Recent seismicity of Central High Atlas and Ouarzazate basin (Morocco

ORIGINAL PAPER

Recent seismicity of Central High Atlas and Ouarzazate basin(Morocco)

Parfait Noel Eloumala Onana • El Arbi Toto •

Lahcen Zouhri • Abdelkader Chaabane •

Azelarab El Mouraouah • Aomar Iben Brahim

Received: 4 April 2010 / Accepted: 30 December 2010 / Published online: 22 February 2011

� Springer-Verlag 2011

Abstract The paper describes the tectonic evolution of

the Central High Atlas and the Ouarzazate basin in Mor-

occo, noting that many of the tectonic phenomena follow

the previous structural lines. Although the area is one of

relatively low seismicity, its location in an intra plate part

of the African Plate and its high morphology, the effects of

an earthquake could be considerable. This study combines

seismic records from 1900 to 2007 with the integration of

existing tectonic and structural information to better define

the seismogenic zones. A newly identified cluster of epi-

centers in a NNW-SSE linear zone may be related to the

Imilchil fault, where the earthquakes with magnitudes of

4–5 at depths of 5–20 km have been identified. Recent

work in the Ouarzazate basin indicated that the Amekc-

houd fault ramp could generate seismic events with mag-

nitudes as large as 6.4 Mw.

Keywords Central High Atlas � Ouarzazate basin �Tectonic structures � Diffuse and moderate seismicity

Resume L’article decrit l’evolution tectonique du Haut

Atlas central et du bassin de Ouarzazate au Maroc,

observant que de nombreux phenomenes tectoniques sui-

vent des traits structuraux preexistants. Bien que la region

presente une faible sismicite, les effets d’un seisme pour-

raient etre considerables du fait de sa position au sein d’une

intra-plaque de la plaque africaine et du fait de son relief.

Cette etude combine des enregistrements sismiques de

1900 a 2007 tout en prenant en compte des informations

disponibles d’ordre tectonique et structural afin de mieux

definir les zones sismogenes. Un ensemble d’epicentres

dans une zone lineaire NNW-SSE, peut etre relatif a la

faille d’Imilchil, a ete nouvellement identifie, avec des

seismes de magnitude 4 a 5 et des foyers a des profondeurs

de 5 a 20 km. Des travaux recents dans le bassin de

Ouarzazate ont indique que la faille d’Amekchoud pourrait

generer des evenements sismiques avec des valeurs de

magnitude Mw aussi importantes que 6,4.

Mots cles Haut Atlas central � Bassin de Ouarzazate �Structures tectoniques � Sismicite diffuse et moderee

Introduction

The study area is between latitude 30.5� and 32.5�, longi-

tude 7.30� and 5.30�. It consists of the Central High Atlas

and the Ouarzazate basin and is generally considered a

region of low-seismic risk as no large magnitude earth-

quakes have been recorded. Nevertheless, with the occur-

rence of the last devastating earthquake in north eastern

Morocco, near Al Hoceima city, concerns have been raised

regarding the potential for moderate to large magnitude

earthquakes to occur in the study area.

The Central High Atlas is a unit of the North African

intercontinental range formed during the Cenozoic

(Mattauer et al. 1977; Teixell et al. 2003). Located between

the Anti-Atlas and the High Atlas, the Ouarzazate basin,

like the High Atlas range, experienced the same geological

P. N. E. Onana � E. A. Toto � A. Chaabane

Sciences Faculty, IBN Tofail University of Kenitra,

B.P. 133, Kenitra, Morocco

L. Zouhri (&)

Departement Geosciences, Institut Polytechnique LaSalle

Beauvais, 19 Rue Pierre Waguet, 6000 Beauvais, France

e-mail: [email protected]

A. El Mouraouah � A. Iben Brahim

National Institute of Geophysics, CNRST,

B.P. 8027, Agdal NU, Rabat, Morocco

123

Bull Eng Geol Environ (2011) 70:633–641

DOI 10.1007/s10064-011-0361-z

evolution history, particularly related to the NE-SW faults.

Indeed, the Moroccan Atlasic range has been affected by

intense orogenic activities, as evidenced by the uprising of

the High Atlas mountain, ductile deformation and thrust

faulting, accompanied by important syn-orogenic sedi-

mentation in the Ouarzazate basin (Gil et al. 2008).

Neo-tectonic and palaeo-seismic studies in this region

have allowed the identification of several tectonic struc-

tures (Ait Brahim 1990; Benammi et al. 2001, 2005;

Serbier et al. 2006). Detailed analysis of the Quaternary

deformed geological units allowed an estimation of the slip

rate on several fault segments of about 0.1 mm/year in the

Ouarzazate basin (Serbier et al. 2006). The compressive

displacement in the study area was accommodated along

the E–W and N50–N70 thrust fault network (Ait Brahim

1990) with strike slip faults displacing even the basement

rocks. The local shortening rate of this compressive

deformation evaluated on different sections amounts to

about between 18 and 24% (Teixell et al. 2003; Missenard

2006). From the geological and geomorphological evi-

dence, a 0.3 mm/year shortening and 0.2 mm/year uplift

have been estimated in the area (Gil et al. 2008).

Although the structures of the Central High Atlas and

Ouarzazate basin have been the subject of several previous

papers, to date, there has been little to confirm the faults

identified adequately indicate the vulnerability of the study

area to real seismic hazard. This study characterizes the

recent seismicity of the Ouarzazate basin and examines

the influence of the structure on the overall seismicity of

the region. The correlation of seismic, geological and

geophysical data allows a better seismotectonic analysis

and understanding of the implications for seismic hazard.

Geodynamic setting

The Moroccan High Atlas were formed during the Meso-

zoic and Cenozoic (Fig. 1) related to the convergence of

the African and Eurasian plate. The High Atlas tecto-

genetic cycle started with discordant Permo-Triassic

deposits accumulating on the Precambrian and Palaeozoic

basement (Michard 1976). According to Missenard (2006),

two processes are important in the origin of the High Atlas:

(a) a lithospheric phenomenon which operated on a NE-

SW elongated strip and affected the entire Moroccan

structural domain; its origin could be attributed to a

hot spot or to a secondary convective system related

to the West-African Craton.

Fig. 1 Simplified geological map of the Central High Atlas, the Ouarzazate basin and the neighbouring area. a Location of the Atlas Mountains

in the North African foreland, b geological map of the Central High Atlas

634 P. N. E. Onana et al.

123

(b) crustal shortening expressed as several decollement

levels related to compression within the African plate

which closed the Atlasic rift.

As a consequence of the North Atlantic opening, Europe

and Africa began to converge at a velocity of less than

10 cm/year (Rosenbaum et al. 2002). This movement was

absorbed by different processes (in space and time) such as

the resorption of the Tethys by subduction and the conti-

nental collision in the Alps. Dewey et al. (1989) assumed

that the alpine high-pressure metamorphism was an indi-

cator of the onset of the Eurasia-Africa plate convergence.

Since the Jurassic, Morocco (bounded by the Tethys in the

north east and the Atlantic in the west), has witnessed the

development and the disappearance of the Atlasic Rift

system. Owing to crustal thinning at the beginning of the

Mesozoic, this rifting episode produced four structurally

and mechanically weak domains in Morocco: the Occi-

dental High Atlas, the Central High Atlas and the Oriental

High Atlas, which are all characterized by E–W to the

ENE-WSW tectonic events, whereas the Middle Atlas is

related to NE-SW structures. The deformation is essentially

localized on structures of Hercynian age which were tec-

tonically inverted during the Cenozoic (Laville et al. 2004).

The main compressive phase took place in the Pyrenean

and in the Atlasic intra-continental range during the late

Eocene and the Oligocene, (Verges et al. 1995; Meigs et al.

1996; Frizon de Lamotte et al. 2000). These periods were

characterized by deformation, which was concentrated in

the distant High Atlas hinterland, whereas the marginal

thrust belt of the sub-Atlas zone was scarcely deformed

(Teson et al. 2006).

From the early Miocene, the convergence has slowed to

velocities of less than 10 km/Myr (Missenard (2006)).

Movement towards the forelands created complex

sequences of synchronous thrusting resulting in the sub-

Atlas zone in the Pliocene and the deposition of the earliest

terrigenous and lacustrine sediments with numerous

angular unconformities (Teson et al. 2006).

Previous works in the study area have suggested a

decrease in shortening from the east to the west; e.g.,

Missenard (2006) computed shortening of 24% for Midelt–

Errachidia in the east, 18% for Imilchil in the center and

15% for the Demnate sections in the west.

Geology and seismotectonic setting

The Central High Atlas formation by lithospheric short-

ening of about 20 km, between the Middle Atlas in the

north and the Anti-Atlas in the south, resulted in folding

and faulting of the Jurassic and Cretaceous rock sequences

(Missenard 2006). Almost all the faults were reactivated as

reverse faults during the Cenozoic compression (Fig. 2).

Though less frequent, the ductile deformations are also

observed in the southern limits of the High Atlas complex.

In the present study, which concentrates on the Central

High Atlas and Ouarzazate foreland basin, four geody-

namic domains are distinguished (Figs. 1, 2):

(1) the Central High-Atlas domain, constituted by the

Paleozoic basement and its thick Mesozoic cover

([2,000 m), is characterized by horst and graben

systems. It is limited by the North Atlas fault system

(NAF) in the north and the South Atlas fault system

(SAF) in the south (Fig. 3). This domain was

developed as a subsiding basin, where depocenters

are clearly individualized. The Jurassic sedimentation

was controlled by both local tectonic and global

eustatic factors (Igmoullan et al. 2001),

(2) the sub-Atlasic domain constitutes the southern border

of the High Atlas range. It is formed by hills and highly

deformed rocks that overthrust the Neogene and

Quaternary deposits to the south. Since the Late

Cretaceous, the tectonic gravity-related High Atlas

uprising (Zylka and Jacobshagen 1986; Laville et al.

1977) has become active with fault ramps dipping

between 15� and 20� and shallow detachments formed

above the ramps of deep-seated thrusts in the intra-

basement rocks. This is consistent with the compressive

deformation of the younger rocks during the Hercynian

and even the Pan-African Orogeny (Benammi et al.

2005). A detailed analysis of the relationship between

tectonics and sedimentation (cross-cutting relation-

ships, unconformities), complemented by the available

dating of the sedimentary formation, suggests that the

main shortening activity in the sub-Atlasic thrust belt

spanned virtually, continuously, from the Oligocene to

the Pliocene, and continued at a lower rate up to the

recent times (Teson and Teixell 2006),

(3) the Ouarzazate basin domain is the most easterly

depression between the High Atlas and the Anti Atlas

(Fig. 1). It is limited by the South Atlas fault system

(SAF) in the north and by the Precambrian outcrops

in the south. Its origin is related to the tectonic

structuring during the Atlasic rifting some 700 Ma bp

(Villeneuve and Dallmeyer 1987; Tuduri 2005). It

extends for roughly 180 km from the eastern side of

the Siroua Plateau to the outlet of the Todra canyon

(5�30W), with a maximum width of some 30 km and

altitudes ranging between 1,150 m at Ouarzazate and

1,600 m at its northern edge at the contact with the

High Atlas. The center of the basin, filled by Neogene

and Quaternary sediments (Fig. 2), forms a tabular

plain cut by rivers (Agoussine et al. 2004; El Harfi

1994; Serbier et al. 2006),

Seismicity Central High Atlas, Morocco 635

123

(4) the Anti-Atlas, representing the stable domain, is

formed by Precambrian and Palaeozoic basement

rock. This, gently, sloping basement exhibits a

conspicuous onlap geometry of the sedimentary

deposits from the Eocene to the Miocene (Jossen

and Filali Moutei 1988). It is considered to be a

Palaeozoic basin that evolved in a mountain range.

The direction of shortening, which varies from NW-

SE to N–S and at times to NE-SW, has given rise to

large scale structural features, which extend over

distances between 100 m and 10 km (Burkhard et al.

2006).

Major tectonic structures

The Atlasic complex corresponds to an intra-continental

mountain system which developed grabens, horsts, and

faults which were superimposed on older structures. The

fault systems which influence the geodynamic evolution of

the Central High Atlas and Ouarzazate basins can be

considered in four main groups (Ait Brahim 1990; Zouine

1993; Benammi et al. 2005):

(a) the NE-SW (N040�–50�) fault set, the most frequent

of the structural trends, was developed at the end of

the Cretaceous (Fig. 4). Almost all the faults in this

set were reactivated as reverse faults with a left-lateral

strike slip component and generally displaced the

main anticline ridges. The N50 and N70 faults which

dominated the Mesozoic evolution of the Central

High Atlas were reactivated during the Neogene and

Quaternary,

(b) the transverse NW-SE and NNW-SSE (N110–120)

fault set evolved as reverse thrusts with a left-lateral

strike slip component. It is made up of the Demnate

fault zone (ZDF), Imhiwacch-Tawrirt-Aghbala fault

(ITA) and Bertat fault zone (ZFB) in the Central High

Atlas and the Tensift fault network in the Ouarzazate

(Ait Brahim 1990),

(c) the E–W fault set in the Central High Atlas is

inherited from the Hercynian structures. These faults

were the main influence in the Alpine evolution of the

Fig. 2 Cross-section across the axial zone of the High Atlas, the Ouarzasate basin and the Anti-Atlas showing the change in the topography and

structural styles of the four domains of the study area (From Jossen and Filali Moutei 1988)


123

High Atlas range which evolved as reverse faults with

a right-lateral strike slip component (Beauchamp

et al. 1999). They follow the northern border of the

Central High Atlas from Arbala to Midelt (Ait Brahim

1990), in contact with the High-Moulouya basin,

(d) the N–S fault set is less significant in the Central High

Atlas and is represented by neo-formed faults such as

the Zat and Rdat. These faults cross all the morpho-

logical/structural domains and hence indiscriminately

interrupt all the pre-existing tectonic units. Ait

Brahim (1990) reports that in the Haouz basin, these

faults influenced the continued uprising of the Atlas

Mountains during the Quaternary.

In the High Atlas, the anticlines are tight and sepa-

rated by large recumbent synclines, often associated with

faults and/or magmatic intrusions. Owing to the Atlasic

compression, the NE-SW and ENE-WSW faults corre-

spond essentially to reverse faults and thrusts with a

significant left-lateral strike slip component. The trans-

verse NNW-SSE and NW-SE faults, oblique to the

general direction of the range, are right-lateral strike slip

faults (Benammi et al. 2005). Within the sub-Atlas zone,

the structures are characterized by folds and thrusts

detached at two different levels. In the internal (northern)

part, the detachment is located in the Triassic rocks,

whereas in the external part the detachment level is

Fig. 3 Major structural features of the Central High Atlas and spatial

distribution of epicenters, according to the magnitudes of the

earthquakes. M.St. fault: morpho-structural fault, PA preferential

alignment according to magnitudes between 4 and 5. ITA Imhiwach-

Tawrirt-Aghbala fault, SAF South Atlas fault system, NAF North

Atlas fault system, ZDF Demnat fault zone, AF Amekchoud fault


123

located within upper Cretaceous rocks (Teson and Teixell

2006).

On the northern border of the High Atlas, the seismic

profiles show overlapping structures converging toward the

north. On the southern border, the sub-Atlasic zone is

strongly deformed by tangential tectonics expressed by the

thrusting of Meso-cenozoic deposits over the Neogene

Ouarzazate basin (Benammi et al. 2005).

Ouarzazate basin

The Ouarzazate basin is a foreland basin developed

along the southern front of the Central High Atlas. It

corresponds to an asymmetrical depression whose north-

ern limit is more readily recognizable than the southern

one. In fact, the northern limit is materialized by the

WSW-ENE striking South Atlas fault zone. The strike of

this major tectonic feature swings NE-SW along the

northern boundary of the Siroua Plateau. Further to the

south, as in the Souss basin, the South Atlas fault zone

corresponds overall to a south-verging reverse fault

associated with a south-facing monoclinal fold. The

southern border of this basin, however, is marked by a

gradual basinward down-dropping of the basement along

the Anti-Atlas Mountains. As a consequence, obvious

onlap geometry of sedimentary deposits developed from

the Eocene to the Miocene.

Fig. 4 Seismic activity in the Central High Atlas and Ouarzazate

basin superimposed on the geological structural map with the seismic

events plotted according to depth. PA observed preferential alignment

according to depth from 10 to 20 km (See Fig. 3 for the other legend

explanations)


123

During the Cenozoic, the Ouarzazate basin underwent a

phase of elastic deformation contemporaneous with a slow

rate of flexural subsidence. Indeed, the total Cenozoic

sedimentary fill in the basin does not exceed 1 km of

thickness. Serbier et al. (2006) analyzed the accumulative

deformation in the Neogene and Quaternary deposits and

inferred the smallest deformational potential throughout

geological time.

At present, this foreland subsidence appears to be no

longer active. Moreover, the basin is mainly dominated by

erosion, and sedimentation is restricted to alluvial depres-

sions where fan deposits occur. According to Serbier et al.

(2006), the shortening amounts to about 0.4 mm/year in the

basin as a whole with a slip rate in the order of 0.1 mm/

year on a fault ramp associated with the Amekchoud

anticline. As a result of the slip accumulation on this fault,

Quaternary alluvium was deposited (Serbier et al. 2006).

Seismicity in the study area from 1900 to 2007

The Central High Atlas and Ouarzazate basin have been the

subject of several geotectonic studies, e.g., Ait Brahim

(1990), Teixell et al. (2003), Benammi et al. (2005),

Burkhard et al. (2006), Missenard (2006) and Serbier et al.

(2006). According to these studies, the deformation during

the Hercynian involved rocks as deep as the crust and the

lithosphere and occurred in bands of several hundreds of

kilometers thick. The available geotectonic data, combined

with recent 2D seismic profiles, have allowed an

improvement of the knowledge of the seismotectonics in

the study area.

For this study, the seismic data was extracted from a

variety of sources including the Moroccan catalogue

established by Cherkaoui (1988, 1991) and Ibenbrahim

et al. (2004), and the global catalogue for the main

Moroccan earthquakes from 1045 to 2005 prepared by

Pelaez et al. (2007). Overall, these catalogs cover the

period from 1045 to 2007.

The seismic activity is distributed over the whole area of

the Central High Atlas and Ouarzazate basin (Figs. 3, 4),

decreasing from the east to the west and with a more dense

concentration in the vicinity of strike-slip and thrust faults.

From the spatial distribution of the epicenters, the seismic

activity is seen to be denser in the Central High Atlas than

in the Ouarzazate basin (Fig. 4). From the magnitudes and

density of the spatial distribution of seismic events, the

seismicity of the study area (Fig. 3) has been characterised

as follows:

(1) Throughout the Central High Atlas domain, where

strike-slip and thrust faulting occurs, the seismic

events are generally of low-magnitude. However,

some seismic events with magnitudes between 4 and

5 were seen to follow the more frequent 3–4

magnitude events. Despite the seeming diffuseness

in seismicity, the epicenters of events with magni-

tudes between 4 and 5 in the eastern Central High

Atlas appear to be aligned along a structural feature

which probably corresponds to the Imilchil fault.

(2) Characteristic zone: earthquakes with magnitudes of

between 4 and 5 in the High Atlas generally

propagate in an NNW-SSE direction. The epicenter

spatial distribution according to depth (Fig. 4) is

consistent with this alignment, to which the NNW-

SSE Imilchil transverse fault (ITA) is the closest.

Importantly, this alignment is coincident with the

junction zone between the Middle-Atlas and the

High-Atlas. From a structural point of view, the North

and South Atlas fault systems and the observed fault

segments in the Imilchil area are either interrupted or

offset by this alignment; the shift appears clearly

there. Along this junction, the Central High Atlas

seems to be mechanically de-coupled from its oriental

part.

(a) In the Ouarzazate basin, where shallow faults are not

well developed (Fig. 2), the sedimentary cover con-

sists of geological layers ranging in age from

Neogene to Quaternary. Seismicity in the basin is

also variable with 2–3 magnitude events most com-

mon, although magnitude 3–4 events also occur

frequently in the southern and western reaches of

the basin. In general the basin seismicity becomes

stronger at its northern and southern limits. On the

west of the Ouarzazate basin, splays of NW-SE and

other faults form a sort of local tectonic junction. The

seismic activity is relatively denser here and higher; a

5.3 magnitude has been recorded.

(b) Figure 5 shows the seismic events based on their

focal depths. It can be seen that 57.34% of the events

occur at depths of\10 km, 28% at depths between 10

and 20 km and only 9.32% at depths between 20 and

40 km, i.e.,[94% of the events are at shallow depths

in the Ouarzazate basin sedimentary cover and in the

overlapping structures in the Central High Atlas.

Importantly only 4.36% of events have epicentres at

[70 km; in each case in the Imilchil area (Fig. 4) and

probably related to seismic activity on the deep fault

ramps described by Giese and Jacobshagen (1992).

As noted above, the maximum magnitude recorded in

the study area was 5.3 Mw and events with magnitudes\4

account for 95% of the activity. Although this might sug-

gest a weak to moderate seismicity, the proximity of the

Ouarzazate basin to the major tectonic structures of the

Central High Atlas means, it is an important seismogenic


123

source zone as these major tectonic structures are also able

to generate destructive seismic events. The most recent

palaeo-seismological studies carried out by Serbier et al.

(2006) have allowed the characterization of the Amekc-

houd fault ramp near the cities of Ouarzazate and Skoura,

and indicate earthquakes with magnitudes as large as 6.4

could occur on this ramp.

Impact on the risk management in the Ouarzazate

region

Over the last few decades many parts of Morocco have

shown the potential to generate earthquakes with a magni-

tude of 8 or more. Although the recorded seismic activity is

not very significant in the Ouarzazate area at the foot of the

High Atlas, the results of the present study cannot preclude

the possibility of destructive seismic events occurring. In

addition to its 40,000 inhabitants, very ancient historical

buildings, numerous hotels of high-standard and a newly

built international resort, the area hosts one of the most

important cinema industries. Moreover, as the revenue from

tourism is expected to be significantly higher in the

upcoming years, the Moroccan government has imple-

mented various development plans, recognizing that natural

hazards, such as earthquakes, could be a major challenge to

a sustainable development of the study area.

The presence of buildings of all types from mud build-

ings to reinforced cement concrete framed structures and

steel construction and evidence of improper and low-

quality construction practices makes the Ouarzazate area

vulnerable even against average earthquakes. This study is

a first step toward a comprehensive evaluation of the

seismic hazard analysis in the area such that earthquake

resistant design and appropriate seismic safety assessments

can be properly taken into account in the Moroccan Code

for Paraseismic Construction (RPS 2000 modified and

Revised RPS2010).

Conclusion

The Moroccan Atlasic domain is affected by many local

and regional tectonic structures. This work, which inte-

grates the most recent seismic data, was undertaken in an

attempt to analyze and interpret the seismic activity in the

Central High Atlas and the Ouarzazate basin.

It has been shown that whilst the seismicity is hetero-

geneous and diffuse over the whole study area, the distri-

bution of seismic activity is relatively denser in the Central

High Atlas where deeper and stronger magnitude earth-

quakes have occurred. Elsewhere, the seismicity, as

induced by the tectonic activity in the region, is moderate

to weak with earthquakes generally below magnitude 4 and

the largest having a magnitude of 5.3.

Based on an analysis of earthquake magnitudes and

depths, there is a relatively high density of events around

the tectonic structures of the South Atlas Fault System, the

Imhiwach-Tawrirt-Aghbala fault and the Amekchoud fault.

To the west of the Ouarzazate basin, the South Atlas

Fault System forms a splay with NW-SE and other faults,

creating a local ‘‘tectonic junction’’ or seismic focal area.

The Amekchoud fault ramp extends for [10 km and is

close to the cities of Ouarzazate and Skoura. It is considered

that this fault ramp has the potential to generate seismic

events with magnitudes as high as 6.4 Mw which must be

taken into account in the relevant construction codes.

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Recent seismicity of Central High Atlas and Ouarzazate basin (Morocco

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