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Tedesco D.,Office for the Coordination of Humanitarian Affairs | Tedesco D.,Naples 2 University | Tassi F.,CNR Institute of Environmental Geology and Geoengineering | Tassi F.,University of Florence | And 6 more authors.
Journal of Geophysical Research: Solid Earth | Year: 2010

On 17 January 2002, the city of Goma was partly destroyed by two of the several lava flows erupted from a roughly N-S oriented fracture system opened along the southern flank of Mount Nyiragongo (Democratic Republic of Congo), in the western branch of the East African rift system. A humanitarian and scientific response was promptly organized by international, governmental, and nongovernmental agencies coordinated by the United Nations and the European Union. Among the different scientific projects undertaken to study the mechanisms triggering this and possible future eruptions, we focused on the isotopic (He, C, and Ar) analysis of the magmatic-hydrothermal and cold gas discharges related to the Nyiragongo volcanic system, the Kivu and Virunga region. The studied area includes the Nyiragongo volcano, its surroundings, and peripheral areas inside and outside the rift. They have been subdivided into seven regions characterized by distinct He/He (expressed as R/Rair) ratios and/or δ13 C-CO2 values. The Nyiragongo summit crater fumaroles, whose R/Rair and δ13 C-CO2 values are up to 8.73 and from -3.5% to -4.0% VPDB, respectively, show a clear mantle, mid-ocean ridge basalt (MORB)-like contribution. Similar mantle-like He isotopic values (6.5-8.3 R/Rair) are also found in CO2-rich gas emanations (mazukus) along the northern shoreline of Lake Kivu main basin, whereas the 13δC- CO2 values range from -5.3% to -6.8% VPDB. The mantle influence progressively decreases in (1) dissolved gases of Lake Kivu (2.6-5.5 R/R air) and (2) the distal gas discharges within and outside the two sides of the rift (from 0.1 to 1.7 R/Rair). Similarly, δ13 C-CO2 ratios of the peripheral gas emissions are lighter (from -5.9% to - 11.6% VPDB) than those of the crater fumaroles. Therefore, the spatial distribution of He and C signatures in the Lake Kivu region is mainly produced by mixing of mantle-related (e.g., Nyiragongo crater fumaroles and/or mazukus gases) and crustal-related (e.g., gas discharges in the Archean craton) fluids. The CO2/He ratio (up to 10×10) is 1 order of magnitude higher than those found in MORB, and it is due to the increasing solubility of CO2 in the foiditic magma feeding the Nyiragongo volcano. However, the exceptionally high 40Ar */He ratio (up to 8.7) of the Nyiragongo crater fumaroles may be related to the difference between He and Ar solubility in the magmatic source. The results of the present investigation suggest that in this area the uprising of mantle-originated f luids seems strongly controlled by regional tectonics in relation to the geodynamic assessment of the rift. These fluids are mainly localized in a relatively small zone between Lake Kivu and Nyiragongo volcano, with important implications in terms of volcanic activity. Copyright 2010 by the American Geophysical Union.

Balagizi C.M.,University of Liege | Balagizi C.M.,The Second University of Naples | Darchambeau F.,University of Liege | Bouillon S.,Catholic University of Leuven | And 3 more authors.
Geochemistry, Geophysics, Geosystems | Year: 2015

We report a water chemistry data set from 13 rivers of the Virunga Volcanic Province (VVP) (Democratic Republic of Congo), sampled between December 2010 and February 2013. Most parameters showed no pronounced seasonal variation, whereas their spatial variation suggests a strong control by lithology, soil type, slope, and vegetation. High total suspended matter (289-1467 mg L-1) was recorded in rivers in the Lake Kivu catchment, indicating high soil erodibility, partly as a consequence of deforestation and farming activities. Dissolved and particulate organic carbon (DOC and POC) were lower in rivers from lava fields, and higher in nonvolcanic subcatchments. Stable carbon isotope signatures (δ13C) of POC and DOC mean δ13C of -22.5‰ and -23.5‰, respectively, are the first data to be reported for the highland of the Congo River basin and showed a much higher C4 contribution than in lowland areas. Rivers of the VVP were net sources of CH4 to the atmosphere (4-5052 nmol L-1). Most rivers show N2O concentrations close to equilibrium, but some rivers showed high N2O concentrations related to denitrification in groundwaters. δ13C signatures of dissolved inorganic carbon suggested magmatic CO2 inputs to aquifers/soil, which could have contributed to increase basalt weathering rates. This magmatic CO2-mediated basalt weathering strongly contributed to the high major cation concentrations and total alkalinity. Thus, chemical weathering (39.0-2779.9 t km-2 yr-1) and atmospheric CO2 consumption (0.4-37.0 × 106 mol km-2 yr-1) rates were higher than previously reported in the literature for basaltic terrains. © 2015. American Geophysical Union. All Rights Reserved.

Homuth B.,Goethe University Frankfurt | Lobl U.,Goethe University Frankfurt | Batte A.G.,Makerere University | Link K.,Johannes Gutenberg University Mainz | And 3 more authors.
International Journal of Earth Sciences | Year: 2014

Shear-wave splitting measurements from local and teleseismic earthquakes are used to investigate the seismic anisotropy in the upper mantle beneath the Rwenzori region of the East African Rift system. At most stations, shear-wave splitting parameters obtained from individual earthquakes exhibit only minor variations with backazimuth. We therefore employ a joint inversion of SKS waveforms to derive hypothetical one-layer parameters. The corresponding fast polarizations are generally rift parallel and the average delay time is about 1 s. Shear phases from local events within the crust are characterized by an average delay time of 0.04 s. Delay times from local mantle earthquakes are in the range of 0.2 s. This observation suggests that the dominant source region for seismic anisotropy beneath the rift is located within the mantle. We use finite-frequency waveform modeling to test different models of anisotropy within the lithosphere/asthenosphere system of the rift. The results show that the rift-parallel fast polarizations are consistent with horizontal transverse isotropy (HTI anisotropy) caused by rift-parallel magmatic intrusions or lenses located within the lithospheric mantle-as it would be expected during the early stages of continental rifting. Furthermore, the short-scale spatial variations in the fast polarizations observed in the southern part of the study area can be explained by effects due to sedimentary basins of low isotropic velocity in combination with a shift in the orientation of anisotropic fabrics in the upper mantle. A uniform anisotropic layer in relation to large-scale asthenospheric mantle flow is less consistent with the observed splitting parameters. © 2014 Springer-Verlag Berlin Heidelberg.

Smets B.,European Center for Geodynamics and Seismology | Smets B.,Vrije Universiteit Brussel | Smets B.,Royal Museum for Central Africa | d'Oreye N.,European Center for Geodynamics and Seismology | And 28 more authors.
Bulletin of Volcanology | Year: 2014

This paper presents a thorough description of Nyamulagira's January 2010 volcanic eruption (North Kivu, Democratic Republic of Congo), based on a combination of field observation and ground-based and space-borne data. It is the first eruption in the Virunga Volcanic Province that has been described by a combination of several modern monitoring techniques. The 2010 eruption lasted 26 days and emitted ~45.5 × 106 m3 of lava. Field observations divided the event into four eruptive stages delimited by major changes in effusive activity. These stages are consistent with those described by Pouclet (1976) for historical eruptions of Nyamulagira. Co-eruptive signals from ground deformation, seismicity, SO2 emission and thermal flux correlate with the eruptive stages. Unambiguous pre-eruptive ground deformation was observed 3 weeks before the lava outburst, coinciding with a small but clear increase in the short period seismicity and SO2 emission. The 3 weeks of precursors contrasts with the only precursory signal previously recognized in the Virunga Volcanic Province, the short-term increase of tremor and long period seismicity, which, for example, were only detected less than 2 h prior to the 2010 eruption. The present paper is the most detailed picture of a typical flank eruption of this volcano. It provides valuable tools for re-examining former-mostly qualitative-descriptions of historical Nyamulagira eruptions that occurred during the colonial period. © 2013 Springer-Verlag Berlin Heidelberg.

Mifundu W.,CRSN | Matabaro B.,CRSN | Mateso C.,Goma Volcano Observatory | Kibuye M.,Goma Volcano Observatory | And 6 more authors.
Bulletin of the International Institute of Seismology and Earthquake Engineering | Year: 2013

The lake Kivu basin had experienced an earthquake of local magnitude m 1= 6.1 on February 3rd, 2008 at 7h34' UT, in its south-western part. Many phenomena were generated in the lake Kivu and on the shorelines. At several places were observed land subsidence on the shoreline where several children were killed in D.R. Congo and Rwanda sides. At other places land uplift was observed. According to the fishermen, small tsunami was generated and attacked the coast lines. The highest level accessed by the tsunami was estimated to 4m at Ibindja Island. The small gas escaping generated by the tsunami was observed at several places in the lake Kivu. Many damages were recorded in Bukavu city and surrounding areas in D.R. Congo and Rwanda, characterized by the fissures on the walls or collapse of houses. Total 49 people were killed by the earthquake and aftershocks, more precisely 10 in D.R.C. and 39 in Rwanda. This event was preceded by many foreshocks and followed by a long duration of felt aftershocks. The focal mechanism of this event indicates a normal fault. The maximum intensity was recorded close to the epicentre area and estimated to VIII-IX.

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