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A concentrate of seismic history unearthed by public works in Algeria

South-west of Algiers, a trench opened for public works allowed geologists to establish the record of a sequence of rupture and sedimentation events. These few square meters provided a precious archive of the great earthquakes the north of Algeria experienced over the last millennium.

Bandeau Heddar.jpg

The north of Algeria is among the most seismically active areas in the western Mediterranean. In recent centuries, it was affected by several large earthquakes (magnitude 7 and above) which caused thousands of victims and considerable damage to infrastructures. This seismic activity is the result of the slow convergence between African and Eurasian tectonic plates, at a pace of 4 to 6 mm/year in the NW-SE direction. The resulting compression reactivates faulted and folded structures, on land and offshore. South of Algiers, the Mitidja basin is bounded by several major active structures, which are the sources of potential destructive earthquakes, but also of more moderate ones with no surface ruptures.

During the historical period, several major earthquakes have been described in Algeria, among which those of 1365 (followed by a tsunami and about 500 aftershocks) or of 1716 (which partly destroyed Algiers and caused the loss of 20,000 lives); in some other cases their location and/or intensity remains doubtful. Seismic monitoring in Algeria began in 1980 and was continuous only since 2005; this explains the lack of microseismicity recordings necessary to identify and monitor active structures.

South-west of Algiers lies parallel to the coast a set of hills among which those called the "Algiers Sahel ridge"; this relief is formed by an anticline emerging near the shore and crossed by a long fault dipping to the north-west. The aim of the study was to detect surface ruptures that record earthquakes produced in the past by the Sahel structure activity.

On the southern flank of the Sahel fault-propagation fold, public works unearthed the trace of a 5-meter long and 1.5-m deep depression filled with sedimentary (colluvial) deposits affected by small-scale faults.


The trench wall after cleaning; the white grid is made of 50-cm squares



Even though these faults are too small to cause earthquakes by themselves, they are the result of the underlying seismogenic thrust. The depression was formed by the extrados deformation of the upper compartment of the large thrust (compressive) fault of the Algiers Sahel during the successive seismic events. This site is particularly interesting because it is filled with sediments rich in organic material, thence datable by radiocarbon methods (14C).

The sediments form a well-defined sequence of about ten layers. The oldest eight are separated by flat transitions indicating that they deposited without significant erosion. Two groups are recognized according to their lithological characteristics. Whereas the first three layers are made of millimetre to centimetre gravels in a silty clay matrix and less than 22-cm thick, the following ones mainly contain silty clay with remains of shells and plants (charcoal). The last sedimentary unit is unconformable above the older ones, indicating a dominant control of erosion. Finally, a brown surface layer 40 cm thick covers the whole sequence. This shows that the depression was filled step by step as a result of erosion by local heavy precipitation, with temporary stabilization allowing organic matter accumulation.
Eleven shell or charcoal samples were collected in the sedimentary units and dated with 14C. Their age ranges from the ninth century (778-897) and the modern period (>1750).


Sedimentary units and faults (red lines) observed on the trench wall

Five steep normal (extension) faults can be seen in the depression, three northwards and two southwards; one of them splits in two. Their displacement ranged from 6 to 30 cm and resulted into shear zones and folds. This pattern shows that the depression is a graben, i.e. the subsidence of the compartment located between two extension faults. According to sediment structure and composition, eight rupture events could be identified.

The retrospective analysis of deformations enabled restoring stratigraphic units to their inferred original geometries by graphically reversing the sense of displacement on faults. Each event caused displacements along one or several faults. The oldest events which could be dated occurred between 778 and 1222; the surface layer was formed after the agricultural reform of modern Algeria (1963).

Reconstitution of the sequence of sequence of deformation, sedimentation and erosion events at the trench

These events can be put in relation with the major earthquakes in the Algiers region, whose historical record extends discontinuously back over 700 years. However, matching the first two events with known earthquakes was impossible by lack of historical data older than the Ottoman Empire. The two major historical earthquakes (1365 and 1716) are good candidates to correspond to one of the rupture events 3, 4 or 5, but dating is not precise enough to conclude. The 1842 earthquake, which is certain, could correspond to one of the three recent surface rupture events, but those of 1802, 1804 and 1860, less known, could also be related to them.

This paleoseimologic record is nevertheless likely to be incomplete, as moderate or even important events may have caused ruptures which did not reach the surface and thus cannot be observed. Estimating recurrence interval of major earthquakes associated with the Sahel structure is therefore not possible; this interval could be around 300 years during a first period (778-1727) then three to four times smaller after 1727. This discrepancy could be explained by the overestimation of the age of event 3 (the oldest dated), by the aftershocks following some major earthquakes, or by a pulse of activity recently undergone by the Sahel structure.


The paper

Heddar A., Authemayou C., Djellit H., Yelles A.K., Déverchère J., Gharbi S., Boudiaf A., Van Vliet Lanoe B., 2013. Preliminary results of a paleoseismological analysis along the Sahel fault (Algeria): New evidence for historical seismic events. Quaternary International 302 : 210-223.
See the first page


The authors

This work is the fruit of a collaboration of researchers of the laboratory Domaines océaniques of IUEM, of the Centre de recherche en astronomie, astrophysique et géophysique (CRAAG) in Algiers and of the University of Montpellier.


The journal

Quaternary International was founded in 1989 by the International Union for Quaternary Research. It publishes papers on all the full spectrum of the physical and natural sciences that are commonly employed in solving Quaternary problems.



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