Browsing by Author "Deschamps, Anne"

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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.
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    Seismic Waves Attenuation in Continental Lithosphere under Extensional Condition: Comparison of the East African and Baikal Rift Systems
    (2012) Dobrynina, A. A.; Albaric, Julie; Deschamps, Anne; Perrot, Julie; Ferdinand, Richard W.; Déverchère, Jacques; San'kov, Vladimir; Chechelnitsky, V. V.
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    Shallow and Deep Earthquake Sequences Captured in the North Tanzanian Divergence, East Africa: Inferences on Seismogenic Processes and Rheology
    (2009) Albaric, Julie; Perrot, Julie; Déverchère, Jacques; Deschamps, Anne; Richard, Ferdinandi; Le Gall, Bernard
    Using a temporary local seismic network of 35 stations deployed in North Tanzania (SEISMOTANZ'07 experiment) during 6 months in 2007, we captured two earthquake sequences (Gelai and Manyara) occurring respectively in the southern end of the Kenya rift and in the North Tanzanian Divergence (NTD). None of the sequences depicts typical swarm or mainshock-aftershock patterns. Although distant of only ~150 km, their triggering mechanisms appear to be different. They highlight a major change in the magmatic/tectonic nature of the rift where the eastern branch of the Est African Rift enters the Tanzanian craton. Both depict similar shape and long-axis, emphasizing 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 processes involving slow slip on a normal fault and dyke intrusion within the upper crust, and an interaction with the eruption of the nearby Oldoinyo Lengai volcano. At Manyara, the sequence reveals a long-lasting seismic activity deeply rooted (~20-35 km depth), possibly indicative of stress loading transmitted laterally. Focal solutions demonstrate a mixture of normal and strike slip faulting on sub-vertical inherited structures striking N60°E. The yield stress envelope modelled from the depth frequency distribution of earthquakes in Manyara 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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    Strain Accommodation by Slow Slip and Dyking in a Youthful Continental Rift, East Africa
    (2008) Calais, Eric; d’Oreye, Nicolas; Albaric, Julie; Deschamps, Anne; Delvaux, Damien; Déverchère, Jacques; Ebinger, Cynthia; Ferdinand, Richard W.; Kervyn, François; Macheyeki, Athanas S.; Oyen, Anneleen; Perrot, Julie; Saria, Elifuraha; Smets, Benoit; Stamps, Sarah D.; Wauthier, Christelle
    Continental rifts begin and develop through repeated episodes of faulting and magmatism, but strain partitioning between faulting and magmatism during discrete rifting episodes remains poorly documented. In highly evolved rifts, tensile stresses from far-field plate motions accumulate over decades before being released during relatively short time intervals by faulting and magmatic intrusions. These rifting crises are rarely observed in thick lithosphere during the initial stages of rifting. Here we show that most of the strain during the July-August 2007 seismic crisis in the weakly extended Natron rift, Tanzania, was released aseismically. Deformation was achieved by slow slip on a normal fault that promoted subsequent dyke intrusion by stress unclamping. This event provides compelling evidence for strain accommodation by magma intrusion, in addition to slip along normal faults, during the initial stages of continental rifting and before significant crustal thinning.