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Nyamulagira and Nyiragongo are two of the most active volcanoes in Africa, but their eruptive histories are poorly known. Assessing lava flow volumes in the region remains difficult, as field surveys are often impossible and available Digital Elevation Models (DEMs) do not have adequate spatial or temporal resolutions. We therefore use TerraSAR-X add-on for Digital Elevation Measurement (TanDEM-X) interferometry to produce a series of 0.15 arc sec (∼5 m) DEMs from between 2011 and 2012 over these volcanoes. TanDEM-X DEMs have an absolute vertical accuracy of 1.6 m, resulting from the comparison of elevation with GPS measurements acquired around Nyiragongo. The difference between TanDEM-X-derived DEMs from before and after the 2011-2012 eruption of Nyamulagira provides an accurate thickness map of the lava flow emplaced during that activity. Values range from 3 m along the margins to 35 m in the middle, with a mean of 12.7 m. The erupted volume is 305.2 ± 36.0 × 106m3. Height errors on thickness depend on the land covered by the flow and range from 0.4 m in old lavas to 5.5 m in dense vegetation. We also reevaluate the volume of historical eruptions at Nyamulagira since 2001 from the difference between TanDEM-X and SRTM 1 arc sec DEMs and compare them to previous work. Planimetric methods used in literature are consistent with our results for short-duration eruptions but largely underestimate the volume of the long-lived 2011-2012 eruption. Our new estimates of erupted volumes suggest that the mean eruption rate and the magma supply rate were relatively constant at Nyamulagira during 2001-2012, respectively, 23.1 m3 s-1 and 0.9 m3s-1.

Earthquake source parameter determination is of great importance for hazard assessment, as well as for a variety of scientific studies concerning regional stress and strain release and volcano-tectonic interaction. This is especially true for poorly instrumented, densely populated regions such as encountered in Africa, where even the distribution of seismicity remains poorly documented. In this paper, we combine data from satellite radar interferometry (InSAR) and teleseismic waveforms to determine the source parameters of the Mw 5.9 earthquake that occurred on 2008 February 3 near the cities of Bukavu (DR Congo) and Cyangugu (Rwanda). This was the second largest earthquake ever to be recorded in the Kivu basin, a section of the western branch of the East African Rift (EAR). This earthquake is of particular interest due to its shallow depth and proximity to active volcanoes and Lake Kivu, which contains high concentrations of dissolved carbon dioxide and methane. The shallow depth and possible similarity with dyking events recognized in other parts of EAR suggested the potential association of the earthquake with a magmatic intrusion, emphasizing the necessity of accurate source parameter determination. In general, we find that estimates of fault plane geometry, depth and scalar moment are highly consistent between teleseismic and InSAR studies. Centroid-moment-tensor (CMT) solutions locate the earthquake near the southern part of Lake Kivu, while InSAR studies place it under the lake itself. CMT solutions characterize the event as a nearly pure double-couple, normal faulting earthquake occurring on a fault plane striking 350° and dipping 52° east, with a rake of –101°. This is consistent with locally mapped faults, as well as InSAR data, which place the earthquake on a fault striking 355° and dipping 55° east, with a rake of –98°. The depth of the earthquake was constrained by a joint analysis of teleseismic P and SH waves and the CMT data set, showing that the earthquake occurred in the shallow crust, at approximately 8 km depth. Inversions of ENVISAT (Environment Satellite) and ALOS (Advanced Land Observation Satellite) data place the earthquake at 9 km. A comparison of the scalar moment (9.43 ± 0.06 × 1017 Nm from seismology and 8.99 ± 0.010 × 1017 Nm from the joint InSAR solution) shows good agreement between the two data sets. Such an agreement is in contrast to the large discrepancies observed (up to an order of magnitude) in other places along the EAR where similar earthquake sequences are associated with magmatic intrusion. From this, we infer that the rupture was brittle and occurred with little aseismic deformation as might be expected from magma injection. Our results provide insights into the style of rifting occurring in the South Kivu Volcanic Province and hence will aid future studies on seismic risk in the context of Lake Kivu and underline the importance of systematic monitoring of the EAR area.

A seismic crisis with a series of moderate earthquakes started on July 12th 2007 in the Natron area in North Tanzania. According to the USGS-NEIC earthquake catalogue, it lasted up to the 8th of September and 80 teleseismic earthquakes were recorded, with the strongest one on July 17th (Mw 5.9). Soon after the main event of July 17th, the presence of open surface fissures was reported on the ground, on the southern flank of the recent (th event shows the presence of two surface fracture systems delimiting a NNE-trending narrow graben in the southern flank of the Gelai volcano. These features were subsequently mapped in relative detail in the field in October 2007 before the following rain season. They turn to be subvertical open fissures, arranged in en-échelon way and displaying horizontal dilation as well as vertical offset. They are interpreted to evolve at depth into 60-75° dipping normal fault systems. According to their calculated dip, these fault systems converge at depth where they should either merge or intersect. Assuming a 50 m high vertical portion for the open fracture, the depth and horizontal coordinates of the fault intersection point are calculated for a series of profiles drawn across the graben structure.It is suspected that the July-August moderate-magnitude earthquake swam crisis might correspond to a volcano-tectonic event during which a magmatic dyke was injected at depth under the southern flank of the Gelai volcano. In this scenario, the observed graben structure bounded by the open fracture network and the inferred normal faults at depth are similar to the volcano-extensional structures that commonly form in the upper crust above a thin subvertical dyke injection. The top of this inferred dyke should correspond approximately to the intersection of the lines prolonging the two normal faults (even if physically the faults did not join the top of dyke), with an average depth of 4000m under the topographic surface.

The Rukwa rift basin is presently a closed hydrogeological depression containing a shallow lake (max 20 me deep) with its surface at an altitude around 810 m above sea level. Lacustrine terraces and paleo-shorelines are known up to 980 m above sea level, an altitude at which it reach the overflow sill towards Lake Tanganyika. Both Lakes Tanganyika and Nyasa (Malawi) are presently overflowing, but as their lake level fluctuates, they have been disconnected from their outlet in the recent past. High resolution seismic profiling in both Lakes Rukwa and Malawi has show the presence of active fault systems underneath the lake floor. Some of these fault systems appear to have had a cyclic activity, with alternating periods of high tectonic activity/sedimentation and periods of tectonic quiescence. The accommodation zone between Lake Rukwa and Nyasa is occupied by the Rungwe volcanic Province, with the Ngozi, Rungwe and Kiejo volcanoes presenting signs of recent volcanic activity. The Rungwe Province is cross-cut by several directions of faults, which clearly control the location of the volcanic vents.In our work, we reviewed the available data on recent (Late Pleistocene – Holocene) volcanic eruptions, in the Rungwe area itself, in the drill cores from the surrounding lakes and from aerial observations up to 300 km away from the Rungwe Province. We performed morphotectonic and paleoseismic investigations of the Kanda fault, a major normal fault between lakes Rukwa and Tanganyika. We investigated lacustrine deposits of the Rukwa basin corresponding to the two last cycles of high lake level. The chronological framework was established using 30 new radiocarbon dating and the most prominent volcanic tephra layers were used as a reference in the correlations. The results are still preliminary, but a good correlation already appear between climatically induced lake level change (in Lake Rukwa), seismo-tectonic activation of the regional fault network (underneath Lake Rukwa and the Kanda fault between Lakes Rukwa and Tanganyika) and the timing of the recent strong volcanic eruptions in the Rungwe Volcanic Province since the last 40.000 years. This relation is explained taking into account that Lake Rukwa is very sensitive to climate change as it occupies a flat depression and its overflow outlet is 180 m above its present-day level. Its lake level rises rapidly when the climate becomes more humid as it was the case during the Last Glacial Maximum and during the Younger Dryas event. Increase in lake level means increasing of the load in the basin and perturbation of the ambient tectonic stresses. In most of the Rukwa rift, the tectonic stress is of extensional (normal faulting) regime, with the maximum principal stress axis (sigma 1) subvertical. In these conditions, increasing the vertical load will increase the shear stress on the existing normal faults, triggering (seismogenic) normal faulting deformation. As the architecture of the active volcanoes in the Rungwe Province is tectonically controlled, activation of the faults, together with a greater pressure of water in the tectonic discontinuities are likely to trigger large volcanic eruptions, strongly explosive.

The 160 km-long Kanda normal fault system is running longitudinally across the Ufipa tilted horst between the Tanganyika and Rukwa rift basins in the western branch of the East African Rift System. It is likely to have been affected the Ms 7.4 earthquake in 1910 whose instrumental epicentre was located approximately near Sumbawanga town, along the northern portion of the fault. Remote sensing, morphotectonic, and palaeoseismological investigations together with structural geology and electric resistivity profiling allow to characterise the main seismological parameters of this major active fault.In order to reconstruct the along-trend displacement profile and fault segmentation, topographic profiles were made across the fault scarp using data from differential GPS, Hand GPS and the 90 m resolution STRM DEM. Comparison of topographic profiles from the same site using the different topographic data provides some insight on the methodology and validity of topographic profiles extraction. Due to the large dimensions of the fault scarp (10-30 m high) the strong magnitude of the potential earthquakes (Ms. 7.5) and the large associated co-seismic slip (2-4 m), classical paleoseismic trenching is difficult to apply. Instead, we used existing exposures such as natural trenches, road cuts and outcrops, supplemented by a dense topographic survey (5000 points on differential GPS over less than 1 km2) to produce a 2 m resolution digital elevation model, performed detailed geological mapping, airphoto interpretation, electric resistivity profiling, small drillings, and C14 dating. Efforts were concentrated on a promising site where river sediments and peat deposits are interbedded with volcanic tephra layers and largely exposed on the footwall. Results show that the Kanda fault had different cycles of activity, separated by tectonically quite periods. The last period of activity probably started by 11 cal Ka BP, soon after the deposition of a marked tephra layer, but sedimentation on the footwall continued up to 5 Ka cal BP. Since then, the minimum offset is estimated at 17.9 m, which gives a minimum slip rate of 3.58 mm/year, of an equivalent of one earthquakes generating 3.58 m slip per 1000 years (Ms in the range of 7.0 - 7.5).

The Ufipa Plateau is a tilted horst between the Tanganyika and Rukwa rift basins in the western branch of the East African Rift System. It is cut longitudinally by the 160 km-long Kanda normal fault system, the main active fault affecting the centre of the plateau. The region was hit by a Ms 7.4 earthquake in 1910 and the instrumental epicenter (although not precisely determined) was located near Sumbawanga town, along the northern portion of that fault. The Kanda fault largely reactivates an older strikeslip fault zone, itself affecting mylonites from a ductile shear zone developed in the Ubendian Paleoproterozoic belt. Fault-slip indicators for the recent movements along the fault scarp show almost pure top-to E/ENE normal faulting. Differential GPS topographic profiles were made every 5-10 km across the Kanda fault scarp in order to evaluate the vertical offset since the development of the last morphological surface, covered partly by a hard laterite crust. Offset associated with that morphological scarp reaches up to 50 m, with a mean value of 15 m. The offset values evolve along-trend, with 3 portions of maximal offset separated by 2 minima. This along-trend variation suggests that the entire fault system can be separated into 3 different fault segments with different multi-event faulting histories. According to the empirical relations between earthquake magnitude and maximum fault length, activation of the entire Kanda fault (165 km) would be able to generate a Ms 7.6 earthquake.Electric resistivity profiling show that the total vertical offset associated with the Kanda fault, taking the top of weathered basement as a reference, is much higher than the visible topographic offset of the morphological surface alone. High-resolution digital elevation extraction and modelling combined with Paleoseismic trenching and C14 dating shows that significant vertical movement occurred along that fault segment during the last 10.000 years.

The word “ mazuku ” in Swahili means “evil wind”. It corresponds to lowland (depressions) where carbon dioxide is released and, being heavier than air, accumulates at high – often lethal – concentrations (10 vol.% of CO 2 in atmosphere can be considered as the deadly threshold, even for a short time exposure). Mazuku are abundant in Goma and surrounding areas and particularly in the area south of the large volcanic edifices of Nyiragongo and Nyamulagira volcanoes located in the most eastern part of DR Congo, W branch of the East African Rift System (EARS). Our extensive field surveys have indicated that mazuku are concentrated within to and around the densely populated city of Goma close to the N shores of Lake Kivu, mainly near fault or fissure networks. At a more local scale, depressions allowing CO 2 -rich gas accumulation are created by lava flow superposition, lava tunnels or cavity collapses, or directly associated with open fractures. People are killed by mazuku every year. Given political and social unrest coupled with the current important demographic and urban growths around Goma, the risks associated to mazuku are increasing accordingly. Mazuku are currently the most important natural risk in terms of human loss for the area and there is an urgent need for further research, more systematic mapping and monitoring of mazuku and for appropriate risk management to be implemented. This paper summarizes the current scientific knowledge on mazuku as well as new advances and a preliminary risk assessment performed recently in the frame of the GORISK project.