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Tectonophysics 367 (2003) 187–201

Seismic tomography from local crustal earthquakes beneath eastern

Rif Mountains of Morocco

Inmaculada Serranoa,b,*, Dapeng Zhaoa, Jose Moralesb,c, Federico Torcalb,d

aDepartment of Earth Sciences, Ehime University, Bunkyo-cho 2-5, Matsuyama 790-8577, Japanb Instituto Andaluz de Geofısica, Universidad de Granada, Granada 18071, Spain

cDepartamento de Fisica Teorica y del Cosmos, Facultad de Ciencias, Universidad de Granada, Granada, SpaindDepartamento de Ciencias Ambientales, Universidad Pablo de Olavide, Km. 1, Sevilla 41013, Spain

Received 1 November 2001; accepted 13 March 2003

Abstract

We applied a tomographic method to image an aseismic strike–slip fault in North Morocco and found that the occurrence of

earthquakes is not only controlled by the state of tectonic stress but also by material heterogeneity in the crust. We have

constructed an integrated model of seismic, electric, magnetic and heat flow properties across northeastern Morocco primarily

based on a tomography inversion of local earthquake arrival times. The seismic images obtained show a pronounced low-

velocity zone at 5 km depth parallels to the Nekor fault, coinciding with an anomalously high conductive and low gravity

structure, which is interpreted as a fault gouge zone and/or a fluid-filled subsurface rock matrix. Below 10 km depth, a weak

positive velocity zone indicates that the fault gouge is stable. The seismicity and the seismic velocity results for the Al-

Hoceimas region show that the concentrations of earthquakes are confined in the high velocity area. This anomaly is interpreted

to be a brittle and competent layer of the upper crust that sustains seismogenic stress. On the eastern coast line of Morocco, we

infer that a high density, high velocity body exists in the shallowest layers of the upper crust, probably formed by Miocene

volcanic rocks.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Strike–slip fault; Gouge zone; Seismicity; Low-velocity zone; Tomography; Alboran Sea

1. Introduction

During recent years, different seismic tomo-

graphic studies have been performed to explore

the crustal–mantle structure of the Betic–Rif Cor-

0040-1951/03/$ - see front matter D 2003 Elsevier Science B.V. All right

doi:10.1016/S0040-1951(03)00100-8

* Corresponding author. Instituto Andaluz de Geofısica, Uni-

versidad de Granada, Calle del Observatorio, n. 1 Campus

Universitario de Cartuja, Granada 18071, Spain. Tel.: +34-

958248912; fax: +34-958160907.

E-mail address: [email protected] (I. Serrano).

dilleras, using local, regional and teleseismic events.

However, the lack of seismic stations in the Alboran

Sea and North Morocco has frequently hindered the

achievement of enough resolution at the seismic

images to explain the accurate meaning of the

strong velocity anomalies obtained in the region,

and correlate them with some of the most important

geological structures, such as faults or volcanic

outcrops. In this paper, we show the results of a

seismic tomography study based on local earth-

quakes in North Morocco. We obtain higher reso-

s reserved.

I. Serrano et al. / Tectonophysics 367 (2003) 187–201188

lution in some small areas than in previous studies.

We have compared the seismic images obtained

with the geological studies, aeromagnetic surveys

and the most important electrical conductivity and

Bouguer gravity anomalies obtained in the same

area during the last years. Likewise, the relationship

between the seismic velocity and the seismicity has

been investigated from the seismological observa-

tions of this and previous studies, in order to know

better the origin of the earthquakes in this region.

1.1. Geological setting

The present study area is a part of the complex Rif

chain, which runs along the northern coast of

Morocco forming the southern branch of the Betic–

Rif arc (Fig. 1). This arc is the westernmost termi-

nation of the perimediterranean Alpine chains and can

be considered as the result of deformation of the small

Alboran block between northwest Africa and Iberia

since the early Tertiary. As a consequence of the

Fig. 1. (A) Tectonic sketch of the Betic–Rif Cordilleras on the upper left s

of North Morocco, the geology is from Frizon de Lamotte et al. (1991),

Gibraltar. The geological legend is shown on the upper right side of the f

NNW–SSE convergence between Africa and Iberia,

this block is being expulsed to the west–southwest

(Rebaı et al., 1992). The Alboran block is delimited to

the north by the Cadiz–Alicante fault in Spain and to

the southeast by the Nekor fault in Morocco. The

relative motion of the Alboran block took place first

(before the late Miocene) along the transform zone,

which trends N80j near Temsamane, and only after

the late Miocene did slip occur along the Nekor fault

(Frizon de Lamotte, 1987). The westward motion of

the Alboran block stopped in the Pliocene, and at

present we are observing the development of N–S

trending normal faults with E–W extension (Morel,

1989; Aıt Brahim and Chotin, 1989).

The Rif chain comprises three main zones: an

Internal Zone which belongs to the Alboran Domain,

an Intermediate Flysch Zone and External Zone which

is a foreland fold-and-thrust belt formed from the

Mesozoic and Cenozoic sedimentary cover of the

African margin. The Internal Rif and the eastern

Flysch Zone are separated by the left-lateral Jebha–

ide of the figure. (B) The bottom shows a simplified geological map

the faults are from Groupe de Reserche Neotectonique de l’Arc de

igure.

I. Serrano et al. / Tectonophysics 367 (2003) 187–201 189

Chrafate fault zone (JCFZ). To the east, the External

Rif units are crossed by a NE–SW trending linea-

ment, also a left-lateral fault (Cherkaoui, 1991). The

Nekor fault (Fig. 2) is one of the greatest disconti-

nuities in the Rif, as has been shown by geological

observations, and as such it could have played an

important role in the geodynamics of the region. At

present, the Nekor fault is imagined for a strongly

marked topography, although it is not possible to find

evidence of Quaternary slip and cannot be identified

on seismic reflection profiles (Gensous et al., 1986).

The northern Nekor fault region is fractured by dis-

tributed NE–SW to N–S striking high-angles faults

that exhibit apparent left-lateral strike–slip offsets and

by a conjugate NW–SE oriented group of more minor

faults with apparent right-lateral slip component (Aıt

Fig. 2. Geological map of the eastern Rif showing the boundaries betw

Pliocene sediments. 2: Volcanic rocks. 3: Internal Zones. 4: Flysch nappes.

nappe. 9: Senhadja nappe. 10: Olistostromes. From Frizon de Lamotte (1

Brahim et al., 1990; Fetah et al., 1987; Saadi et al.,

1984a).

1.2. Aeromagnetic survey, heat flow, electrical con-

ductivity and Bouguer gravity anomaly

A detailed aeromagnetic survey in northern Mo-

rocco was carried out in 1969, which outstanding

result was a narrow and elongated positive anomaly

trending NE–SW in the Temsamane–Nekor region

(Demnati, 1972). Afterwards, Michard et al. (1992)

made computations from these aeromagnetic data

including polar reduction to estimate the depths of

the magnetic sources and found it to be associated

with the Beni Malek serpentinised peridotite massif

that belongs to the External Zones of the Maghrebides

een the most important units. Legend: 1: Quaternary, Messinian–

5: Ketama unit. 6: Temsamane area. 7: African foreland. 8: Aknoul

985).


belt. The largest positive anomaly comprises a main

elongated part, extending from the high Nekor valley

to the Alboran Sea, and a superimposed part compris-

ing two relatively short-wave features. These authors

inferred that the main positive anomaly results from a

deep source, probably some peridotitic bodies. The

two subsidiary anomalies may be corresponding to a

peridotite outcrop and lens of ultrabasic rocks at

shallow depth under the Quaternary deposits. The

main conclusion is that the source of the serpentinite

clasts extended roughly parallel to the NE–SW trend

of the North African margin.

Menvielle and Rossignol (1982) made a thorough

examination of anomalous transient variations of the

geomagnetic field and investigated their tectonic

implications in northern Morocco. The main result

obtained was a conductive structure that electrically

connects the Atlantic Ocean with the Alboran Sea.

These authors suggest that the conductive structure

corresponds to a major tectonic crustal fault, the

Nekor fault. Based on electromagnetic data they

proposed the existence of a fault system which is

continuous from the Atlantic coast to the Mediterra-

nean coast, in accordance with the outline of the

Nekor fault in northeastern Morocco. Below the

Nekor fault, the conductivity anomaly is well individ-

ualized and may correspond to a region where the

conductivity is two times higher than background

conductivity. These authors concluded with the evi-

dence that this structure, highly conductive, can be

delineated by the eastern and southern boundaries of

the Alboran continental block.

Subsequently, Rimi et al. (1998), on the basis of oil

exploration data, define the thermal regime of the Rif

area. The result concerning the External Rif shows a

tendency of increasing HFD towards northeastern

Morocco and the Alboran Sea.

The Bouguer gravity map of the Alboran Sea and

surrounding mountain belts shows a negative anomaly

that extends on both sides of the Alboran Sea. More-

over, numerous reversed and nonreversed refraction

profiles have been carried out in the region (Hatzfeld

and Ben Sari, 1977; Makris et al., 1985). According to

these studies, the Moroccan crust is relatively thin

beneath the Rif (f 30 km) and reaches its maximum

thickness beneath the High Atlas (f 40 km). Seismic

refraction data and two-dimensional gravity modeling

in the Betic–Rif–Alboran Sea region show that the

crust becomes thin from the Internal Zones of the Rif

Chain to f 15–20 km beneath the central Alboran

Sea (Hatzfeld and the Working Group for Deep

Seismic Sounding, 1978; Banda and Ansorge, 1980;

Torne and Banda, 1992; Banda et al., 1993).

1.3. Seismological observations

The seismic framework of this region belongs to

the plate boundary between Eurasia and Africa. The

boundary extends from the Azores islands to the Strait

of Gibraltar and it continues to the east through

southern Spain, the Alboran Sea and northern

Morocco, Algeria and Tunisia (Buforn et al., 1995).

The seismicity in the studied area is characterized

by a continuous activity of moderate to low magni-

tude earthquakes (M < 5) and by larger events sepa-

rated by longer intervals of time. The latest large event

was the Al-Hoceima earthquake, of 26 May 1994

(Mw= 6.0), which caused great damage in an elon-

gated corridor trending NNE–SSW, where 80% of the

constructions were destroyed (El Alami et al., 1998).

The major cluster of earthquakes in Fig. 3, located in

northern Morocco, belongs to the sequence associated

with the 1994 Al-Hoceima earthquake. The Al-

Hoceima region has been a place of large earthquakes

during the past and is one of the most seismically

active areas in Morocco (Cherkaoui, 1991). Analyses

of seismological and seismic reflection profiles sug-

gest that seismic deformation in the Al-Hoceima

region is characterized by predominantly sinistral

strike–slip and normal faulting. Deformation occurs

over a distributed zone with individual faults having a

dominant NNE–SSW to N–S orientation (Calvert et

al., 1997).

The seismicity of the whole area has been exam-

ined methodically by several researches. Among the

most valuable works are: Hatzfeld and Ben Sari

(1977), Frogneux (1980), Vidal (1986), Aıt Brahim

et al. (1990), Cherkaoui et al. (1990), Buforn and

Udias (1991), Medina and Cherkaoui (1992), Asebry

et al. (1993), Hatzfeld et al. (1993), Medina (1995),

and Seber et al. (1996). A greater number of these

works are in accordance with the existence of an

important shallow microseismicity in the northern part

of the Nekor fault and a moderate seismicity in the

southern part. Although some authors find correla-

tions between the distribution of regional seismicity

Fig. 3. Distribution of seismic stations (triangles) used in this study.

The squares denote the hypocenters of earthquakes given by the

standard locational program. The circles denote the hypocenters of

earthquakes given by tomography program. The crosses denote the

hypocenter in which case the difference between depth from the

standard locational program and the tomography program is greater

than 5 km. Lines show the boundaries between the Alboran Sea and

Spain–Morocco.


and geological structures, we find this non-docu-

mented.

Calvert et al. (1997), using digital data collected by

the Moroccan seismological network, show the results

of relocated earthquakes in the Al-Hoceimas region.

They did not locate seismicity along the Nekor fault

and the Alboran Ridge during their study or previous

studies, and they concluded that this fact might be an

indication that slip is no longer occurring along these

faults, possibly due to the recent plate convergence

direction (NW–SE). It is also possible that the frac-

tured Al-Hoceima region may be allowing the transfer

of slip from the Alboran Ridge fault to the Nekor

fault. In this case, slip is not occurring aseismically

while strain may be accumulating along these major

faults.

2. Data selection

The area selected for the tomographic study is

located between 34j00VN and 37j48VN and from

2j00VW to 6j00VW, the focus of our attention is the

outlined small area in Fig. 1, comprising 156 km

(latitude)� 237 km (longitude). From the first step of

performing this study, we bore in mind to attain an

enough good resolution in this area to be able to

connect seismic velocity anomalies with significant

faults in Northern Morocco. Seismic stations and

earthquakes for the inversion have been selected

accurately.

We have used P wave arrival times from digital

and analogical data recorded by Seismic Networks

which belong to different institutions: Centre National

de Coordination et de Planification de la Recherche

Scientifique et Technique (CNCPRST) in Rabat

(Morocco), Physique du Globe at Mohamed V Uni-

versity (MOH V), in Rabat (Morocco), Instituto

Andaluz de Geofısica (IAG) in Granada (Spain),

Instituto Geografico Nacional (IGN) in Madrid

(Spain) and Real Observatorio de la Armada (ROA)

in San Fernando (Spain). In order to take advantage of

station distribution and provide additional data cover-

aged beneath the Rif mountains, it was necessary to

use data from analog recordings of Moroccan seismic

stations. Most of the seismic stations used are short-

period vertical component. Fig. 3 shows the distribu-

tion of seismic stations used in this study, strongly

unbalanced between Spain and Morocco.

We have selected from IGN, IAG and CNCPRST

databases exclusively the local earthquakes which

have been recorded at least in five stations placed in

the African continent and whose hypocentral locations

are located inside the area drawn in Fig. 3. In the

beginning we selected 1200 events, recorded during

the period 1993–1999, for which we have enough

accuracy of the hypocentral locations. We had to

remove the majority of these earthquakes because of

the error in first arrival times or the small number of

recording stations. Subsequently, we selected solely

Fig. 4. Different velocity models used in this study. The green line

indicates the velocitymodel used by CNR (ResearchNational Center,

Rabat, Morocco) and the blue line the model used by RSA

(Andalusian Seismic Network, Granada University, Spain). The

black line corresponds to the model obtained from the Wadati

Diagrams. The red line is the model used in this study. (For colours

see the online version of this paper, http://dx.doi.org/10.1016/S0040-

1951(03)00100-8).


those events whose hypocentral location deviation is

not further than 10 km from locations by different

networks. This procedure helps in the repicking of

some anomalous phases that allowed the authors to

determine the reading uncertainty and determination

of mindful catalog data set. Finally, we selected a total

of 221 earthquakes, whose hypocenters are located

using the method of Lienert et al. (1986). The depth of

the selected earthquakes ranges from 1 to 105 km. The

majority of the events have focal depths shallower

than 20 km and only the events located beneath the

Western Alboran Sea are deeper. However, because of

difficult availability of arrival times from seismic

stations placed in north Morocco, the reliability of

the locations is not very good. Finally we have a root

mean square (rms) arrival time residual calculated

from hypocenters of 0.75 s, and only the 51% of the

earthquakes selected have rms smaller than 0.8 s.

Next, the database was relocated using Zhao et al.’s

(1992) program, after removal of any arrival time

differences that exceeded a certain threshold value

(1.0 s). The events were relocated in six different

velocity models with Vp from 5.8 to 6.9 km/s in the

upper crust, from 6.4 to 6.8 km/s in the lower crust

and from 7.9 to 8.1 km/s in the upper mantle.

Comparing the hypocenters from different models

with the hypocenters obtained from the standard

location program (initial locations), we can observe

that the highest correlation coefficient (0.9) between

both databases corresponds to the model: 6.0, 6.5 and

8.1 km/s. Moreover, this last model shows the mini-

mum (lowest) rms arrival time residual in the program

of Zhao et al.’s (1992) program. The results are shown

in Fig. 3, where the circles denote the hypocenters of

earthquakes given by tomography program, the

squares indicate the hypocenters of earthquakes given

by the standard locational program and the crosses

mark the hypocenter in which case the difference

between depth from standard locational program and

the tomography program is greater than 5 km. We

attain a decrease of rms arrival time residual down to

0.43 s after the inversion.

The accuracy of time picking of the P digital

arrivals may be estimated in the most favourable cases

as F 0.01s. In the case of less impulsive arrivals and/

or poor signal-to-noise ratio, the accuracy is degraded,

but not more than 0.1s. For the analog recordings, the

accuracy is about F 0.5 s.

3. Methodology and resolution

We used the tomography method of Zhao et al.

(1992, 1994) to determine the 3-D P-wave velocity

structure. Although the conceptual approach of this

method derives from that of Aki and Lee (1976), it has

some additional features. The technique can deal with

the complex geometry of seismic velocity disconti-

nuities and it uses a 3-D ray tracing scheme to

compute travel times and ray paths. We set three-

dimensional grid nets independently for every layer to

express the three-dimensional velocity structure for

layers that are bounded by two adjacent discontinu-

ities. Velocities at grid points are taken to be unknown

parameters and a velocity at any point in the model is

calculated by linearly interpolating the velocities at

the grids surrounding that point.

We set up a 3-D grid in this study with a grid spacing

that changes from 18 to 45 km in the horizontal

direction and from 5 to 10 km in depth, hence four

layers of grid nodes are set up at 0, 5 and 10, 30 km

depth. The small grid spacing is around the Al-Hocei-

mas region and it is increasing with depth, from 5 km in

the two first layers to 10 km in the rest. The poor

seismic distribution in the area and the fact that nearly

http:\\dx.doi.org\10.1016\S0040-1951(03)00100-8

Fig. 5. (A) Trade-off curve between the variance of the solutions and travel time residuals for different damping, from 1 to 100. The selected

value for the final result is marked within the rectangle. (B) Trade-off curve between the variance of the solutions and travel time residuals for

different thresholds of earthquake location rms, from 0.2 to 2 s. The selected value for the final result is marked within the rectangle. (C) Trade-

off curve between the number of observations and travel time residuals for different thresholds of earthquake locational rms, from 0.2 to 2 s.



56% of earthquakes is shallower than 10 km determine

the grid space.

The selected initial velocity model for the tomo-

graphic inversion is derived from results of the velocity

independent methods (Wadati diagram technique). To

estimate Vp, Vs and Vp/Vs we have used earthquakes

located inside the studied area and recorded by at least

five stations with P and S picks at a wide range of

depths. We only selected earthquakes with Vp/Vs

between 1.67 and 1.79, in the estimations of the origin

time. Fig. 4 reveals the obtained results. However,

bearing in mind how few rays were used for this study,

we consider the aforementioned results of seismic

surveys carried out in the region. Moreover, some

inversions were conducted by using the same tomog-

raphy technique and data set, but by changing the P-

velocity gradually from 5.9 to 6.1 km/s in the upper

crust and from 6.4 to 6.8 km/s in the lower crust, with

an interval of 0.1 km/s. The P-velocity of 6.0 km/s for

upper crust and 6.5 km/s for lower crust give the

minimum rms residual. This model is in agreement

with the aforementioned model, which gives maximum

correlation coefficient between hypocenters from the

location program and hypocenters from the tomogra-

phy program. Finally, we have selected the final

velocity model shown in Fig. 4 (best fit). The P-wave

velocity (Vp) for the upper crust, lower crust and the

uppermost mantle is 6.0, 6.5 and 8.1 km/s, respectively.

Vp in the upper mantle has a vertical gradient of 0.005

km/s per km. Vp/Vs is set to be 1.7 in the initial model.

We used a realistic model with lateral depth

changes in the Moho and mid-crust discontinuity.

The Moho and mid-crust discontinuity geometries

were constructed by referring to the results of last

surveys carried out by different researchers in the

region (Galindo-Zaldivar et al., 1997, 1998; Soto et

al., 1996; Comas et al., 1999; Torne et al., 2000, etc.).

The mid-crust discontinuity depth ranges from 10 to

12 km, and the Moho depth ranges from 14 to 34 km.

Then we conducted a number of inversions for the

case when the Moho and the Conrad have lateral

depth variations and, also, for the case when they are

Fig. 6. (A) Fractional P-wave velocity perturbations (in percentage) at the

value of the inverted velocity at each layer. The depth of the layer is shown

and slow velocities, respectively. The velocity perturbation scale is shown

Alboran Sea and Morocco, the black lines are faults. (B) Distribution of the

at the first four depth layers. The hit count scale is shown at the bottom

flat. Next we compared the results for both of the

cases. We found that the general patterns of velocity

distributions in both cases are almost the same. There

are some differences in amplitudes of the velocity

anomalies for the upper and lower crust. The reason

can be that the selected area is very small and the

influence of the lateral depth variations on the final

results is not very important.

The damping parameter for the inversion was

selected based on an empirical approach (Eberhart-

Phillips, 1986). A number of inversions were run with

different damping values. Afterwards, the reduction in

travel time residual is compared to the variance of the

solutions and we draw a trade-off curve between them.

The selected value of the damping parameter is the one

which gives the optimal residual reduction and the

solution variance. Fig. 5A shows the trade-off curve

between the variance of the solutions and travel time

residuals for different damping, from 1 to 100. The

selected value for the final result is marked with the

circle. On the other hand, a threshold of 1.0 s in the

earthquake set with the damping previously selected

was found to significantly reduce the rms residual,

solving the inverse problem for 1800P-velocity param-

eters at the grid nodes with hit counts (number of rays

sampling a cell) greater than 10 (Fig. 5B and C).

Expecting a poor ray coverage, we analyze seismic

rays crossing the whole area to obtain an indication of

the resolution in the studied area. Regions that contain

many crossing rays are better resolved than regions

that contain a few rays. Fig. 6B shows the distribution

of the number of P wave rays passing through each

grid node, ‘‘hit counts’’, for the four layers. The most

important feature in the shallow layers is the pro-

nounced difference of ray path coverage throughout

the whole area. In this figure we can see that the

coverage is good down to a depth of 20 km in the Al-

Hoceimas and Nekor areas where we observe the

highest values of hit counts. For the Alboran Sea,

the absence of seismic stations results in very poor

control of its shallow velocity structure. The other

deeper layers display similar features in the ray cover-

first four depth layers. The velocity perturbation is from the mean

at the right lower part of each map. Green and red colours denote fast

at the bottom of the figure. The blue line is a boundary between the

number of P wave rays passing through each grid node (hit counts)

of the figure.


age except in the Western Alboran Sea where it is

possible to observe a progressive improvement of ray

coverage in the lower crust and upper mantle.

Afterwards, we applied a checkerboard resolution

test (Zhao et al., 1992, 1994) to examine the reso-

lution scale of the present data set; these tests are

usually used to assess resolution in tomographic

inversions. We assigned positive and negative velocity

anomalies of 3% to all the 3-D grid nodes. The results

of the checkerboard test are not reliable throughout

the area, but we can see that in the selected area the

resolution is enough for the first four layers (down to

30 km). In the same way, the synthetic tests show

good resolution at 5 km depth (Fig. 7), in the area

where the most important anomaly are imaged

(around Nekor fault).

Fig. 7. Results of synthetic tests at 5 km depth. The upper image

shows the input synthetic model. The inverted image is shown in the

lower figure.

4. Results and discussion

Traditionally, zones of fluids, areas with varying

degrees of brecciation, fracturing, high density bodies

and volcanic regions have been identified by electrical

methods and aeromagnetic, gravity and thermal sur-

veys. During the last two decades, one of the greatest

advances in geophysical methods have been made in

seismic tomography using local earthquakes, which

has proved to be one of the most powerful tools to

provide detailed three-dimensional velocity images.

We presented a local integrated model of seismic

velocity, electric, thermal, gravimetric and magnetic

properties through the eastern Rif northern of

Morocco. We have made an effort to correlate and

combine geophysical data from the results obtained by

different surveys carried out in the region to interpret

the main tectonic–geologic features in the studied

area. However, taking into account the lack of seismic

information and the poor resolution on some areas, we

have interpreted only the areas where we think our

information is robust.

4.1. Nekor fault

Themost robust feature imaged in the first layer, at 5

km depth, is the pronounced low-velocity region

trending NE–SW in Northern Morocco. In Fig. 6A,

we can see that this large velocity anomaly is imaged

along the southeast segment of the Nekor fault with

two minimum values, the lowest one (� 6%) placed

close to surface trace of the fault and the second

(� 4%), extending towards the NE, below the coast

line. Bearing in mind the dispersed seismicity in this

region and the important role played by the Nekor fault

in the recent tectonic evolution of Ibero–Mogrebi

region, it makes it absolutely necessary to think care-

fully about the physical conditions and properties of

the material involved in this low velocity zone. The

northern part of this fault constitutes the eastern edge of

the post-nappe, Tortonian–Messinian deposits of the

Boudinar basin. Southeast of the Nekor fault corre-

sponds to the Temsamane domain; west of the linea-

ment the Ketama domain is recognized. There is no

significant difference between the main stratigraphic

and lithologic successions of these two domains.

In agreement with Eberhart-Phillips et al. (1995),

the combination of fractures, breccia, clay, and cata-


clasites in the fault zone and the potential presence of

fluids at high pore pressures should produce large

contrasts in observable geophysical properties. Usu-

ally, low seismic velocities can be ascribed to severe

fracturing and cracking or fault gouge formation, for

example fault gouge is thought to lower the velocity

of rock by about 20%. The place where there is a

significant fault gouge and/or fracturing with con-

tained fluids, the fault zone will exhibit low velocity,

low resistivity and high attenuation. In our context,

the electrical conductive structure detected in relation

to the Nekor fault can be associated with the low

velocity zone and high-attenuation zone (Seber et al.,

1996). Moreover, the high surface heat flow and the

upheaval of the isotherms in the region (Rimi et al.,

1998) mean that the depth of the conductive structures

does not exceed a few tens of kilometres. In addition,

this velocity anomaly occupies the same relative

position that the NE–SW elongated part of negative

gravity values extending along SE boundary of the

Nekor fault. These features can be interpreted as a

fault gouge zone and/or a fluid-filled subsurface rock

matrix in the upper crust. Otherwise, it is clear that the

Nekor fault is a sinistral strike–slip fault that was

active during the Miocene (Leblanc and Olivier, 1984)

and also it is widely recognized that low-velocity

zones are a feature of some old or active strike–slip

zones (Stern and McBride, 1998).

Similarly, Gupta et al. (1996), from broad band

magnetotelluric soundings, reveal the presence of an

anomalously high conductivity zone in the Latur

region (India) at a shallow depth range of 6–10

km. Consistent with this result is the observation of

a low velocity layer at 7 to 10 km depth. They

interpret this high conductivity and low velocity

anomaly as a fluid filled fractured rock matrix.

Usually, in seismic tomography, the existence of a

low velocity layer in continental crust support the

presence of fluids.

However, from 10 km depth downwards (Fig. 6A),

a weak positive velocity zone is imaged along the SE

Nekor fault in accordance with results of previous

tomographic studies (Calvert et al., 2000). Geological

and geochemical evidence suggest that fault gouge is

stable only to depths of 8–12 km (Wang, 1984) and at

these depths cracks and fractures are annealed, a fact

that can explain the existence of a low velocity zone

only at shallow depth.

At present, we believe that the shallow low veloc-

ity zone along the SE Nekor fault can be associated

with the previous activity of this fault even though

that actually has no significant seismic activity or

tectonic evidence of Quaternary motion.

4.2. Seismic velocity in relation with Temsamane

magnetic anomaly

As previously mentioned, according to Michard et

al. (1992), the NE–SW trending magnetic anomaly in

the Eastern Rif (Temsamane anomaly) is in relation to

the ultramafic outcrop belonging to the Beni Malek

serpentinised peridotite massif. This positive magnetic

anomaly parallels the Nekor fault to the northwest.

From our results, we cannot observe anomalous

values in the area, but the perturbation values increase

in the NW side of the Nekor fault with regard to the

SE side (Fig. 6A). This fact does not imply the

existence of high-density bodies but could indicate

from 2.5 to 7.5 km depth, high density lithologies that

can influence the rise of average velocity of the

material involved. If the ultramafic bodies do not

have enough volume, it can be possible that although

they have an influence on the magnetic properties they

do not affect the seismic velocity too much. But most

likely these bodies are placed in the shallowest kilo-

metres of the upper crust, above the first slice where

we have results, and for this reason, their influence on

seismic velocity is very limited.

4.3. Al-Hoceimas region

In the first slice of Fig. 8, we can see shallow

seismic velocity variations imaged in the epicentral

area of the Al-Hoceimas seismic sequence. The slow-

est seismic anomaly (� 4%) in the area coincides with

the triangular lower Nekor basin, which is bounded to

the west and to the east by faults that have been active

since the late Miocene (Morel, 1989). The lower

Nekor basin, although it is occupying the same area

in space as the low velocity anomaly, at present is

filled with 400 m. of Quaternary sediments (Frizon de

Lamotte, 1982), which indicates these sedimentary

rocks are not in relation with the low velocity zone

obtained at 5 km depth.

Fig. 8 shows a S–N cross section where it is

possible to observe that the main cluster of earth-

Fig. 8. The upper part of the figure shows the fractional P-wave velocity perturbations in the Al-Hoceima region at the first depth layer (5 km).

The empty circles denote earthquakes, the blue line is a boundary between the Alboran Sea and Morocco, the black lines are faults. The middle

part of the figure shows NW–SE (A–AV) and SW–NE (B–BV) vertical cross sections of P wave velocity perturbations indicated in the upper

figure. The lower part is a vertical cross section along South–North direction (C–CV) showing the relationship between the earthquakes

(occurred within a width of 35 km along the profile) and P-wave velocity perturbations. LC: coastline.



quakes is located in the transition zone between fast

and slow velocity anomalies. Vp is low in the southern

area of the earthquake zone and is high in the northern

zone.

The association between P-wave velocity and seis-

micity in the upper crust has been a relevant topic

during the last 20 years. According to Lees and Malin

(1990), in many regions the strong correlation between

high-velocity anomalies and seismicity is clear, sug-

gesting that major earthquakes in strike–slip regimes

occur in zones of higher velocity and that represent

asperities along the fault, where stress is accumulating

before large earthquake rupture. In contrast, low

velocity regions may represent either higher degrees

of fracture, high fluid pressure, or higher temperatures

where deformation is more likely to be aseismic.

Compositional variations may be also responsible for

velocity variations (Zhao and Kanamori, 1993). In our

case, the high velocity zone could be associated with

the seismic activity, although the relation is not very

clear. However, seismic activity in this area ends at 20

km depth, which coincides with the high–low velocity

boundary (see Fig. 8). In the area where the Al-

Hoceimas earthquakes occurred, high velocity zone

attains the end at 20 km depth and the low velocity

zone begins at this depth and extends downwards. The

seismicity distribution and seismic velocity may indi-

cate that the most important concentrations of earth-

quakes are confined to the high velocity area. Indeed

we can interpret this higher velocity area to be a brittle

and competent part of the upper crust which sustains

seismogenic stress. The seismic rupture zones in the

upper crust around strike–slip fault zones, such as the

Al-Hoceimas faults, are generally characterized by

high velocities, and near the end of the rupture zones

low-velocity structures are usually seen.

4.4. High velocity below Kebdana Mountains

The eastern coast line of Morocco spreads with a

fast velocity anomaly ( + 3%) at 5 km depth, in

relation with a disperse shallow seismic activity.

However, it seems clearly the NE–SW trend of this

anomaly in the shallowest layer, and at greater depths

the positive seismic anomaly disappears. In the mag-

netic map of North Morocco (Demnati, 1972), we

observe a weak positive anomaly drawn in SE Melilla,

and the shape of the isolines may indicate the exis-

tence of a shallow body with a NE–SW trend.

Usually, the connection between a positive magnetic

and fast seismic anomalies at shallowest layers of the

upper crust can be in relation with high density

bodies, emplaced in the past at shallow layers of the

crust. Around this area, there are important neogene

volcanic outcrops and in the western part of the

studied area ultramafic outcrops also exist. On the

other hand, we observe the existence of a volcanic

center in this area (late Miocene potassic volcanism)

and a NE–SW regional trend of the volcanic outcrops

in whole Ibero–Mogrebi region. Supporting this

hypothesis, the heat flow density and thermal gradient

calculated for this area show some of the highest

values, 85 mW/m2 and 37 jC/km across the North

Morocco (Rimi et al., 1998). From geological, mag-

netic, thermal and seismic features we can infer that

from at least 2.5 to 7.5 km depth, a high density body

exists probably formed by mid Miocene calc-alkaline

or late Miocene potassic volcanic rocks.

4.5. Low velocity beneath 20 km depth

The most significant feature beneath 20 km depth

is a low seismic velocity obtained in the studied area.

This slow velocity anomaly has its lowest values

(� 5%) at 30 km depth, in the western area.

5. Conclusions

At 5 km depth, a pronounced NE–SW trending

low-velocity region can be associated with the elec-

trical conductive structure detected in relation to the

Nekor fault and coinciding with high-attenuation zone

(Seber et al., 1996). The high surface heat flow and

the upheaval of the isotherms in the region (Rimi et

al., 1998) mean that the depth of the conductive

structures does not exceed a few tens of kilometres.

Moreover, this velocity anomaly occupies the same

relative position as the NE–SW elongated part of

negative gravity values extending along the SE boun-

dary of the Nekor fault. These features could be

interpreted as a fault gouge zone and/or a fluid-filled

subsurface rock matrix in the upper crust. From 10 km

depth downwards, a weak positive velocity zone is

imaged along the same area in accordance with the

depth that fault gouge is stable (about 8–10 km).


In the Al-Hoceimas region, most of the earth-

quakes occur from the southern boundary of the low

velocity area southwards into the high velocity area.

In addition, the seismic activity in this area ceases at

20 km depth, which coincides with the high–low

velocity boundary. We can interpret this higher veloc-

ity area to be a brittle and competent part of the upper

crust which sustains seismogenic stress.

The eastern coast line of Morocco spreads with a

large fast velocity anomaly at 5 km depth trending

NE–SW, in relation to a disperse shallow seismic

activity. From geological, magnetic, thermal and seis-

mic features, we can infer that from at least 2.5 to 7.5

km depth, a high density body exists probably formed

by mid Miocene calc-alkaline or late Miocene potas-

sic volcanic rocks.

Finally, the most significant feature below 20 km

depth is the low seismic velocity imaged through the

studied area in relation to the occurrence of inter-

mediate earthquakes in the region. In the same way

that has been observed in the Betic ranges by Serrano

et al. (1998) and Morales et al. (1999), the intermedi-

ate-depth earthquakes are related to significative low-

velocity anomaly in the upper mantle.

Acknowledgements

This work has been supported by the Comision

Interministerial de Ciencia y Tecnologıa, proyecto

REN2001-2418-C04-04/RIES y FEDER and Grupo

de Investigacion RNM-104. The first author (I.

Serrano) thanks the Universidad de Granada (Spain)

and Ministerio de Educacion, Cultura y Deporte

(Direccion General de Universidades) for two post-

doctoral fellowships at Ehime University (Japan).

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