Lithologie, evolution structurale et geodynamique des terrains precambriens d'Abankor-Timgaouine (Hoggar occidental, Algerie)

SIGLET 55648 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Active Coastal Thrusting and Folding, and Uplift Rate of the Sahel Anticline and Zemmouri Earthquake area (Tell Atlas, Algeria)

International audienceMajor uplifts of late Quaternary marine terraces are visible along the coastline of the Tell Atlas of Algeria located along the Africa-Eurasia convergent plate boundary. The active tectonics of this region is associated with large shallow earthquakes (M≥6.5), numerous thrust mechanisms and surface fault-related fold. We conducted a detailed levelling survey of late Pleistocene and Holocene marine notches in the Algiers region that experienced 0.50 m coastal uplift during the 2003 Zemmouri earthquake (Mw 6.8). East of Algiers, Holocene marine indicators show three pre-2003 main notch levels formed in the last 21.9 ka. West of Algiers on the Sahel anticline, the levelling of uplifted marine terraces shows a distinct staircase morphology with successive notches that document the incremental folding uplift during the late Pleistocene and Holocene. The timing of successive uplifts related to past coseismic movements along this coastal region indicates episodic activity during the late Holocene. Modelling of surface deformation in the Zemmouri earthquake area implies a 50-km-long, 20-km-wide, NE-SW trending, SE dipping fault rupture and an average 1.3 mcoseismic slip at depth. Further west, the 70-km-long Sahel fold is subdivided in 3 sub-segments and shows ~0.84- 1.2 mm/yr uplift rate in the last 120-140 ka. The homogeneous Holocene uplift of marine terraces and the anticline dimensions imply the possible occurrence of large earthquakes with Mw≥7 in the past. The surface deformation and related successive uplifts are modelled to infer the size and characteristics of probable future earthquakes and their seismic hazard implications for the Algiers region

H/V ambient noise for a rapid assessement of an unstable zone geometry : road slide case in Lakhdaria (Algeria)

Following torrential rainfall that struck the area of Lakhdaria during the winter of 2006, a slide road slope carried away half of the road on forty meters length, leaving a five meters high scarp. The remaining part of the road and houses located upstream were threatened by a potential second slip as showed by the cracks and the déformations observed around the scarp. Ambient vibration recordings were carried out 3 days after this event, with the aim of circumscribing the unstable zone around the scarp, characterized by a strongly disturbed soil. The results of this study showed that in the investigated area, H/V curves exhibit a frequency peak around 6 Hz behind the edge of the scarp and another one around 2 Hz, along the road, on both sides of the scarp. The first peak is related to the disturbed and unstable section of the soil, which is about 6m thick (slip surface depth), in good agreement with field observations and confirmed by geotechnical investigations. The second peak is related to a deeper geological interface. Upstream of the road, where there are some houses, H/V curves are flat, indicating the absence of the strongly disturbed soil section. These results allowed us to rapidly evaluate the lateral extension and thickness of the unstable zone. This study shows the reliability of H/V ambient vibration method to characterize the geometry of "fresh" sliding zones or unstable disturbed soil masses

Heavier Damages without Site Effects and Site Effects with Lighter Damages: Boumerdes City (Algeria) after the May 2003 Earthquake

International audienc

Upper Cretaceous-Cenozoic uplifts and tectonics within a Precambrian shield — insight from the Hoggar (Algeria) local sedimentary cover

International audienceIn order to better constrain the Mesozoic-Cenozoic evolution of the Precambrian Hoggar shield, a paleomagnetic study, combined with detailed fieldwork, was carried out to date its detrital local cover, the Serouenout Formation. Thermal demagnetization yields, only in a few samples, the characteristic remanent magnetization carried by hematite. Post-tilting remagnetization was obtained in sites located along a fault affected by intense fluids circulation. The paleomagnetic directions recorded at seven widespread other sites are on the contrary associated with a positive fold test. It provides a relatively well-defined paleomagnetic pole (A95 = 4.1°), sufficient to estimate the age of the Serouenout Formation. The comparison of this pole with the reference curve of Africa suggests two possible age windows, Triassic and Upper Cretaceous-Lower Paleocene, while the discovery in the uppermost levels of the Serouenout Formation of a fern-rich level with Weichselia reticulata (Bathonian to Cenomanian; Blanco-Moreno et al., 2018) imposes a deposition during the Cenomanian. The presence of a detrital formation at least 350 m thick, with a basal conglomerate containing large pebbles, implies the existence, during this time period, of a tectonic event that generated differential uplift. In addition, structural observations indicate that the Serouenout Formation recorded later brittle tectonics, dominated by a network of vertical N–S dextral faults. The horizontal displacement generated by one well-developed fault has been estimated to be at least 1 km. This activity is related to the known Alpine reactivations of the N–S Pan-African mega-structures, which are still at the origin of the current intraplate seismicity

THE 20 MARCH 2006 EARTHQUAKE OF LAALAM (KABYLIA, ALGERIA) (Mw=5): GEOLOGICAL AND GEOPHYSICAL PROSPECTIONS (AMBIENT VIBRATION NOISE) AND CORRELATIONS WITH DAMAGES. LE SÉISME (Mw = 5) DE LAALAM (KABYLIE, ALGÉRIE) DU 20 MARS 2006 : PROSPECTIONS GÉOLOGIQUES ET GÉOPHYSIQUES (BRUIT VIBRATOIRE AMBIANT) ET CORRÉLATIONS AVEC LES DOMMAGES

url: http://www.mem-algeria.org/actu/comn/pubt/bulletin_ANGCM/Bulletin_SGN_V20-3.pdfInternational audienceGeological and geophysical (ambient vibrations) prospections conducted after the moderate Laalam earthquake (Mw = 5, USGS), demonstrate that structure destructions following this event are not related to site effects. Nevertheless, damages and human losses (4 dead, 68 injured) may be explained by vulnerable habitations and the epicenter proximity, either directly (ground shaking) or indirectly, by induced effects. Induced effects are constituted of soil and road cracks, rock falls, and a landslide reactivated by the earthquake. However, the surficial cracks are not expressing the fault on the surface, because of the moderate magnitude of the event. This study is comforting the need of moderate earthquake induced effects and raises the important problem of damages caused by low to moderate earthquakes in Algeria and buried fault location and mapping