Journal of Seismology3: 95–104, 1999.© 1999Kluwer Academic Publishers. Printed in the Netherlands.

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Neotectonics and associate seismicity in the Eastern TellianAtlas of Algeria

A. Harbi, S. Maouche & A. AyadiC.R.A.A.G., BP 63, Bouzar´eah 16340, Algiers, Algeria

Received 16 January 1998; accepted in revised form 2 September 1998

Key words:Eastern Tellian Atlas, seismicity, neotectonic, submarine fault, focal mechanism, seismic profiles

Abstract

Seismicity in the Eastern Tellian Atlas of Algeria is active of moderate to low magnitude. The direct identificationof active fault is often a difficult task. In fact, in this region, despite the intense seismicity, only the Constantineearthquake of 27 October 1985 (Ms = 6.0) and the Kherrata earthquake of 17 February 1949 (Mb = 4.9), havegenerated surface ruptures. Hence, the integration of both geological, historical and instrumental seismic data areimportant in order to characterise the most important seismogenic structures. This paper presents a preliminaryoverview of the identified neotectonic faults that we consider active in the Eastern Tellian Atlas of Algeria.Thus, seismicity and neotectonic maps are presented and the faults which are active or potentially active froma neotectonic point of view are shown in relation with the main seismic groupings. This study based mainly onavailable seismic and bibliographic data and several unpublished marine seismic data enable us to suspect a fault asthe eventual source of the Jijeli earthquake of 21 August 1856 that destroyed the Jijeli town and its surroundings.The results inferred from this work represent a starting point for more detailed studies in seismogenic areas.

Introduction

The Tell Atlas occupies the southern side of theMediterranean Basin (Figure 1) and it has been foldedand faulted since the early Cenozoic by compressionwith a NNW–SSE shortening direction. The inter-mountains Neogene post-nappes basins correspond toE–W elongation. These structures are characterisedby compressional deformation during the Quaternaryperiod. These neotectonic features are constituted ofseveral seismogenic zones which correspond to E–W to NE–SW striking fold and reverse fault whichaffected the young Quaternary deposits.

During the last two decades, the neotectonic andseismotectonics of the western part of the Tell At-las have been the subject of a number of stud-ies. Indeed, the earthquake of 10 October 1980offered a good opportunity to improve our knowl-edge on neotectonic and seismotectonic of this part(Meghraoui, 1982; Thomas, 1985; Meghraoui etal., 1986, 1988; Meghraoui, 1988; Yielding, 1989;Dewey, 1990; Bezzeghoud et al., 1994, 1995; Aoudia

and Meghraoui, 1995; Lammali et al., 1997, etc.).On the other hand several seismogenic zones of theEastern Tellian Atlas remain unexplored despite theiractive seismicity (we note few studies: Rothé, 1950;Guiraud, 1977; Meghraoui, 1988; Bounif et al., 1987;Deschamps et al., 1991), and it is necessary to firstmake an overview of the neotectonic and seismicity ofthis region.

In this study, an attempt is made to correlate theseismic data with some of the identified neotectonicfaults in the Eastern Tellian Atlas and surroundingareas (35◦N–38◦N, 4◦E–9.5◦E) on the basis of ge-ological information (Vila 1980, Meghraoui 1988,Bounif et al., 1987), geophysical data (Harbi 1996)and previous works (Hatzfeld, 1978; Deschamps etal., 1991; Benouar, 1993; Mokrane et al., 1994;Bezzeghoud et al., 1996). Additional seismicity datafiles (C.R.A.A.G, IGN, ISC, USGS/NEIC) have alsobeen used for this purpose.

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Figure 1. General geological setting of the Algerian Tell Atlas (after Meghraoui, 1988). (1) Paleozoï; (2) Jurassic; (3) Cretaceous-Oligocene flyschs; (4) Jurassic basement and tellianCretaceous; (5) Neogene volcanism; (6) Neogene and Quaternary Basins.

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Figure 2. The distribution of earthquake epicenters in the North eastern Algeria between 1357 and 1996.

Seismicity and focal mechanism of the EasternTellian Atlas

In the Eastern Tellian Atlas there is an active seis-micity but of moderate to low magnitude (M≤ 6.3).The last large event in this area is the Constantineearthquake of 27 October 1985 (Ms = 5.7, mb = 5.4;Benouar, 1993) and the larger off-shore event is theJijeli earthquake of 21 August 1856, (I = VIII, Am-braseys 1982, Benhallou 1985; Mokrane et al., 1994).Historical seismicity reveals that destructive earth-quakes have occurred in this region (Rothé, 1950; Hée,1950; Roussel, 1973; Benhallou, 1985; Mokrane etal., 1994). The analysis of Figure 2, which representsthe seismicity map of the north-east of Algeria for the

period 1357–1996 (Io≥ VII and M ≥ 4), shows thatepicentres distribution has a tendency for concentra-tion around many zones. From east to west, it can beseen that the significantly important seismic groups onthis map are those of Guelma, where the 10 February1937 event occurred (Ms = 5.2); Constantine Basin,where the October 27 1985 earthquake occurred (Ms

= 6.0); vicinity of Batna where the Mac-Mahon earth-quake of 16 March 1924 (mb = 5.6) occurred; Kherrataand its surrounding areas (Kherrata earthquake of the17 February 1949,Mb = 4.9), Mansourah region wherethe Mansourah earthquake of 24 November 1973 (Ms= 5.1) occurred and M’sila region, where the 1 Jan-uary 1965 event (Ms = 5.5) occurred. Moreover, inthe Kherrata region, we observe a permanent micro-

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Figure 3. Tectonic map of the Eastern Tellian Atlas of Algeria (modified from Meghraoui, 1988). (1) volcanism; (2) Paleozoï; (3) Jurassic-Cretaceous and lowerCenozoic basement; (4)Neogene post nappe deposits; (5) Plio-Quaternary deposits; (6) Quaternary deposits; (7) fault [(A, B, C, D, E, F) Offshore recent faults (from Harbi,1996)]; (8) Anticline; (9) Reverse fault;(10) Strike-slip fault; (11) Normal fault.

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Figure 4. Criteria for recognition of active submarine faults (after Yonekura, 1983, In The Research Group for Active Faults of Japan, 1992).

activity along years. Many epicentres are located nearthe coast and may be related to the offshore contin-uation of the features in land or to submarine faults;particularly, we note the following events: the 10 Oc-tober 1930 earthquake (M = 5), the 19 September 1935event (mb = 5.1), the 24 November 1947 earthquake(M = 4.7), the 28 December 1954 (M = 4.4) and the3 December 1980 event (M = 4.3) which occurredrespectively offshore the coasts of El Kala, Skikda,Collo, Jijel and the Gulf of Bejaïa.

Since the installation of the seismological station(the oldest in this region is situated in Setif) andat present one modern seismological network cover-ing the whole country is in operation (since 1990,Bezzeghoud et al., 1994), it became possible, to im-prove epicentre localisation and to determine the focalmechanisms. In order to study the focal mechanism ofearthquakes in this region, we have selected only thosesolutions that have been well studied by some authors(Hatzfeld, 1978; Deschamps et al., 1991). In Figure 6,we can observe the following characteristics of focalmechanisms: earthquake 2 has a solution correspond-ing to normal fault with large strike-slip component;more to the west, earthquake 4 has a mechanism of

reverse faulting and all remaining earthquakes (1, 3,5) have strike-slip mechanisms.

Neotectonic data and active faults

In a previous study about the eastern Algerian mar-gin (Harbi 1996), some submarine faults have beenidentified from seismic profiles analysis. In fact, over4000 km of seismic reflection profiles (in 24 fold cov-erage, 8s two-way travel time), collected from twoseismic surveys carried out in 1973/1974 and 1977by the Algerian oil company SONATRACH, allowedus to prepare a simplified structural scheme for theeastern Algerian margin. The structural map (Harbi,1996) showed us that six submarine faults affected thePliocene and even Quaternary sediments and there-fore moved during the neotectonic period. Most ofthem trending NE–SW and affect the slope (faults A,C and D, Figure 3) or the continental shelf (faultsB, E and F, Figure 3). Clearly, it is very difficult torecognise an active submarine fault. However, takinginto account the criteria for the recognition of activesubmarine faults (Yonekura, 1983, in The ResearchGroup for Active Faults of Japan, 1992), we consid-ered the mapped submarine faults (Figure 3) to be

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Figure 5. Seismic profile across the marginal plateau of Bejaïa (after Harbi, 1996). (a) Interpreted seismic line. (b) Real section of the contouredpart showing the faults E and F and the strong accumulation of the Quaternary strata.

active. Thus, the faults B, E and F are active follow-ing the first criterion (Figures 4 and 5) and the faultsA and C are actively following the second criterion(Figure 4). Furthermore, in the Gulf of Béjaïa Plio-Quaternary submarine faults (E and F, Figures 3 and 5)form a graben and generate landslides of the Quater-nary strata on the sea bottom and a strong sedimentaryaccumulation (Harbi, 1996). Auzende, 1970 analysinga seismic reflection profile, observe a subsidence inthe Gulf of Béjaïa. According to this author, the strongsedimentary accumulation at the end of the slope mustbe related to the Soumam river. This river is going onsea by the canyon of Béjaïa. In fact, the landslidesand the strong accumulation in the Golf of Béjaïa asdepicted on the seismic reflexion profiles are probablyrelated both to the Plio-Quaternary tectonic activity inthis region and the Soumam river drainage. Otherwise,

neotectonic faults affecting Plio-Quaternary depositsare well known in some areas in land. In the M’silaregion, some Plio-Quaternary anticlines have been ob-served by several authors (Greandjean et al., 1966;Guiraud, 1973, 1977; Vila, 1980) in the north-eastof Chott el Hodna (Figure 3). Among these anticlinesMeghraoui (1988) studied the fold of Bou Taleb ofChott el Hammam. This author shows that the reversefault which limits the south-east limb of this anticlineis active. After the Kherrata earthquake of 17 Febru-ary 1949, Rothé (1950) observed surface ruptures andmapped the fault relating to an anticline trending N070 E. Later, Meghraoui (1988) confirms the existenceof the active fault of Kherrata (Figure 3) on the basisof some geomorphological study comparatively withEl Asnam case study. In the Constantine region, AinSmara fault and Sigus fault moved during the neotec-

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Figure 6. Seismotectonic map of the Eastern Tellian Atlas of Algeria. Focal mechanism are from Hatzfeld (1978) except this of the Constatine earthquake of 27 October 1985 which is fromDeschamps et al., 1991. Open square: Historical seismicity (1357–1910), open circles: Instrumental seismicity (1911–1996), blanked square: cities, (1) Focal Mechanism, (2) Neotectonic fault,(3) Reverse fault, (4) Strike-slip fault, (5) Anticline.

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Figure 7. Isoseismal map of the earthquakes of 21 and 22 August 1856 of Jijel (simplified from Ambraseys, 1982) showing the macroseismicepicentre of the main event and the submarine fault E.

tonic period (Vila, 1980). According to Bounif et al.(1987), these faults which are oriented, respectively,in NE–SW and E–W direction suggest the presence ofactive tectonics. The surface ruptures which were gen-erated by the Constantine earthquake of 27 October1985 may correspond to the north-eastern continuationof the Ain Smara fault (Figure 3). The Guelma Basin isa seismogenic zone, it is en ‘pull-apart’ structure and itis situated between east-west dextral strike-slip faults(Meghraoui, 1988); particularly, we note in this basinthe Bouchegouf fault and the Hammam N’baïlis fault.These faults are related to the hydrothermal springsand affected the Quaternary sediments (Vila, 1980)and therefore may be active (Figure 3).

Discussion and conclusion

The epicentral distribution of the whole earthquakeseries allows the establishment of a correlation be-tween the seismic activity and the system of activeor potentially active faults. Therefore, to make goodcorrelations, we use events with intensity above Io≥VII or magnitude higher than 4.0. In Figure 6 the dis-

tribution of the epicentres in the Eastern Tellian Atlasis shown in relation with the main seismogenic zonescorresponding to the main geological faults whichare considered active from both the neotectonic andseismic standpoints. Taking into account the spatialepicentre determination errors (of about 10 km for thelocalisation before 1980), the seismic events cannotbe used to identify the faults in a region with a sig-nificant precision. Nevertheless, there is particularly agood correlation between the main seismic groups inland indicated above and the position of neotectonicand active faults. Thus, faults of Chott El Hammam,Bouchegouf and Hammam N’Baïlis may be active andcorrespond, respectively, to M’sila and Guelma seis-mogenic zones; Kherrata Ain-Smara and Sigus faultsare active from seismotectonic point of view since thesurface ruptures have been, respectively, observed af-ter the Kherrata earthquake of 17 February 1949 andthe Constantine earthquake of 27 October 1985. Theexistence of active faults in the vicinity of Batna is tobe considered. On the other hand, it is obvious thatthe seismicity of the Gulf of Bejaïa and surrounding

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areas must be related to the network of fractures in thiszone (Kherrata fault and submarines faults (E and F)).The relationship between the submarine fault E andthe location of the epicentre of Jijeli earthquake of 21August 1856 is particularly noteworthy. According toRothé, 1950, the 21 August 1856 event has generateda tsunami along the coast of Jijeli. Regarding to theisoseismal map established by Ambraseys, 1982 (Fig-ure 7), the seismic source could be located offshoreJijeli with an approximately NE–SW direction.

The stress field acting along the northern Algeriagenerates tectonic structures in the NE–SW directionand E–W direction and with a slip rate of 7.6 mm/yr.,obtained by using seismic moment of the major seis-mic events in the western part of Algeria (Lammaliet al., 1997). The African plate moves to the northtowards the Eurasian plate with a NNW direction andwith a translation to the east. This translation explainsthe presence of the strike-slip faults observed in differ-ent seismogenic regions (Thomas, 1976; Bounif et al.,1987; Deschamps et al., 1991).

The focal mechanisms presented in Figure 6 arefault planes solutions determined from first motionsof P-waves (Hatzfeld, 1978) except this of 1985 Con-stantine earthquake which is obtained from body wavemodelling (Deschamps et al., 1991). From that figure,we can easily see that the rupture mechanism consid-ering the important events shows the presence of astrike-slip component for the approximately all the fo-cal solutions except for the 25 November 1973, wherewe have a pure reverse fault (according to Hatzfeld,1978). This can suggest the existence of the transla-tion component associated to the NNW compressionregime between the African and the Eurasia plates(Thomas, 1976, 1985). We also note a good correla-tion with fault-plane orientation obtained from focalmechanism and trend of geological faults. The 28 June1974 event localised in Kherrata region presents a nor-mal fault with a large strike-slip component. As shownin Figure 6 the Kherrata fault may not be the sourceof this event. Taking into account the precision of lo-calisation the event could be associated with anotherfault segment present in the region. On the other hand,the structures of the intermountains Neogene basinsare characterised by reverse faulting in the westernand central part of the Tellian atlas. The strike slipin the eastern part is more pronounced and the basinsare completely closed. The change in the regime ofdeformation could be in relation with the closure ofthese basins.

Acknowledgements

This study has been promoted by the Centre of As-tronomy, Astrophysics, and Geophysics (C.R.A.A.G);we are very grateful to Pr. H. Benhallou, Director ofC.R.A.A.G who has encouraged the authors to publishthe results obtained. The providing of marine seismicreflection profiles was made possible by the gener-ous collaboration of SONATRACH-EXPLORATION;grateful thanks are due to this institution for havingkindly allowed the processing and the interpretation ofthese profiles. Dr M. Bezzeghoud, Dr D. Benouar andthe anonymous referee are thanked for careful readingof the manuscript and for useful corrections.

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