Spatial variation of coda wave attenuation using aftershocks of the Al Hoceima earthquake of 24 February, 2004, Morocco

doi:10.4236/ns.2013.58A1009

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Vol.5, No.8A1, 72-77 (2013) Natural Science

http://dx.doi.org/10.4236/ns.2013.58A1009

Spatial variation of coda wave attenuation using

aftershocks of the Al Hoceima earthquake of 24

February, 2004, Morocco

Abderrahim Boulanouar1*, Lahcen El Moudnib1,2, Mimoun Harnafi2, Taj-Eddine Cherkaoui2,

Abdelaali Rahmouni1, Mohamed Boukalouch1, Jamal Sebbani1

1Physics Department, Faculty of Science, Mohammed V University-Agdal, Rabat, Morocco;

*Corresponding Author: [email protected]

2Earth Science Department, Scientific Institute, Mohammed V University-Agdal, Rabat, Morocco

Received 6 July 2013; revised 6 August 2013; accepted 13 August 2013

License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

On 24th February 2004 a significant earthquake

(Md = 6.4) occurred in the north of Morocco

causing great damage in the vicinity of Al Ho-

ceima region. This area is characterized by a

complex faulting system as a result of com-

pressional tectonic forces. Three short period

seismic stations are set in this area of interest

and recordings from these stations were used in

this study. In order to complete our knowledge

of attenuation, 60 local earthquakes are re-

corded a few days after the great earthquake

with magnitude Ml 2.6 - 5.0 to estimate seismic

attenuation. For this purpose, we applied the

single backscattering model of Aki & Chouet

1975 in the frequency range for 1 to 8 Hz. The

study of coda waves was limited to a relatively

short lapse time (20 Seco nds) in order to sample

the earth’s crust only. The values of Qc esti-

mated for all the three stations show a strong

frequency dependent relationship of the form Qc

= Q0fn, where Q0 is Qc at 1 Hz , and n represents

the degree of frequency dependence, and re-

flects the level of crustal heterogeneities to

varying degrees. The average frequency de-

pendent attenuation relationship has been ob-

tained which indicates that the attenuation is

high in this region. Finally to conclude our work,

the values of Q0 suggest that Al Hoceima area is

highly heterogeneous and the n parameter in-

dicates a meaning frequency dependence of Qc.

Keywords: Attenuation; Coda Waves; Single

Backscattering Model; Al Hoceima; Morocco;

Seismic; Earthquak e

1. INTRODUCTION

Al Hoceima is located among the most active seismic

zones in Morocco between the African and Eurasian

plates [1]. During the last 10 years, Al Hoceima has been

affected by two strong earthquakes [1,2], the first on

May 26th, 1994 (Mw = 6.0) and the later on February

24th, 2004 (Mw = 6.4). That’s why, the study of its seis-

micity is very important in order to establish the seismic

hazard assessment for this important area. Although this

is a very active seismic zone, little is known about the

seismic attenuation.

Attenuation, inverse of quality factor, is one of the

most fundamental proprieties of seismic waves from

which a lot of information about the earth can be de-

duced [3]. This parameter is an important factor for un-

derstanding the physical mechanism of seismic wave

attenuation in relation to the composition and physical

condition of the Earth’s interior and it is also an essen-

tial parameter for the quantitative prediction of strong

ground motion for the viewpoint of engineering seis-

mology [4]. This is also important for seismic hazard

measurement [4].

The attenuation can be described as the decay of the

seismic waves’ amplitude [5]. The single backscattering

model given by Aki and Chouet [5], has been used in this

study. This method is widely applied to studies in the

crustal structures in different parts in the world [3,6-11].

In the present study, the seismic attenuation was inves-

tigated in Al Hoceima region by analyzing a large num-

ber of short period local seismograms and comparing it

with other regions of the world [8]. The results of this

study are discussed and compared with previous results

A. Boulanouar et al. / Natural Science 5 (201 3) 72-7 7 73

and correlate with tectonic structures. These results may

be helpful for seismic risk and earthquake engineering

purposes.

2. TECTONIC SETTING OF THE

STUDIED AREA

The study area is located in the northern part of Rif

chain. The seismotectonic framework of the region is

characterized by generally diffuse seismicity, with clus-

ters of important earthquakes. This area has experienced

several destructive earthquakes such as the 26th May,

1994 and 24th February 2004 shockes. The latter pro-

duced several massive damage (about 600 deaths and

more than 2500 buildings destroyed) in Al Hoceima and

the surrounding areas.

Four structural units can be distinguished [2] within

this system (Figur e 1):

● Bokkoya regions, containing nappe of Palaeozoic

terranes and covering with Mesozoic-Cenozoic.

● The Ketama metamorphic unit, which consist of Cre-

taceous flysch and lime- stones.

● The middle Miocene Ras-Tarf andesites, which con-

sists of Volcanic rocks.

● The Tiziren unit, comprising Middle Jurassic to.

Early Cretaceous.

The study area is exploded by faults such as the Nekor

fault [2], which is the major transverse structure and The

Imzouren (NNW-SSE) and Trougout (N-S) normal faults

and The Jbel Hammam fault system (NNW-SSE).

3. DATA SET AND METHODS

For the estimation of the Coda waves quality factor, a

group of 60 earthquakes recorded in are used, with mag-

nitudes ranging from 2.6 to 5. These events were re-

corded during the period from 6 to 8 March 2004 around

the Al Hoceima Area by a three station network operat-

ing in the study area .Data were recorded digitally at 20

samples/sec. The details of these events are plotted in

Figure 2. The depths of these events are generally lower

than 14 Km. The seismograms (Example of seismograms

recorded by the network on Figure 3) having the signal

to noise ratio below 3 were discarded for obtaining reli-

able of quality factor values. For this study only the ver-

tical components of the seismograms have been used.

Recordings of Al Hoceima earthquakes have been

used to determine the attenuation of coda wave, from the

rate of coda amplitude decay.

In the present study, the coda wave Qc is estimated

following single backscattering model of Aki and Chouet

[3,5,9,10,12]. Coda waves of local earthquakes can be

considered as backscattered S and S waves generated by

numerous heterogeneities present randomly but uni-

formly in the crust and upper mantle [3]. From this

model the coda amplitude represented as:









,..exp..



ftC ftftQ



 (1)

where C(f) represent the coda source factor, Q the quality

Figure 1. Structural map of the Al Hoceima area and main faults [2].

A. Boulanouar et al. / Natural Science 5 (201 3) 72-7 7

Figure 2. Location map of earthquakes and stations used for this study [14].

Figure 3. Example of seismograms recorded by the network for the event of 07 October 2004, 04:29 hr.

Units of velocity are cm/sec. BHZ, vertical component; BHN, north-south component; BHE, east-west

component.

factor.

Taking the logarithm of Eq.1 gives:



ln, .ln.

fttc bt







 (2)

The quality factor is then determined by a least

squares fit of



ln, .

ftt





for validation of the model that the source of the earth-

quake and receiver are coincident.

Figure 4 shows an example of a coda Q plot for an

earthquake recorded of 3th March 2004 by HOC3 station

using SEISAN Software [13].





versus t for Eq.2. Accord-

ing to Rautian and Khalturin [11] the above relation Eq.2

is valid for lapse time greater than twice the S-wave

travel time for avoiding the data of the direct S-wave and

4. RESULTS AND DISCUSSION

The quality factor values are estimated filtering the

coda waves of 60 local earthquakes in frequency band

A. Boulanouar et al. / Natural Science 5 (201 3) 72-7 7 75

Figure 4. A codaq plot for an earthquake recorded on 3th March 2004 by HOC3 station [13].

123456789

100

150

200

250

300

350

400

450

500 A graph of the Average Q values against frequency

for the Al Hoceima events. Vertical error bars are shown

R=0,97

Average Q

Frequency (Hz)

Figure 5. Shows the distributions of Qc values with frequency

for coda window lengths 20 seconds.

centered at 1, 1.5, 2, 3, 4, 5, 6 and 8 for lapse time win-

dow length of 20 seconds.

The Qc measurements estimated from 20 sec coda

window length are about 79 at frequency 1 Hz and 385 at

4 Hz. The distributions of Qc values with frequency are

shown in Figure 5.

Tab le 1 shows the mean values of Qc at different cen-

tral frequencies. Given also in this table are the standard

deviation and the number of observations for each cen-

tral frequency. These are used for the calculation of Qc

the frequency-dependent relations.

It is observed from the general trend (Figure 5) that Qc

values follow a power law of the from Qc = Q0fn, where

Q0 is the quality factor at 1 Hz and n is the frequency

dependent coefficient.

The frequency dependence of Q relationship for the Al

Hoceima area:





0.786 0.053

8.461.07 .







(3)

The graph of log(Q) against ln(f) is plotted and the

following calculations are made. This is shown in the

Ta bl e 1 . Average quality factor, Qc and estimated standard de-

viation at different frequencies. In the column heading, S.D.

indicates the standard deviation and N is the number of obser-

vations made for each central frequencies.

Frequency (Hz) QC S.D N

1 79 44 12

1,5 106 52 13

2 158 72 18

3 232 77 16

4 270 101 16

5 300 96 20

6 340 119 17

8 385 131 10

Figure 6.

From the above results, it is observed that Qc values

obtained for the seismograms are high frequency de-

pendent (Figure 6). Both Qc values and frequency in-

crease simultaneously. The high frequency dependent

characteristics of the Qc values may be due to different

heterogeneity present in the propagating media [12]. This

study shows the lowest values of Q0 and the highest n

components in 20 s lapse time. Most studies show that

the value of quality factor on 1 Hz, Q0, is less than 200

for tectonically and seismically active regions [8] such as

Yunnan, China, Washington State, whereas larger have

been observed for inactive or stable regions such as New

England, South India and North Iberia (Figure 7). Thus

our area is tectonically and seismically active regions

such as Yunnan and China [3,8,9]. Low coda values for

this area (Q0 = 84.6) seem to be associated with tectoni-

cally and seismically active regions.

The values of the quality factor parameters (Q0 and n)

for each seismic station site are shown in Table 2. A

comparison between these values for the three station

sites can characterize the attenuation property in each

local seismic station site. In fact, we calculated Q0 in

each station separately to investigate the lateral variation

of upper lithosphere attenuation structure in our study

A. Boulanouar et al. / Natural Science 5 (201 3) 72-7 7

Figure 6. A graph of log(Q) against ln(f) for the Al

Hoceima events.

Figure 7. A detailed comparison of coda-Q studies for different

regions of the World [9].

area. The frequency dependent values range from 0.529

to 1.02 along the Al Hoceima area. The highest Q0 value

(133.18) is observed in Region where the station 1

(HOC1) is deployed while lowest Q0 (58.27) is observed

in Region HOC3 (Tab le 2). The mean Qc values calcu-

lated along the Al Hoceima, taking into account all the

data that followed the quality criteria is Qc = 133.18f0.529.

The difference in the quality factor values between

HOC1 seismic station site and other two station seismic

site is great which may be considered probably due to

real crustal differences in terms of coda Q (Figure 8).

This indicates that the average attenuation properties and

the scatters in the study area have different pattern [3].

5. CONCLUSIONS

We analyzed 60 aftershocks waveforms of the moder

ate earthquake, February 2004, recorded on three short

Table 2. The values of Q0 and n for the three seismic stations.

Stations Q0 n

HOC1 133.18 ± 1.11 0.529 ± 0.078

HOC2 77.73 ± 1.11 0.886 ± 0.083

HOC3 58.27 ± 1.03 1.02 ± 0.066

0123456789

100

150

200

250

300

350

400

450

500

550

Average Q

Frequency (Hz)

station HOC 1

Station H O C 2

Station H O C 3

Figure 8. Comparison of the Qc relations obtained in different

tectonic and different stations in the Al Hoceima region.

period seismic network deployed in Al Hoceima region

to study lateral variation of attenuation structure using

coda waves. The coda-Q (Qc) has been estimated for Al

Hoceima area using a single Backscattering model 1975

of S-coda envelopes for one lapse time (20 seconds), at 8

frequency bands with the central frequency in the range

of 1 Hz to 8 Hz. The estimated Qc values for the lapse

time 20 sec vary from 57 to 101 at 1 Hz and from 319 to

450 for 8 Hz which indicate that Qc value increases as

frequency increases. The results in the Table 1 described

clearly that Qc value is a function of frequency in this

region and showed a significant lateral variation between

the different seismic stations. And the average Qc fre-

quency relation is given as

for this region.





0.786 0.053

8.461.07 .







This work suggests that the quality factor values are

frequency dependent in the Al Hoceima area (Eq.2) and

the value of frequency dependencies is about n = 0.79,

which indicates a high degree of heterogeneity. Qc shows

a scatter which may be due to the different sampling re-

gions of coda waves during their propagation, indicating

possible lateral variations in the lithosphere in the study

area. The comparison of the low of quality factor on Al

Hoceima region indicates that the attenuation is higher

than other regions of the world.

6. ACKNOWLEDGEMENTS

We gratefully acknowledge the Scientific Institute of Rabat for pro-

A. Boulanouar et al. / Natural Science 5 (201 3) 72-7 7

[7] Havskov, J. (1989) Coda Q for the state of Washington.

Bulletin of the Seismological Society of America, 79,

1024-1038.

viding us with the data from Al Hoceima. The authors also wish to

thank Dr. Amin Esmail Khalili, Geology Department, Helwan Univer-

sity, for his valuable comments that improved significantly this work.

[8] Ma’hood, M. and Hamzehloo. H. (2009) Estimation of

coda wave attenuation in East Central Iran. Journal of

Seismology, 13, 125-139.

doi:10.1007/s10950-008-9130-2

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