Processus gravitaires et évaluation de la stabilité des pentes (approches géologique et géotechnique)

La marge au large d Alger n avait fait l objet d aucune campagne scientifique majeure jusqu à la réalisation de la campagne Maradja en 2003. Du coup, le travail de thèse sur la zone d Alger a d abord concerné la caractérisation morpho-sédimentaire de la zone et la cartographie des instabilités et des dépôts gravitaires. Dans un deuxième temps, il s est concentré sur la caractérisation des propriétés physiques et mécaniques des sédiments, l identification des mécanismes responsables de l initiation des instabilités observées dans la zone d étude et l évaluation de la stabilité actuelle de la zone d étude. Plusieurs résultats importants ont été obtenus : (1) les processus sédimentaires sont très clairement influencés par la morphologie de la marge, elle-même contrôlée par l activité tectonique. La sédimentation sur la marge algérienne est donc très hétérogène, contrastée et contrôlée par deux processus : (a) les processus gravitaires incluant les courants turbiditiques et les glissements sous-marins et (b) la sédimentation hémipélagique (2) de nombreux glissements ont été observés et décrits sur la zone d étude. La majorité des glissements est caractérisée par une surface inférieure à 0.2 km2 et un volume moyen de 0.003 km3. Seul, un glissement au large de la ville de Dellys est hors norme, caractérisé par un volume très supérieur à la moyenne et égal à 0.18 km3 (3) la présence de couches de silt et sable susceptibles de se liquéfier est un facteur prépondérant d initiation des glissements. L évaluation du potentiel de liquéfaction pour les couches de sable, montre une possibilité de liquéfaction pour des séismes soumettant la colonne sédientaire à un PGA de 0.2 g, une valeur qui a pu être atteinte par les séismes historiques connus. (4) l évaluation de la stabilité de la même zone montre l instabilité et la rupture de la pente pour un PGA de seulement 0.1 g, valeur encore plus facilement atteinte par les séismes historiques connus. Au contraire de la marge algérienne, la zone de Nice avait déjà fait l objet de nombreuses études décrivant le contexte géologique lié à l accident de 1979 et cherchant à comprendre son fonctionnement. Le travail présenté ici a d abord concerné une synthèse de l ensemble des données disponibles et en particulier des mesures géotechniques nouvelles réalisées dans la zone, ainsi que de nouveaux prélèvements de terrain. Ceci a permis de mieux modéliser la stabilité de la pente niçoise près de l aéroport et de simuler la stabilité du prodelta du Var avant et après la construction de l aéroport. Une nouvelle explication du déclenchement du glissement est proposée: ce serait le fluage d une couche d argile sensible sous l effet de la charge de l aéroport qui aurait initié la rupture. Cette hypothèse semble très solide au regard des nouvelles données obtenues. Le poids des différents facteurs déclenchant de l accident de 1979 est aussi discuté et sert de cadre pour une estimation des risques actuels.BREST-BU Droit-Sciences-Sports (290192103) / SudocPLOUZANE-Bibl.La Pérouse (290195209) / SudocSudocFranceF

Relations entre déformation active, rhéologie et magmatisme dans un rift continental (étude sismologique de la divergence nord-tanzanienne, rift est-africain)

Cette étude vise à mieux comprendre le rôle respectif et l interaction entre les différents mécanismes qui contrôlent l initiation et le développement des rifts (magmatisme, faille, fabrique lithosphérique, rhéologie). Pour ce faire, le premier réseau local de 35 stations sismologiques a été déployé dans la Divergence Nord-Tanzanienne, la portion la plus jeune du Rift Est-Africain, pendant une période de six mois. Les signaux des séismes proches et lointains sont utilisés comme indicateurs de l histogénèse, de la résistance crustale, de champs de déformation et de contraintes, et renseignent aussi sur la structure et l anisotropie lithosphérique. Une crise sismique enregistrée au Nord de la Tanzanie (Gelai) illustre la co-existence de processus magmatiques et tectoniques, avec mise en place d un dyke et e comportement à la fois asismique (glissement lent) et sismique (séisme de magnitude Mw 5.9) de failles normales. La crise de Gelai indique le rôle majeur et auparavant Insoupçonné des processus asismiques dans l accommodation de la déformation. Des séismes crustaux remarquablement profonds ont été observés plus au sud, dans la région du Lac Manyara. La sismicité de Manyara est associée à des glissements en faille normale et en décrochement probablement déclenchés par des fluides en base de croûte. L ensemble de la sismicité enregistrée dans la DNT indique l influence de l héritage structural, ce qui s observe aussi à l échelle lithosphérique, par le biais de l anisotropie sismiqueThe objective of this study is to better understand the role and the interaction between the different mechanisms that control rift initiation and development (magmatism, fauting, lithospheric fabric, rheology). To this end, a local seismic network has been deployed for the first time in the youngest part of the East African Rift, the North Tanzanian Divergence, for six months. Seismic signal was analyzed to characterize earthquake triggering mechanisms, crustal strength, strain and stress field, and seismic anisotropy. A seismic crisis occurred in North Tanzania (Gelai) involving dyking, seismic and aseismic slip. The Gelai crisis highlights the potential major role of aseismic processes in strain accommodation. Deep crustal earthquakes were recorded in the Lake Manyara region, Manyara seismicity is associated with strike-slip and normal faulting most Iikely triggered by fluids at depth. The influence of structural inheritance on rifting is indicated at lithospheric scale by seismicity and seismic anisotropy.BREST-BU Droit-Sciences-Sports (290192103) / SudocRENNES-Géosciences (352382209) / SudocSudocFranceF

Déformation active et récente, et structuration tectono-sédimentaire de la marge sous-marine algérienne

La convergence lente entre plaques pose d importants problèmes d évaluation du risque sismique, moins faciles à résoudre que dans le subductions. Ainsi l'Afrique du Nord est un chantier majeur pour étudier la réactivation en compression d'une marge passive cénozoïque complexe. Cette étude basée sur les campagnes MARADJA 03 et MARADJA2/SAMRA'05 (bathymétrie MF, sismique-réflexion, réflectivité, CHIRP, gravimétrie, SAR) permet de caractériser pour la premiére fois la structure multiéchelle de la marge sous-marine algérienne. L enregistrement sédimentaire (dont instabilités sédimentaires) et tectonique (géomorphologie, plis, failles) révèle de grande structures récentes et actives, ainsi que l héritage géologique de la marge. Deux grands styles tectoniques sont identifiés: décrochant à l ouest, et inverse au centre et à l est, où des failles aveugles néoformées (plio-quaternaires) à pendage S (opposé aux structures préexistantes), sub-perpendiculaires à la direction de convergence et souvent en échelon, génèrent des plis asymétriques. Ces failles ( vitesse minimale long-terme de raccourcissement de 0.1-0.Ømm/an) peuvent générer des séismes de M=6-7.5 (dont la taille de Khayr al Din près d Alger). Parmi elles, celle associée au séisme de Boumerdès (21/5/2003, Mw:6.8) se prolongerait à la surface par des replats rampes, créant des bassins en piggy-back j(usquà 60 km au large) ou rollover (sur la pente). Une grande part de la déformation liée à k convergence Afrique-Europe NNO-SSE (-5mm/an à la longitude d Alger) est donc accommodée en pied de marge algérienne, ce qui indiquerait, avec la flexion en compression du bassin adjacent, une future initiation de subduction.Important issues in seismic hazard assessment arise from slow convergent plates that are less easy to solve than in subduction zones. North Africa is a major study area in this context to study the reactivation in compression of a complex Cenozoic passive margin. This work based on the MARADJA 03 and MARADJA2/SAMRA'05 cruises data (MB bathymetry, seismic-reflection, side-scan sonar, baokscattering, CHIRP, gravimetry) made possible for the first time a characterization of the multi-scale structure of the offshore Algeriai margin. Sedimentary (among which, sediment instabilities) and tectonic (geomorphology, folds, faults) records reveal large recent and active structures as well as the geological inheritage of the margin. Two main tectonic styles are identified: strike-slip features to the W; and reverse to the centre and E: blind faults (Plio-Quaternary) verging to the N (opposite to preexisting features) expressed as asymmetrical folds, sub-perpendicular to the convergence direction and often en echelon. These faults (minimum long-term shortening rate over ail the structures: 0.1 -0.6 mm/yr) may ail trigger M=6-7.5 events (among them, the Khayr al Din fault near Algiers). Among them the fault associated with the Boumerdes earthquake (21/3/2003, Mw=6.8) would continue to the surface by flats and ramps creating piggy back basins (up to 60 km seaward) or rollovers (on the slope). A large part of the deformation resulting from NNW-SSE Africa-Eurasia convergence (-5 mm/yr at the longitude of Algiers) is accommodated at the foot of the Algerian margin, which could indicate, with the compressionnal flexure of the deep basin, a future subduction inception.BREST-BU Droit-Sciences-Sports (290192103) / SudocPLOUZANE-Bibl.La Pérouse (290195209) / SudocRENNES-Géosciences (352382209) / SudocSudocFranceF

Utilisation des perfluorocarbones en milieu sportif

CHATENAY M.-PARIS 11-BU Pharma. (920192101) / SudocSudocFranceF

Quantifying the role of sandy–silty sediments in generating slope failures during earthquakes: example from the Algerian margin

The Algerian margin is a seismically active region, where during the last century, several large magnitude earthquakes took place. This study combines geotechnical and sedimentological data with numerical modelling to quantitatively assess the present-day slope stability of the Algerian margin. Geotechnical laboratory tests, such as cyclic triaxial tests, oedometric tests and vane shear tests were carried out on sediment cores collected on the study area. The liquefaction potential of a sediment column located about 30 km from the Boumerdès earthquake epicentre of 21st May 2003 was evaluated theoretically for an earthquake of M w = 6.8. We show that thin sand and silt beds such as those described on recovered sediment cores are the main cause of sediment deformation and liquefaction during earthquakes. Numerical calculations showed that the slope failure may occur during an earthquake characterised by a PGA in excess of 0.1g, and also that, under a PGA of 0.2g liquefaction could be triggered in shallow silty–sandy deposits. Moreover, comparison of the predicted slope failure with failure geometries inferred from seafloor morphology showed that earthquakes and subsequent mass movements could explain the present-day morphology of the study area

Factors influencing seismic wave attenuation in the lithosphere in continental rift zones

Attenuation of seismic waves in the crust and the upper mantle has been studied in three global rift systems: the Baikal rift system (Eurasia), the North Tanzanian divergence zone (Africa) and the Basin and Range Province (North America). Using the records of direct and coda waves of regional earthquakes, the single scattering theory [Aki, Chouet, 1975], the hybrid model from [Zeng, 1991] and the approach described in [Wennerberg, 1993], we estimated the seismic quality factor (QC), frequency parameter (n), attenuation coefficient (delta), and total attenuation (QT). In addition, we evaluated the contributions of two components into total attenuation: intrinsic attenuation (Qi), and scattering attenuation (Qsc). Values of QC are strongly dependent on the frequency within the range of 0.2-16 Hz, as well as on the length of the coda processing window. The observed increase of QC with larger lengths of the coda processing window can be interpreted as a decrease in attenuation with increasing depth. Having compared the depth variations in the attenuation coefficient (delta) and the frequency (n) with the velocity structures of the studied regions, we conclude that seismic wave attenuation changes at the velocity boundaries in the medium. Moreover, the comparison results show that the estimated variations in the attenuation parameters with increasing depth are considerably dependent on utilized velocity models of the medium. Lateral variations in attenuation of seismic waves correlate with the geological and geophysical characteristics of the regions, and attenuation is primarily dependent on the regional seismic activity and regional heat flow. The geological inhomogeneities of the medium and the age of crust consolidation are secondary factors. Our estimations of intrinsic attenuation (Qi) and scattering attenuation (Qsc) show that in all the three studied regions, intrinsic attenuation is the major contributor to total attenuation. Our study shows that the characteristics of seismic wave attenuation in the three different rift systems are consistent with each other, and this may suggest that the lithosphere in the zones of these different rift systems has been modified to similar levels

Holocene turbidites record earthquake supercycles at a slow-rate plate boundary

Ongoing evidence for earthquake clustering calls for records of numerous earthquake cycles to improve seismic hazard assessment, especially where recurrence times overstep historical records. We show that most turbidites emplaced at the Africa-Eurasia plate boundary off west Algeria over the past ∼8 k.y. correlate across sites fed by independent sedimentary sources, requiring a regional trigger. Correlation with paleoseismic data inland and ground motion predictions support that M ∼7 earthquakes have triggered the turbidites. The bimodal distribution of paleo-events supports the concepts of earthquake supercycles and rupture synchronization between nearby faults: 13 paleo-earthquakes underpin clusters of 3–6 events with recurrence intervals of ∼300–600 yr, separated by periods of quiescence of ∼1.6 k.y. without major events on other faults over the study area. This implies broad phases of strain loading alternating with phases of strain release. Our results suggest that fault slip rates are time dependent and call for revising conventional seismic hazard models

Turbidite chronostratigraphy off Algiers, central Algerian margin: A key for reconstructing Holocene paleo-earthquake cycles

Northern Algeria is threatened by moderate to large magnitude earthquakes resulting from the slow convergence between the African and European plates. Main active faults are located offshore along the Algerian coast, as exemplified by the 2003 Mw 6.9 Boumerdès earthquake. This event triggered numerous and widespread turbidity currents over ∼ 150 km along strike in the Algerian basin (reaching 2800 m of water depth) and demonstrates the multi-source and multi-path characteristics of earthquake-triggered turbidity flows along this margin segment. We rely on the sedimentological analysis of five cores located at the toe of the Algiers margin, close to the 2003 cable break sites, to explore the potential for Holocene turbidite paleoseismology. Radiocarbon measurements provide age models for hemipelagic sediments. Based on sedimentary facies identification, analysis of depositional sequences (stacking pattern) and a stratigraphic framework established by age models, a first correlation of turbidites between the 5 cores is attempted. The number of turbidites is constant at the base of the continental slope and decreases seawards (over 80 km away from the coast). From turbidite correlations, 36 synchronous events are identified along the Algiers margin segment over the last 9 kyr, and are tentatively interpreted as seismically triggered, providing a 250 yr long mean recurrence interval. The main historical earthquakes in the Algiers area (2003, 1716 and 1365 AD) reasonably correlate with three out of the four last turbidites, strengthening the hypothesis that turbidites are suitable markers for Holocene paleoseismology. Recurrence intervals of turbidites range between 50 and 900 yr, defining quiescence periods exceeding 450 yr. Three quiescence periods lasting about 800, 1400 and 500 yr (7–6.2 ka BP, 5.4–4 ka BP, and 1.5–1 ka BP, respectively) support irregular earthquake cycling. Earthquake-triggered turbidites are more frequent in the study area than in the western adjacent margin segment (offshore El Asnam). This higher frequency could arise from the location of the seismogenic faults beneath the continental slope, whereas they are located several tenths of kilometers onland in the El Asnam area, implying less instabilities of the submarine slope

Results from the SEFASILS Experiment: Evidence for Mantle Exhumation in the Ligurian basin

International audienceThe north Ligurian margin is a stretched continental margin located at the junction of the Western Mediterranean Sea and the Alpine belt. The back-arc Ligurian basin opened from late Oligocene to early Miocene, as a result of the Apulian slab rollback. It has then evolved next to the active Alpine orogen, in a regional compressional setting between the Corsica-Sardinia continental block and mainland Europe, inducing a significant uplift of the north margin by more than 1000 m offshore Imperia (Italy).The SEFASILS project (Seismic Exploration of Faults And Structures In the Ligurian Sea) aims to better understand the Ligurian basin -especially its North margin– to comprehend the mechanisms of the ongoing tectonic inversion of the margin and the crustal-scale tectonic structures –active or not– marking its evolution. Acquiring quality deep seismic data in the Ligurian Sea is challenging due to the complexity of structures beneath the steep margin and to the screening effect of the thick Messinian evaporitic series. To this end, joint acquisitions of deep, long-streamer multichannel seismic (MCS) reflection data and dense sea-bottom wide angle refraction data (WAS) have been carried out along a 150 km long profile offshore Nice perpendicular to the basin’s axis.Based on various geophysical observables, the basin has classically been divided into three domains: a narrow stretched continental margin, a transitional domain of debated nature, and an atypical oceanic domain featuring a less than 5 km-thick crust. Our results from SEFASILS, including pre- and post-stack migrations and some first arrival travel time tomography of MCS and WAS data, confirm this subdivision but point to an absence of crust in the deep basin. In the transitional domain, the crust may also be absent or very thin, overlain by thick sedimentary filling. This apparent mantle exhumation tends to indicate that the continental break-up was not followed by oceanic accretion. It raises some fundamental questions, first on the modalities of break-up and opening, then on those of compressive reactivation of such basin and the ocation of observed seismicity and deformation in such atypical structural setting. On this latter point, SEFASILS results also provide indirect hints of the ongoing compression, even where morphological evidence is less pronounce

Results from the SEFASILS Experiment: Evidence for Mantle Exhumation in the Ligurian basin

International audienceThe north Ligurian margin is a stretched continental margin located at the junction of the Western Mediterranean Sea and the Alpine belt. The back-arc Ligurian basin opened from late Oligocene to early Miocene, as a result of the Apulian slab rollback. It has then evolved next to the active Alpine orogen, in a regional compressional setting between the Corsica-Sardinia continental block and mainland Europe, inducing a significant uplift of the north margin by more than 1000 m offshore Imperia (Italy).The SEFASILS project (Seismic Exploration of Faults And Structures In the Ligurian Sea) aims to better understand the Ligurian basin -especially its North margin– to comprehend the mechanisms of the ongoing tectonic inversion of the margin and the crustal-scale tectonic structures –active or not– marking its evolution. Acquiring quality deep seismic data in the Ligurian Sea is challenging due to the complexity of structures beneath the steep margin and to the screening effect of the thick Messinian evaporitic series. To this end, joint acquisitions of deep, long-streamer multichannel seismic (MCS) reflection data and dense sea-bottom wide angle refraction data (WAS) have been carried out along a 150 km long profile offshore Nice perpendicular to the basin’s axis.Based on various geophysical observables, the basin has classically been divided into three domains: a narrow stretched continental margin, a transitional domain of debated nature, and an atypical oceanic domain featuring a less than 5 km-thick crust. Our results from SEFASILS, including pre- and post-stack migrations and some first arrival travel time tomography of MCS and WAS data, confirm this subdivision but point to an absence of crust in the deep basin. In the transitional domain, the crust may also be absent or very thin, overlain by thick sedimentary filling. This apparent mantle exhumation tends to indicate that the continental break-up was not followed by oceanic accretion. It raises some fundamental questions, first on the modalities of break-up and opening, then on those of compressive reactivation of such basin and the ocation of observed seismicity and deformation in such atypical structural setting. On this latter point, SEFASILS results also provide indirect hints of the ongoing compression, even where morphological evidence is less pronounce