basin at a resolution that is unprecedented for Lake Kivu. New geomorphologic features identified on the lake floor can accurately describe related lake-floor processes for the first time. The late Pleistocene lowstand is observed at 425 m depth, and volcanic cones, tuff rings, and lava flows observed above this level indicate both subaerial and subaquatic volcanic activities during the Holocene. The geomorphologic analysis yields new implications on the geologic processes that have shaped Lake Kivu's basin, and the presence of young volcanic features can be linked to the possibility of a lake overturn.
Three representative basins in the Western rift system of East Africa
are bordered along one side by high-angle normal faults with 2- to 5-km
throws (border faults). In plan view ˜100-km-long systems of
linear border faults form curvilinear border fault segments bounding the
East Kivu, West Kivu, and Rusizi basins. The opposite sides of these
asymmetric basins are bounded by lower relief faulted monoclines or en
echelon ramps. The largely unfaulted rift flanks have been uplifted 2 km
above the 1.3-km-high East African plateau, with uplift narrowing basins
during Quaternary time. Maximum estimates of ˜E-W crustal
extension within basins are less than 25% (< 16 km), and planar
border faults may penetrate the crust. The East Kivu and West Kivu
basins are linked across the rift valley by a horst that serves as a
hinge for subsidence in both basins. The westward tilted East Kivu and
eastward tilted Rusizi border fault segments are linked along the rift
by oblique-slip transfer faults that also accommodate along-axis
differences in elevation. Upper Miocene-Recent eruptive volcanic centers
within the comparatively high-strain interbasinal region (accommodation
zone) generally coincide with the tips of border fault segments and
transfer faults. The orientations of Miocene-Recent dip-slip and
oblique-slip faults show little correlation with Precambrian shear zones
or foliations in metamorphic basement. Differences between the East
Kivu, West Kivu, and Rusizi basins in the age of initial faulting,
subsidence, and age/composition of volcanic products suggest that border
fault segments developed diachronously and propagated along the length
of the rift. This along-axis border fault propagation and the
crosscutting geometry of transfer faults contribute to the segmentation
of the Western rift valley.
The Tanganyika-Rukwa-Malawi (TRM) rift segment in western Tanzania is a
key sector for understanding the opening dynamics of the East African
rift system (EARS). In an oblique opening model, it is considered as a
dextral transfer fault zone that accommodates the general opening of the
EARS in an NW-SE direction. In an orthogonal opening model, it
accommodates pure dip-slip normal faulting with extension orthogonal to
the rift segments and a general E-W extension for the entire EARS. The
central part of the TRM rift segment is well exposed in the Ufipa
plateau and Rukwa basin, within the Paleoproterozoic Ubende belt. It is
also one of the most seismically active regions of the EARS. We
investigated the active tectonic architecture and paleostress evolution
of the Ufipa plateau and adjacent Rukwa basin and in order to define
their geodynamic role in the development of the EARS and highlight their
pre-rift brittle tectonic history. The active fault architecture,
fault-kinematic analysis and paleostress reconstruction show that the
recent to active fault systems that control the rift structure develop
in a pure extensional setting with extension direction orthogonal to the
trend of the TRM segment. Two pre-rift brittle events are evidenced. An
older brittle thrusting is related to the interaction between the
Bangweulu block and the Tanzanian craton during the late Pan-African
(early Paleozoic). It was followed by a transpressional inversion during
the early Mesozoic. This inversion stage is the best expressed in the
field and caused dextral strike-slip faulting along the fault systems
that now control the major rift structures. It has been erroneously
interpreted as related to the late Cenozoic EARS which instead is
characterized by pure normal faulting (our third and last stress stage).
AN unfortunate error has crept into the Annual Report of the Geological Survey of Uganda for the year 1926, whereby the reader is informed that Mr. A. D. Combe, of this Survey, is of opinion that there is a continuous succession through the Transvaal System into the Waterberg System in the Cape Province of South Africa. May I be permitted to make use of the columns of NATURE to correct this error? Perhaps readers of this journal who have copies of the report will kindly turn up page 32, delete line 34 and delete all of line 35 except the last word and the comma in front of it, and re-write thus:
Slip-tendency analysis is a new technique that permits rapid assessment of stress states and related potential fault activity. The tendency of a surface to undergo slip in a given stress field depends on its frictional characteristics (primarily controlled by rock type) and the ratio of shear to normal stress acting on the surface, here defined as slip tendency (determined by orientation of the surface within the stress field). An interactive computer tool displays the stress tensor in terms of its associated slip-tendency distribution and the relative likelihood and direction of slip on surfaces of all orientations. The technique provides easy visualization and rapid evaluation of stress in terms of its potential for causing slip on individual faults or fault populations for use in seismic-risk and fault-rupture risk assessment, exploration for high-risk and earthquake-prone blind faults, selection of likely earthquake focal mechanism solutions, and for use in analysis of compatibility of geologic structures.
Both oceanic and continental rifts show regular along-axis segmentation, but the relationship, if any, between the two is poorly understood. The tectonically active East African Rift system encompasses systematic along-axis variations in extension and magmatism, making it possible to explore the links between strain, magmatism, and the length scales of faulting and depositional systems during rift development. We summarize Quaternary along-axis segmentation within the Afar Rift system, which is transitional to seafloor spreading, and compare it with segmentation in a ``continental'' rift sector, the Main Ethiopian Rift (MER) system to the south. We use high-resolution satellite imagery calibrated by field studies and digital topography data to delineate faults and magmatic centers in unmapped areas and to compare with existing geological reports. From south to north we see a significant and systematic decrease in the lengths of young basin-bounding normal faults (~50 km long in MER to