The Mahelma fault, located upon the Sahel anticline at the tip of the major Sahel fault, is often suspected to be a secondary structure. Yet, to date, the Mahelma fault has displayed no definitive evidences for faulting activity and no unambiguous typology. This paper reviews new data, especially geological and subsurface data, acquired through the Algiers west microzoning project. The data indicate that (1) the Mahelma fault is an antithetic high-angle reverse fault, (2) the interpreted dip is around 80° toward the SSE and thus implies a North-verging fault, and (3) the fault zone has been observed in several fault sites and allowed direct observation of fault displacements and fault throws. (4) Displacements directly related to the Mahelma fault affect clearly young sediments including consolidated Quaternary sands. We suggest that the Mahelma fault forms an active backthrust originating from pre-existing normal fault with a possible linkage and interaction with the major Sahel forethrust. We conclude from the foregoing that the Mahelma fault most likely ruptures in concert with earthquakes on the Sahel main fault. The results also point the activity of the Sahel fault-related fold and hence complicate earthquake hazard analyses.

Significant tectonic clockwise rotations were evidenced in the Tell Atlas (Neogene Algerian Cheliff basin) by previous paleomagnetic studies. For northwestern Africa and in the context of the Africa‐Eurasia convergence, they provided a new argument validating a kinematic model, based on transpression with shortening accommodated by clockwise block rotations. To corroborate this deformation pattern at a larger scale, new paleomagnetic studies were performed on 349 cores in the Mitidja basin, of Mio‐Plio‐Quaternary age, in the Algerian Tell Atlas. This intramontaneous basin is structured by two regional major E‐W to WNW‐ESE dextral shear zones. Primary magnetization data were obtained in 43 out of the 49 sampled sites. This magnetization is carried by Ti‐poor titanomagnetite. Its direction shows that significant tectonic block rotations affected this basin since 16 Ma. Zones located between the E‐W and WNW‐ESE major structures are affected by coherent clockwise rotations (average magnitude of 48°) of large blocks, compartmented by presently associated sinistral NE‐SW faults. Along the shearing structures, smaller blocks, resulting from the fragmentation of the large blocks, show various rotations, many of which are of large magnitude. These rotations, similar to those highlighted previously in the Cheliff basin, are interpreted as resulting from bookshelf, consequence of the Africa‐Eurasia plates convergence in the Tell Atlas.

The purpose of this study was to locate and characterize the Thénia Fault Zone (TFZ) in the urban area of Boumerdes city; geological and electrical resistivity tomography surveys have targeted the Plaisancian marl and its Quaternary cover. As a whole, data indicate a complex near-vertical fault zone with an asymmetric and zoned internal structure of at least 150 m wide and with a straight N120° overall trending. The fault zone is traversed with two elongated parallel fault branches (FB1 and FB2), generally, 70 m distant from each other. These fault branches locate two intense damage zones (IDZs) of 10–15 m thick each, situated at the margin of two damage zones each having a thickness of several tens of meters. Downward sand injections into IDZs during Pleistocene epoch, possible pulverization of Plaisancian marl rocks, systematic deflection of actual stream channels, and vertical displacement of at least 30 m affecting Quaternary alluvial deposits show that the area would have undergone active tectonic driven by the TFZ.

A moderate earthquake with a moment magnitude of Mw 5.5 struck the Sub-Bibanique region of eastern Algeria on 14 May 2010, killing three people, injuring hundreds of others, and causing moderate damages in the epicentral area, mainly in the villages of Beni-Ilmane and Samma. The focal mechanism of the seismic source for the first shock, obtained by near-field waveform modelling, exhibits left-lateral strike-slip faulting with the first nodal plane oriented at N345°, and right-lateral strike-slip faulting with the second nodal plane oriented at N254°. A second earthquake that struck the region on 16 May 2010, with a moment magnitude of Mw 5.1, was located 9 km SW of the first earthquake. The focal mechanism obtained by waveform modelling showed reverse faulting with nodal planes oriented NE–SW (N25° and N250°). A third earthquake that struck the region on 23 May 2010, with a moment magnitude of Mw 5.2, was located 7 km S of the first shock. The obtained focal mechanism showed a left-lateral strike-slip plane oriented at N12° and a right-lateral strike-slip plane oriented at N257°. Field investigations combined with geological and seismotectonic analyses indicate that the three earthquake shocks were generated by activity on three distinct faults. The second and third shocks were generated on faults oriented WSW–ENE and NNE–SSW, respectively. The regional stress tensor calculated in the region gives an orientation of N340° for the maximum compressive stress direction (σ1) which is close to the horizontal, with a stress shape factor indicating either a compressional or a strike-slip regime.

This study is devoted to the analysis of the stress state along the Maghreb region based on the inversion of focal solutions. We have inverted the main shock and aftershock focal mechanisms of the strongest seismic events that occurred in five seismogenic zones, from west to east: Al Hoceima (2004), Cheliff (1980), Tipasa–Chenoua (1989), Zemmouri (2003) and Constantine (1985). Most of the focal mechanisms of the aftershock sequences have been constructed within this study. Compressive stress regime is observed in the central part of Algeria between Cheliff and Zemmouri. On both edges of the Maghreb region, the stress regime becomes strike-slip in the Constantine region and in the Moroccan Rift. These different regimes seem to be linked to the free-edge effect (Ionian slab subduction) and to the dynamics of the Alboran Sea in the eastern and western parts of the study area respectively. The σ1 directions experience an anticlockwise rotation of about 20° from eastern to central Algeria. We observe that the direction of σ1 and the direction of convergence are the closest in central Algeria, where the collision is not perturbed by edge effect.

A moderate earthquake with moment magnitude Mw=5.5 hit the Subbibanique region in the East of Algeria at the locality of Beni-Ilmène on 14/05/2010 at 12h29mn(GMT).The earthquake was located by the CRAAG at latitude 35.99°N and longitude 4.19°E, the depth was 6 km.The Focal Mechanism(FM) of the seismic source obtained by waveform modeling (near-field) shows left-lateral strike-slip for the nodal plane oriented N345° and right-lateral strike-slip for the second nodal plane oriented N254°. A second shock stroke on 16 May 2010 with Mw=5.3. It is localized 9 km SW of the first shock at latitude 35.96°N, longitude 4.06°E and 5 km of depth. The FM obtained by waveform modeling shows reverse faulting with nodal planes oriented NE-SW. A third shock hit on 23 may 2010 with Mw=5.3, localized 7 km at the south of the first shock at the latitude 35.93° N, longitude 4.12° N and 6 km of depth. The FM obtained shows a left-lateral strike-slip plane oriented N355° and a right-lateral plane oriented N85° similar to that of the first shock. Geologic and sismotectonic analysis lead us to conclude that the fault plane of the first shock is oriented NNW–SSE and other shocks were generated by separate faults.

The similar to 60 km-long Sahel ridge west of Algiers (Tell Atlas, north Algeria) is considered as an ENE-WSW fault-propagation fold running along the Mediterranean coast and associated with a north-west dipping thrust. Its proximity with Algiers makes this structure a potential source of destructive earthquakes that could hit the capital city, as occurred in 1365 AD and 1716 AD. The first paleoseismologic investigation on the Sahel ridge was conducted in order to detect paleo-ruptures related to active faulting and to date them. From the first investigations in the area, a first trench was excavated across bending-moment normal faults induced by flexural slip folding in the hanging wall of the Sahel anticline thrust ramp. Paleoseismological analyses recognize eight rupture events affecting colluvial deposits. C-14 dating indicates that these events are very young, six of them being younger than 778 AD. The first sedimentary record indicates two ruptures before 1211 AD, i.e. older than the first historical earthquake documented in the region. Three events have age ranges compatible with the 1365, 1673 and 1716 Algiers earthquakes, whereas three other ones depict very recent ages, i.e. younger than 1700 AD. Potential of these secondary extrados faults for determining paleoseismic events and thrust behaviour is discussed. (c) 2012 Elsevier Ltd and INQUA. All rights reserved.

The present-day crustal deformation in the North African area is mainly driven by the NW-SE to NNW–SSE convergence (4 – 5 mm/yr) between the African and Eurasian plates (Fig. 1- [1]; [2]; [3]). This convergence is accommodated over a wide deformation zone (the Tellian Atlas) implying the existence of significant seismic activity. This domain represents the southern part of the Alpine ranges at the boundary between the African and Eurasian plates. It is characterized by E-W to NE-SW trending folds and thrust belt and is composed by an "external zone" with Mesozoic to Tertiary rocks and an "internal zone" consisting of a thrust stack of metamorphic complexes and Mesozoic calcareous and flysch. Overlying these previous series, Neogene to Quaternary sediments fill the intramountainous basins, limited by E–W and NE–SW faults as evidenced (from East to West) in the “Constantine” basin, the “Soummam” basin, the “Mitidja” basin, the “Chelif” basin and the “Mleta” basin [4]; [5]; [6]. Few studies were dedicated to these faults ([7]; [8]; [9]), consequently the timing and amount of displacement along these faults, as well as their relationship with the Neogene basin development, are still a matter of debate.

On March 20, 2006, an earthquake (M w = 5.3; SED) struck the mountainous region of the Babors chain (Wilaya of Bejaia, northeast Algeria). The seismic epicenter was located near the Kherrata village. This earthquake was felt on a large area of the northeastern part of Algeria. It reached an intensity of VII (EMS scale) at the Laâlam village, situated at about 20 km northeast of Kherrata. Here, many old and recent houses were damaged or collapsed totally, four people died and 68 were injured. Field investigations revealed that these casualties were caused by a landslide triggered by the earthquake. Many fissures were visible on ground throughout the site. They were generated by both sliding and settling phenomena. The Laâlam site is prone to landslide, as revealed by some evidences on old instabilities. This is due to two main factors: local geomorphology and geology. These factors intervene synchronously for reducing the slope instability at the Laâlam village. The March 20, 2006 Kherrata earthquake was the trigger that released the Laâlam landslide.

In this work, we show that the North-Eastern boundary of the Ahnet Paleozoic Basin (North-West Hoggar, Algeria) corresponds to a major dextral transpressive fault, active during the Upper Palaeozoic. The analysis of tectonic structures and microstructures associated with this fault allows characterizing compressive and dextral strike-slip components. The chronology of both components is discussed through two possible models of evolution: (1) counter clockwise deviation of the stress field, with successively East-West then NE-SW shortening; (2) Positive flower structure above a major shear zone, within a NE-SW shortening. The second model seems to be the most realistic, since it agrees with previous studies.