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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.

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.

Rapid urbanization, demographic pressure, and sprawl of cities are key factors in the vulnerability and damage related to geo-hydrological hazards. Dysfunctional urban services that favor informal settlements are at the forefront of elements that increase vulnerability. Cases of cities that suffer from geo-hydrological hazards are increasingly reported in many regions, especially in tropical countries in the Global South. Yet, studies on such examples are rare and commonly overlook the human and societal components of hazard risks. Here, we focus on Bujumbura, a city in Africa that has experienced rapid unplanned growth and sprawl into unserviced areas because of the non-application or a lack of a valid urban planning law. After filling in the gap in data collected using high-resolution field surveys and focus group discussions, this study highlights various factors of vulnerability to geo-hydrological hazards in the urban area. Indeed, 108 events of flood and flash floods and 81 gullies were inventoried in Bujumbura between 1997 and 2021. These geo-hydrological hazards have had a significant impact, particularly on housing, and have caused increasing displacement of the population. This vulnerability is exacerbated by the inefficiency of the rainwater drainage system in the urban space. Our result demonstrates how the failure of the institutions responsible for urban management is at the top of all the causes of the vulnerability of the sprawling city. We anticipate that our empirical approach is an effective way to obtain concrete information to develop practical strategies to prevent and mitigate vulnerability to geo-hydrological hazards in urban sprawling contexts. [ABSTRACT FROM AUTHOR]

Risk perception is an essential element to consider for effective risk management at time of eruption. This is especially the case in densely populated cities close to volcanoes like Goma in the East of the Democratic Republic of Congo highly exposed to volcanic hazards from Nyiragongo. The perception of volcanic risk involves the processes of collecting, selecting, and interpreting signals about uncertain impacts of volcanic hazards. Using a questionnaire survey, this study describes the spatial differences and factors influencing the individual volcanic risk perception of 2,224 adults from height representative neighbourhoods of Goma before the May 2021 Nyiragongo eruption. A composite risk perception indicator was built from the perceived severity and perceived vulnerability. Statistical analysis of survey's results shows that the risk perception varies less with demographic and contextual factors than with cognitive and psychological factors. The spatial analysis shows that respondents from the eastern neighbourhoods, affected by the 2002 eruption, demonstrated a significantly higher level of risk perception than participants living in the western neighbourhood. Therefore, collective memory of past events in the impacted areas does play a role. Evidence from this study will help to develop well-targeted volcanic risk awareness-raising in Goma. [ABSTRACT FROM AUTHOR]

Nyiragongo is one of the rare volcanoes on Earth hosting a lava lake. However, the understanding of its plumbing and lava lake systems remains limited, with, until recently, only sporadic or time‐limited historical observations and measurements. Combining dense accurate lava lake and crater floor level measurements based on 1,703 satellite radar images and topographic reconstructions using photogrammetry, we obtain the first reliable picture and time evolution of intra‐crater erupted lava volumes between the two last flank eruptions in January 2002 and May 2021. The filling of the crater by lava, initiated in 2002 and continued up to May 2021, is seen as an evidence of a long‐term pressure build up of the magmatic system. This filling occurs through irregular pulsatory episodes of rising lava lake level, some of which overflow and solidify on the surrounding crater floor. Pauses of stable molten lava lake level and sudden numerous level drops also marked the summit's eruptive activity. The joint analysis with seismic records available since 2015 revealed that the largest lava lake drops are synchronous with seismic swarms associated with deep magma intrusions, generally preceded by an increase of extrusion rate within the crater. The appearance of a spatter cone in the summit crater in 2016, most likely superficially branched to the lava lake, was a clear marker of the change in eruption dynamics. This first long‐term time series of Nyiragongo's crater topography between two hazardous flank eruptions might further help to better decipher Nyiragongo's past and future behavior using multi‐parameter observations. Plain Language Summary: Nyiragongo volcano, in D.R. Congo, stands close to a densely populated area (∼1 million inhabitants). Its Hawaiian‐style eruptive activity essentially consists in a long‐lived molten lava lake or lava fountains in the summit crater lasting for weeks to decades, interrupted by effusive flank eruptions. Nyiragongo erupted on its southern flank in January 2002 and May 2021, draining the lava lake system and opening fissures producing rapid lava flows toward the city of Goma and Lake Kivu. Between these two flank eruptions, the continuous summit eruptive activity took the form of a persistent lava lake progressively filling in the summit crater. Here, we derive accurate elevation measurements of Nyiragongo's crater floor and lava lake surface between the two last flank eruptions from radar satellite instruments and 3D photogrammetric reconstruction. The lava lake, of which the level fluctuates, acts as a growing piezometer connected to the magmatic system as its rim rose by ∼300 m in about 19 years due to overflowing. A combined analysis with seismic records provides important indications on the origins of the observed variations and helps to better understand the link between the continuous eruptive activity and the deeper magmatic processes. Key Points: Processing of 1,703 satellite radar images leads to the first reliable picture of erupted volumes within Nyiragongo's crater since 2002We provide a multi‐instrumental analysis with topographic reconstructions using photogrammetry and seismic observations at NyiragongoOur approach helps to decipher the origins of surface (intra‐crater) evidences of magmatic pressure variations in the plumbing system [ABSTRACT FROM AUTHOR]