Comment on “Zemmouri earthquake rupture zone (Mw 6.8, Algeria): Aftershocks sequence relocation and 3D velocity model” by A. Ayadi et al.

International audienceAlthough often difficult to characterize, the relationship between a seismic rupture, its aftershock sequence, and cumulative subsurface or surface faulting or folding is an important challenge to modern seismology and seismotectonics. Among other benefits, it helps document fault length, slip, and magnitude relationships, reconstruct the evolution of the rupture process through historical and prehistorical times and identify the complexity of the deformation in its path toward the surface. This approach is a prerequisite to any seismic hazard assessment but is particularly difficult for faults whose surface trace projects offshore. A specific effort to identify and quantify the source parameters of large earthquakes in coastal areas is therefore needed, not only in subduction zones but also in areas of slow rate and/or diffuse deformation

Recent and active deformation pattern off the easternmost Algerian margin, Western Mediterranean Sea: New evidence for contractional tectonic reactivation

International audienceWe describe for the first time a set of large active thrusts and folds near the foot of the easternmost Algerian margin, Western Mediterranean, from swath bathymetry and high-resolution seismic data acquired in 2005 during the Maradja2/Samra cruise. This active system resumes a previous passive margin and creates growth strata deposition on the limbs of large folds, resulting in the development of perched basins at the foot of the margin since less than ~ 1 Ma. They form a set of overlapping fault segments verging toward the Algerian basin, in a way similar to what has been observed off eastern Algiers on the rupture zone of the 2003 Mw 6.8 Boumerdes earthquake. The horizontal shortening rate across large folds is estimated to be of the order of 1 mm/yr. Although no historical earthquakes are reported here, these fault segments could have been responsible for large (M ~ 7.5) events in the past. This young tectonic system further supports the hypothesis of subduction inception of the Neogene oceanic lithosphere in the context of the Africa–Eurasia convergence

Searching for the Africa-eurasia Miocene Boundary offshore western algeria (Maradja'03 cruise)

International audienceWe present new results from the MARADJA'03 cruise depicting the geological structures offshore central and western Algeria. Using swath bathymetry and seismic reflection data, we map and discuss the offshore limits of the Internal Zones corresponding to relics of the AlKaPeCa domain that drifted and collided the African plate during the Miocene. We identify large reverse faults and folds that reactivate part of these limits and are still active today. The morphology of the westernmost NE–SW margin suggests a former strike-slip activity accommodating a westward block translation responsible for the shift of the Internal Zones towards the Moroccan Rif. To cite this article: A. Domzig et al., C. R. Geoscience 338 (2006). Nous présentons les résultats récents de la campagne MARADJA'03, qui visent à mettre en évidence les structures géologiques dans le domaine marin au nord-ouest de l'Algérie. Grâce aux données de bathymétrie multifaisceau et de sismique réflexion, nous cartographions et discutons les limites en mer des Zones internes correspondant aux reliques du domaine AlKaPeCa qui a dérivé, puis est entré en collision avec la plaque africaine au Miocène. De grandes failles inverses et plis, actifs dans le champ de contrainte actuel, réactivent certaines de ces limites. La marge ouest-algérienne, orientée NE–SW, indique la présence d'une ancienne activité en décrochement ayant accommodé la translation des Zones internes vers l'ouest

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

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 (multibeam bathymetry, seismic-reflection, side-scan sonar, backscattering, CHIRP, gravimetry) made possible for the first time a characterization of the multi-scale structure of the offshore Algerian 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 west; and reverse to the centre and east: blind faults (Plio-Quaternary) verging to the north (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 all the structures: 0.1-0.6 mm/yr) may all 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.La convergence lente entre plaques pose d'importants problèmes d'évaluation du risque sismique, moins faciles à résoudre que dans les 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 multifaisceau, sismique-réflexion, réflectivité, CHIRP, gravimétrie, SAR) permet de caractériser pour la première fois la structure multiéchelle de la marge sous-marine algérienne. L'enregistrement sédimentaire (dont instabilités sédimentaires) et tectonique (géomorphologie, plis, failles) révèle de grandes 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 sud (opposé aux structures préexistantes), sub-perpendiculaires à la direction de convergence et souvent en échelon, génèrent des plis asymétriques. Ces failles (à vitesse minimale long-terme de raccourcissement de 0.1-0.6 mm/an) peuvent générer des séismes de M=6-7.5 (dont la faille 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 et rampes, créant des bassins en piggy-back (jusqu'à 60 km au large) ou rollover (sur la pente). Une grande part de la déformation liée à la convergence Afrique-Europe NNO-SSE (~5 mm/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

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

The 1856 Tsunami of Djidjelli (Eastern Algeria): Seismotectonics, Modelling and Hazard Implications for the Algerian Coast

International audienceOn August 21st and 22nd 1856, two strong earthquakes occurred off the seaport of Djidjelli, a small city of 1000 inhabitants, located 300 km east of Algiers (capital of Algeria). In relation to these two earthquakes, an important tsunami (at least one) affected the western Mediterranean region and the eastern Algerian coastline between Algiers and La Calle (Algero-Tunisian border). Based on historical information as well as on data recently collected during the Maradja 2 survey conducted in 2005 over the Algerian margin, we show that the tsunami could have been generated by the simultaneous rupture of a set of three en echelon faults evidenced off Djidjelli. From synthetic models, we point out that the area affected along the Algerian coast extended from Bejaia to Annaba. The maximum height of waves reached 1.5 m near the harbor of Djidjelli