Browsing by Author "Sue, Christian"

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    Contrasted Seismogenic and Rheological Behaviours from Shallow and Deep Earthquake Sequences in the North Tanzanian Divergence, East Africa
    (Elsevier, 2010) Albaric, Julie; Perrot, Julie; Déverchère, Jacques; Deschamps, Anne; Le Gall, Bernard; Ferdinand, Richard W.; Petit, Carole; Tiberi, Caterina; Sue, Christian; Songo, Majura A. M.
    We report preliminary results of a seismological experiment, SEISMO-TANZ’ 07, which consisted in the deployment of a local network (35 stations) in the East African Rift System (EARS), North Tanzania, during 6 months in 2007. We compare two earthquake sequences (Gelai and Manyara) occurring, respectively, in the southern end of the Kenya rift and in the North Tanzanian Divergence (NTD). Only distant of ∼150 km, their triggering mechanisms are different. None of the sequences depicts typical swarm or mainshock–aftershock patterns. They highlight the change in the magmatic/tectonic nature of the rift where the eastern branch of the EARS enters the Tanzanian craton. The similar shape and long-axis of the elongate sequences emphasize the preferred locus of active strain release along NE–SW discontinuities which probably root at depth into steep Proterozoic shear zones. At Gelai, the deformation is dominated by aseismic process involving slow slip on normal fault and dyke intrusion within the upper crust (Calais et al., 2008). The spatial and temporal earthquake distribution indicates a possible correlation between the Gelai crisis and the eruption of the nearby Oldoinyo Lengai volcano. At Manyara, the sequence is more uncommon, revealing a long-lasting seismic activity deeply rooted (∼20–35 km depth) possibly related to stress loading transmitted laterally. The yield strength envelope modelled from the depth frequency distribution of earthquakes in the NTD is consistent with the presence of a mafic lower crust and further supports the strength increase of the rifted crust from south Kenya to the NTD.
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    Focal Mechanisms, Stress Field and Crustal Rheology in the North Tanzanian Divergence (East African Rift) Inferred from Local Seismicity Analysis
    (2010) Albaric, Julie; Godano, Maxime; Déverchère, Jacques; Perrot, Julie; Deschamps, Anne; Sue, Christian; Le Gall, Bernard; Ferdinand, Richard W.; Petit, Carole; Tiberi, Caterina
    We deployed a temporary local seismic network in the North Tanzanian Divergence (NTD) for 6 months in 2007 (35 stations, SEISMOTANZ'07 experiment). The region is characterized by major changes in the magmatic/tectonic nature of the rift, at the place where the eastern branch of the East African Rift enters the Tanzanian craton. More than 200 earthquakes were accurately located south of Lake Manyara
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    Insights into Initial Stages of Rifting from Seismotectonics and SKS Splitting in the North Tanzanian Divergence
    (2010-12) Albaric, Julie; Barruol, Guilhem; Déverchère, Jacques; Deschamps, Anne; Perrot, Julie; Tiberi, Caterina; Ferdinand, Richard W.; Sue, Christian; Le Gall, Bernard; Petit, Carole
    Magmatism and faulting are preponderant processes involved in continental rifting. Their interaction, relative importance, and dependence to the rheological properties of the lithosphere and to the timing of rifting, remain poorly known. To address this question, we have used the results from a seismological experiment, called SEISMO-TANZ (35 stations, broadband and enlarged-band), launched in the North Tanzanian Divergence (NTD) for 6 months in 2007. The region encompasses one of the youngest parts of the East African rift (EAR) and is characterized by the development of the rift into the Tanzanian craton. The NTD is often considered as non-volcanic compared to other places in EAR and the lithosphere is highly resistant. More than 2000 local earthquakes were recorded, highlighting active faults and one magmatic intrusion. Inherited structures play a key role as guides for dykes and slips. 26 Focal mechanisms (double-couple hypothesis) were obtained from P-wave polarities and indicate a transtensive deformation in the southern part of the region (Manyara rift). The stress inversion performed indicates a stable, well-determined σ3 axis striking ESE-WNW. From 25 teleseismic events recorded during the experiment, we have measured seismic anisotropy (SKS splitting) and present here our last results. Fast polarization directions are quite homogeneously NE-SW and delays times increase from the craton (W) to the Mozambique belt (E). Fossilized anisotropy and dykes or melt-filled lenses alignments would both explain the majority of these observations. We finally compare these results with other seismic anisotropy measurements made in EAR and with geodetic and seismotectonic analyses in order to better assess the origin of the strain pattern in this part of the rift, and to discuss the respective role of magmatism, faulting and fabrics in the extending lithosphere.