Preliminary results of a paleoseismological analysis along the Sahel fault (Algeria): New evidence for historical seismic events

International audienceThe ∼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. 14C 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

The Ain Temouchent earthquake of December 22th, 1999

On December 22nd, 1999 an earthquake of Magnitude Mw : 5.7 occurred at Ain Temouchent (northwest Algeria). This moderate seismic event was located in a region characterized by a low seismic activity where few historical events have been observed. The earthquake, with a maximum intensity of VII (MSK scale), caused serious damages to the Ain Temouchent city and its surroundings. In the epicentral area, 25 people died and about 25,000 people were made homeless. Some minor breaks have been observed in several areas in the field. They were mainly related to minor collapses in the landscape or in volcanic cavities. The focal mechanism has been studied by using broadband data at regional and teleseismic distances, and different methods. The fault-plane solution has been estimated from first motions of P wave. Depth and source time function have been estimated from the modeling of body waveforms. Scalar seismic moment and source dimension have been obtained from spectral analysis. Results show thrust motion, with a horizontal pressure axis oriented in a NW-SE direction, a depth of 4 km and a simple source time function with time duration of 5 s. Scalar seismic moment estimated from waveform modeling is 4.7 · 1017 Nm, and spectral analysis gives a value of 1.7 · 1017 Nm and a source radius of 7.5 km

The Beni-Ilmane (Algeria) seismic sequence of May 2010: Seismic sources and stress tensor calculations,

International audienceA 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

New Moscovian palaeomagnetic pole from the Edjeleh fold (Saharan craton, Algeria)

International audienceA palaeomagnetic study was carried out in the Moscovian (~305Ma) formation in the Edjeleh anticline, the only area where important dips can be observed in the Upper Palaeozoic series of the Illizi basin (Algeria). This study shows the existence of three magnetization components. Two of them are interpreted as Cenozoic and Permian remagnetizations; their poles are 88.8°N, 164.0°E, K=262, A95=3.3° and 43.4°S, 61.7°E, K=93, A95=5.9°, respectively. The third component is determined by both well-defined ChRMs and remagnetization circles analysis. Its associated fold test is positive and significant. Because the folding started before or during the Autunian, this third component was acquired very early and is very probably the primary magnetization. Its corresponding palaeomagnetic pole (28.3°S, 58.9°E, K=157, A95=4.2°) is close to the poles from the Saharan platform obtained from neighbouring periods. The positive fold test of this study thus validates these previous Upper Palaeozoic poles. This new result is in agreement with the geodynamical model (Matte 1986; Henry et al. 1992), which proposed the occurrence of a large clockwise rotation of Africa during the Carboniferous; such a motion agrees with the Permian Pangaea A2 reconstruction

Study of some noncooperative linear elliptic systems

summary:Using an approximation method, we show the existence of solutions for some noncooperative elliptic systems defined on an unbounded domain

Juxtaposed and superimposed paleomagnetic primary and secondary components from the folded middle carboniferous sediments in the Reggane Basin (Saharan craton, Algeria)

International audienceA paleomagnetic study was carried out on the Middle Carboniferous sediments of the eastern margin of the Reggane Basin of Algeria. Seven sites (108 samples) in the Lower Serpukhovian and 11 sites (129 samples) in the Upper Serpukhovian, Bashkirian and Lower Moscovian levels were investigated. Besides a common, but generally limited, viscous remanent magnetization (component A) and a recent chemical remanent magnetization of reversed polarity (A‧), two main components were identified: one of these (component B), is characterized by a negative fold test and has been identified as a Lower Jurassic remagnetization. The associated paleomagnetic pole obtained in the seven zones by combining characteristic remanent magnetization directions (ChRM) and great circles (λ=71.1°N, ϕ=251.4°E, A95=3.8°, K=254) lies in the vicinity of the NW African poles of similar ages. The second (component C) displays both normal and reversed polarities. Also determined by the combination of ChRM or stable end points and remagnetization circles, it yields a positive fold test which constrains the magnetization acquisition time and a positive reversal test which argues in favor of a ;non-composite; nature of the component C. The normal polarities observed in the Lower Serpukhovian levels represent the latest normal event observed in Africa before the Kiaman superchron. The paleomagnetic South pole calculated from 10 sites (n=64 data) gathered in four large areas (λ=26.5°S, ϕ=44.7°E, A95=4.7°, K=383) is the first African Carboniferous pole founded on both positive reversal and fold tests. It lies only slightly apart from other Middle Carboniferous poles previously published for the northern part of Africa where no intraformational test were available to constrain the magnetization age

Comment on "Active coastal thrusting and folding, and uplift rate of the Sahel Anticline and Zemmouri earthquake area (Tell Atlas, Algeria)", by S. Maouche, M. Meghraoui, C. Morhange, S. Belabbes, Y. Bouhadad, H. Haddoum. [Tectonophysics, 2011, 509, 69-80]

International audienceBased on geomorphologic analyses and leveling survey of Quaternary coastal indicators (i.e. marine terraces and notches) along of a 50-km-long coastal stretch of the Algerian coast west of Algiers, Maouche et al. (2011) interpret the coastal segment to have undergone high uplift rates, i.e. 0.84-1.19 mm/yr since last interglacial maximum (MIS 5e, 122±6 ka in Table 1, ~140 ka in Maouche et al., 2011) and ~2.5 mm/yr for the last 31 ka. This uplift was said to be due to repeated seismic events that would have occurred during the last ~140 ka, and more particularly during the late Pleistocene. We raise major issues about the interpretation proposed by Maouche et al. (2011). These issues deal with 1) the use of previous chronological data and the chronostratigraphy proposed, 2) processes involved in the creation of coastal staircase morphology on the coast west of Algiers, 3) anomalously high uplift rates compared to other available data on the same geomorphic features (marine terraces) in the same setting of reactivated passive margins, and 4) the fold geometry used for modeling of fold growth and its implications for coseismic surface deformation and uplift estimates. In other words, we contest the statements that coseismic deformation is the cause of staircase morphology on the Mediterranean coast west of Algiers and that very large (M>7) earthquakes have occurred there in the past