Tectonophysics 492 (2010) 88–98

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Tectonophysics

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Attenuation of coda waves in the Aswan Reservoir area, Egypt

H.H. Mohamed a, S. Mukhopadhyay b,⁎, J. Sharma c

a National Research Institute of Astronomy and Geophysics, Aswan Earthquake Research Center, P.O. Box 152, Aswan, Egyptb Department of Earth Sciences, IIT Roorkee, Roorkee-247667, Indiac 70/11-A, Purvawali Ganesh Chowk, Roorkee-247667, India

⁎ Corresponding author.E-mail addresses: haggaghm@yahoo.com (H.H. Moh

(S. Mukhopadhyay), jyotisharma99@rediffmail.com (J. S

0040-1951/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.tecto.2010.05.018

a b s t r a c t

a r t i c l e i n f o

Article history:Received 30 October 2009Received in revised form 18 May 2010Accepted 19 May 2010Available online 26 May 2010

Keywords:Aswan ReservoirEgyptCoda QLapse timeTurbidity

Coda attenuation characteristics of Aswan Reservoir area of Egypt were analyzed using data recorded by alocal earthquake network operated around the reservoir. 330 waveforms obtained from 28 earthquakesrecorded by a network of 13 stations were used for this analysis. Magnitude of these earthquakes variedbetween 1.4 and 2.5. The maximum epicentral distance and depth of focus of these earthquakes were 45 kmand 16 km respectively. Single back-scattering method was used for estimation of coda Q (Qc). The Q0 values(Qc at 1 Hz) vary between 54 and 100 and frequency dependence parameter “n” values vary between 1 and1.2 for lapse time varying between 15 s and 60 s. It is observed that coda Q (Qc) and related parameters aresimilar at similar lapse times to those observed for those for Koyna, India, where reservoir induced seismicityis also observed. For both regions these parameters are also similar to those observed for tectonically activeregions of the world, although Aswan is located in a moderately active region and Koyna is located in atectonically stable region. However, Qc does not increase uniformly with increasing lapse time, as is observedfor several parts of the world. Converting lapse time to depth/distance it is observed that Qc becomes loweror remains almost constant at around 70 to 90 km and 120 km depth/distance. This indicates presence ofmore attenuative material at those depth levels or distances compared to their immediate surroundings. It isproposed that this variation indicates presence of fluid filled fractures and/or partial melts at some depths/distance from the area of study. The Qc values are higher than those obtained for the Gulf of Suez and AlDabbab region of Egypt at distances greater than 300 km from the study area by other workers. The turbiditydecreases with depth in the study area.

amed), sagarfes@iitr.ernet.inharma).

ll rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Reservoir induced seismicity have been reported from many partsof the world (Gupta, 1992; Talwani, 1997) including areas of lowtectonic activity. One such example is that of Aswan Reservoirseismicity in Egypt. Many studies have been carried out on Aswanseismicity and its relation to the reservoir (Kebeasy et al., 1981; Awad,1994; Awad and Mizoue, 1995; Mekkawi et al., 2004; Awad andKwiatek, 2005; Haggag et al., 2009). Aswan High dam was built in1964 on river Nile in Egypt and the resultant reservoir extends∼350 km along the river. It is the second largest reservoir in theworld,the first being Lake Volta in Ghana formed in 1965 by the constructionof the Akosombo dam. A Mw 5.7 earthquake occurred near thewestern margin of the reservoir on an E–W trending fault, known asKalabsha fault, on 14 November 1981 and categorized as a reservoirinduced seismicity by Kebeasy et al. (1981). Ever since, earthquakeskeep on occurring in this region. Fig. 1a shows location of about 1000events that occurred during 2004 to 2007 (Haggag et al., 2009). A 13

station network was installed here to monitor the earthquakes(Fig. 1b). We have used coda waves of 28 events that occurred during2007–2009, mostly along the Kalabsha fault and towards east of the1981 earthquake, to evaluate the attenuation characteristics of thearea (Fig. 1b).

2. Geology and tectonics

Egypt experiences low to moderate amount of natural seismicitycaused by relative motion between African, Arabian and Eurasianplates. Most of this seismicity is confined to its boundary with the RedSea and is accompanied by high heat flow and swarm type of activityat a few places indicating possible subsurface magma movement(Abdel-Fattah et al., 2008).

The study area encompasses a region within 23.25°N to 24°N and32.3° to 33.05°E, i.e. the northern part of Aswan Reservoir. In this areamost of the faults trend either in the E–W or in the N–S direction(Fig. 1). Mostly normal and strike slip faults are present in this area.The most prominent active fault showing right lateral strike slipmovement is the E–W trending Kalabsha fault. It is also the longest(∼300 km) fault where the 1981 Aswan earthquake (Mw 5.7) oc-curred (Kebeasy et al., 1981; Awad, 1994). About 12 km north of the

Fig. 1. Figure showing the tectonic features, a) seismic network and earthquakes that occurred in the study region during 2004 to 2007 (after Haggag et al., 2009) and b) earthquakes(black dots), whose coda waves were used for this analysis and stations (triangles). The reservoir induced Aswan earthquake of 1981 is shown by a star.

89H.H. Mohamed et al. / Tectonophysics 492 (2010) 88–98

90 H.H. Mohamed et al. / Tectonophysics 492 (2010) 88–98

Kalabsha fault lies the E–W trending Seiyal fault (∼90 km) which isalso dominated by right lateral strike slip movement. These two E–Wright lateral strike slip faults encompass a graben structure in betweenthem (Awad, 1994; Awad and Kwiatek, 2005). The N–S faults arenearly parallel to the main course of the Aswan Reservoir. Amongthese, the prominent ones are the Khour El_Ramla fault lying closestto the E–W trend of the reservoir, the Kurkur, Alburqa, Seiyal, Gazalleand Abu Dirwa faults (Fig. 1). The Kurkur fault (∼35 km long) and theKhour El-Ramla fault (∼65 km long) are dominated by left lateralstrike slip motion, and are associated with small folds and steeplydipping beds (Issawi, 1978, 1982). The Gazalle fault (∼35 km long)and the Abu Dirwa fault (∼15 km long) are also dominated by leftlateral strike slip motion.

The area shows generally flat topography with relief varyingbetween 150 and 350 m. Sedimentary rocks of thickness ∼500 moverlies granitic/metamorphic basement. The sedimentary rocksconsist of the Nubian sandstone, Quaternary calcite and Nile deposits(Issawi, 1982). The igneous and metamorphic rocks are exposed inseveral localities along the western side of the Reservoir. The Nubiansandstone bed is locally folded in some locations into small anticlinesand synclines along the fault traces (Issawi, 1978). Geologically thearea is complex with crustal heterogeneity, presence of criss-crossingfault systems, small scale folding of sedimentary rocks and regionaluplift. The crustal thickness in Aswan area ranges from 30 km to35 km (Kebeasy et al., 1992).

3. Data analysis

Coda of 28 earthquakes that occurred mostly along the Kalabshafault was analyzed for estimation of coda Q (Qc) for the study area.These events were recorded by a 13 station network operating in thestudy area. 3-component broadband sensors (Trillium40 broad bandfrom Nanometrics) were used in the network. Data were recordeddigitally at 100 samples/s. The velocity response is flat over thefrequency range 0.5 to 50 Hz. The earthquakes occurred within thenetwork with maximum epicentral distance of 45 km and focal depth

Table 1Details of the events whose data are used for the coda Q analysis. Mag and Rms represent

Year Month Date Hour Minute Second

2007 3 24 8 18 40.762007 4 10 10 44 12.272007 4 12 10 39 58.372007 4 12 10 58 47.852007 4 12 13 25 35.952007 4 12 13 56 22.942007 4 12 14 17 41.242007 4 12 14 19 22.392007 4 12 15 16 7.122007 4 12 21 27 0.692007 4 30 9 34 31.262007 5 16 6 41 23.62007 7 23 8 23 17.912007 10 7 11 23 31.142007 10 15 11 49 16.762008 7 2 21 40 1.732008 7 15 15 32 1.522008 8 11 5 34 23.072008 10 23 21 28 28.812009 1 14 12 45 48.752009 1 24 21 32 32.522009 1 26 3 14 54.72009 2 4 8 16 27.092009 2 8 13 6 48.92009 2 11 0 30 19.42009 3 13 5 24 35.392009 3 18 11 31 17.452009 3 25 22 29 21.29

of 16 km. The local magnitudes of these events vary between 1.4 and2.5. The details of these events are given in Table 1.

The single back-scattering method of Aki and Chouet (1975) wasused for estimation of frequency and lapse time dependence of Qc forthe Aswan Reservoir area. Single-back-scattering hypothesis isconsidered to be a first order approximation of attenuation char-acteristics of the real earth and is widely used for routine deter-mination of quality factor of coda waves. For this reason we have usedit for a preliminary investigation of attenuation characteristics ofAswan Reservoir region. A brief description of the method is givenbelow.

Assuming that coda waves are composed of single back-scatteredwaves from randomly distributed heterogeneities, the coda amplitudecan be approximately expressed by the following formula (Aki andChouet, 1975)

A f ; tð Þ = A0 fð Þt−1 exp −πft = Qc½ � ð1Þ

where A( f,t) is the coda amplitude for a central frequency “f” over anarrow bandwidth signal at lapse time t, A0( f) is the coda sourcefactor at frequency f. This is used for the estimation of quality factor Qc

of coda waves representing the average attenuation properties of themedium for a given region. Rewriting (1) as

ln A f ; tð Þt½ � = ln A0 fð Þ½ �−πft =Qc ð2Þ

we can calculate Qc by applying a linear regression analysis betweenln[A(f,t)t] and time t for each frequency.

The Qc values were calculated using SEISAN software package(Havskov and Ottemoller, 2005). Qc was estimated at nine centralfrequencies (1.5, 2, 3, 4, 6, 8, 12, 16 and 18 Hz) so that its variationwith frequency could be studied reliably. All the seismograms wereband pass filtered at the chosen central frequencies. An increasingfrequency band was used (Table 2) with increasing central frequencyto avoid ringing and to take constant relative bandwidths for gettingequal amount of energy into each band as suggested by Havskov andOttemoller (2005) and Rautian and Khalturin (1978). In order to

magnitude of earthquakes and rms error in seconds respectively.

Latitude Longitude Depth Mag Rms

18.68 41.07 12 2.1 0.1244.20 13.01 0.1 2 0.1339.97 59.04 11.6 2.1 0.2658.80 48.83 9.2 2.1 0.1825.60 36.38 11.4 2.3 0.2656.38 23.88 11.8 2.3 0.2617.69 41.53 10.5 2.1 0.2419.37 22.71 11.5 2.3 0.2616.12 7.39 10.1 2.3 0.2927.01 1.14 8.7 1.9 0.0334.52 31.84 14.4 2 0.2241.39 24.29 15.9 2.3 0.2323.30 18.30 3.8 2 0.0923.52 31.53 16.3 2.1 0.1149.28 17.58 6.4 2.1 0.1940.03 2.40 2.7 2.3 0.1932.03 2.05 6.5 2.5 0.2234.38 23.64 3.6 2.1 0.1628.48 29.28 0.2 2.4 0.1445.81 49.51 2.3 1.4 0.2832.54 33.06 1.8 2.1 0.2214.91 54.95 5 2.3 0.1716.45 27.36 2.7 2.1 0.186.82 49.01 2.6 2.1 0.17

30.32 19.91 2.5 2.2 0.2124.59 35.80 7 2.1 0.1131.29 17.97 10 2.1 0.229.35 21.78 2.6 2.2 0.21

Table 2Table showing central frequency and bandwidth used for estimation of Qc.

Central frequency (Hz) Bandwidth (Hz)

1.5 12 1.343 24 2.666 48 5.3412 816 10.6618 12

Fig. 3. Plot of number of observations at a given lapse time.

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study the lapse time dependence of Qc, coda windows of length W of20 s were selected at lapse times tstart (i.e. the time of the codawindow after origin time) of 15 to 60 s with an interval of 5 s. Thelapse times were chosen such that they were always greater thantwice the S-wave travel time ts to avoid the data of the direct S waves(Rautian and Khalturin, 1978; Havskov and Ottemoller, 2005). Fig. 2shows an example of a seismogram whose data were used for thisanalysis.

The filtered seismic traces within these coda windows aresmoothed by calculating root mean square (rms) values of codaamplitudes of the filtered seismograms with a sliding window oflength equal to 5 cycles of each central frequency. The smoothingwindow is slid along the coda in steps of half the window length andthe value at each step in the same frequency band is evaluated. Theserms values constitute a smoother envelope of the coda.

The rms amplitude of the last 5 cycle length of lapse time windowis divided by noise data of the same length before the onset of the Pwave to calculate the signal to noise (S/N) ratio. The calculated Qc

values at given central frequencies were accepted only when cor-relation coefficients (C) for the best-fit line for the coda decay slopewith respect to lapse time were greater than 0.5 and signal to noiseratio (S/N) was greater than 2 for a given data set. The Qc values forsuch data set were averaged for each central frequency. Total numberof waveforms used for this analysis for each lapse time is shown inFig. 3. The results are discussed in the next section.

4. Results and discussion

Fig. 4 shows the plot of Qc versus frequency for different lapsetimes. The frequency relation parameter for Qc can be represented asQc=(Q0±ΔQ0)f(n±Δn), where Q0 is Qc at 1 Hz frequency, n is thefrequency relation parameter and ΔQ0 and Δn are the respectiveerrors. The (Q0±ΔQ0) and (n±Δn) values are shown in the plots forall the lapse times and given in Tables 3a and 3b. In order to checkhow Qc varies with lapse time Qc versus average lapse time (tc) valuesare plotted in Fig. 5, where tc= tstart+W/2, where W is the window

Fig. 2. Vertical component seismogram showing record of an earthquake that occurred on 16recorded at AHD. Maximum amplitude is 180491 in counts. Origin time (OT) and P- and S-awindow length 20 s is also shown with a box.

length. The minimum and maximum volumes sampled by the codadepend on tstart and tstart+W respectively. The average volumesampled can be assumed to be represented by the average lapse timetc (Havskov et al., 1989). A plot of Qc versus tc gives an idea about howthe expanding volume of material from which coda reaches a stationaffects Qc. For larger lapse times larger volumes are sampled by thecoda waves. Larger volume incorporates effect of deeper regions.Hence these variations in Qc reflect the variation of coda attenuationfrom larger volume/deeper zones of the study area. It is observed fromFig. 5 that Qc values in general increase with increasing lapse time.However, the rate of increase is not uniform. At some frequencies andlapse times the Qc values decrease with increasing lapse time orremain almost same. Such discrepancy is observed around 45 s and65 s average lapse times for most of the frequencies.

It has been observed by numerous seismologists that Qc varieswith lapse time and generally increases with increasing lapse time(Aki and Chouet, 1975; Sato, 1977, 1978; Roecker et al., 1982; Pulli,1984; Rhea, 1984; Jin and Aki, 1986; Phillips and Aki, 1986;Gagnepain-Beyneix, 1987; Havskov et al., 1989; Kvamme andHavskov, 1989; Del Pezzo et al., 1990, 1995; Ibañez et al., 1990;Nishigami et al., 1990; Akamatsu, 1991; Kanao and Ito, 1991; Kosuga,1991; Del Pezzo and Patané, 1992; Hellweg et al., 1992; Giampiccoloet al., 2002, 2004; Tuvè et al., 2006; Mukhopadhyay and Tyagi, 2007;Mukhopadhyay et al., 2008). They think that a possible explanation ofthe observed lapse time dependence of Qc could be attributed todepth-dependent variation in this parameter. Depth-dependentvariation in attenuation is indicative of variation in level ofheterogeneity with depth. Wennerberg (1993) showed that thistrend cannot be explained by multiple scattering model of Zeng(1991), but requires variation in intrinsic and/or scattering

May 2007 at 6 h 41 min and 23.6 s, located at 23.541°N, 32.616°E and 15.9 km depth andrrival times are shown with vertical arrows. Coda window for a lapse time of 30 s and

Fig. 4. Qc versus frequency plots for lapse time tstart of a) 15 s, b) 20 s, c) 25 s, d) 30 s, e) 35 s, f) 40 s, g) 45 s, h) 50 s, i) 55 s and j) 60 s. The Qc frequency relations and standarddeviations are also shown in each plot.

92 H.H. Mohamed et al. / Tectonophysics 492 (2010) 88–98

attenuation within the expanding volume involved in the codageneration process. Abubakirov and Gusev (1990) and Hoshiba(1991) too suggested the same. According to Roecker et al. (1982),Kvamme and Havskov (1989), Ibañez et al. (1990), Wennerberg

(1993), Del Pezzo et al. (1995), Woodgold (1994) and Akinci et al.(1994), the plausible cause of increase in Qc with lapse time isdecrease in intrinsic attenuation with depth. On the basis of specialtests Del Pezzo et al. (1990) came to the conclusion that the observed

Fig. 4 (continued).

93H.H. Mohamed et al. / Tectonophysics 492 (2010) 88–98

lapse time dependence of Qc can be explained by depth-dependentvariation in attenuation, although some effect is also due toinappropriate assumptions in the single back-scattering model. Thenon-uniform variation of Qc with lapse time for our case indicates thatin the study area, Qc does not increase uniformly with depth and mayeven be lower at certain depths compared to surrounding materials.

According to Woodgold (1994), the variation in Qc with lapse timecan be caused by any of several other factors such as (i) considerationof non-zero source–receiver distance with non-isotropic scattering,

Table 3aTable shows the Q0 and n values with standard deviations for different lapse times.

Lapse time tobeginning ofwindow(s)

Averagelapse timetc (s)

Averagedepth “h”(km)

Q0 ±ΔQ0 n value ±Δn

15 25 50 54 6 1.16 0.0720 30 58 54 3 1.2 0.0325 35 67 64 3 1.14 0.0230 40 77 70 1 1.06 0.0135 45 86 68 4 1.05 0.0440 50 95 73 3 1.09 0.0245 55 104 86 2 1.05 0.0150 60 113 95 7 1.02 0.0355 65 122 82 2 1.07 0.0160 70 129 100 4 1 0.02

(ii) use of a 2-D model instead of a 3-D model, (iii) use of a singlescattering model where multiple scattering is significant. When lapsetime is much larger than the S-wave travel time, the effect of the firstassumption is insignificant. Besides it is observed that when mediumis considered isotropic (Aki and Chouet, 1975; Pulli, 1984; Del Pezzoand Patané, 1992; Hellweg et al., 1992; Akinci et al., 1994; Del Pezzoet al., 1995; Gusev, 1995; Akinci and Eydo˘gan, 1996; Baskoutas,1996; Giampiccolo et al., 2002, 2004; Tuvè et al., 2006; Mukhopadhyayand Tyagi, 2007; Mukhopadhyay et al., 2008) Qc shows lapse timedependence. However, even when non-isotropic scattering is as-sumed (Abubakirov and Gusev, 1990; Gusev, 1995) Qc shows lapsetime dependence. Thus it is generally accepted that lapse time de-pendence of Qc represents variation of attenuation in the medium.Kopnichev (1977) and Gao et al. (1983) demonstrated that the effect

Table 3bQ0 and n values for Koyna region (after Gupta et al., 1998).

Lapse time (s) Q0 n

20 66 1.1630 96 1.0940 131 1.0450 148 1.0460 182 1.02

Fig. 5. a) to j) Qc versus average lapse time/depth plots for frequencies of 1, 1.5, 2, 3, 4, 6, 8, 12, 16 and 18 Hz frequencies. k) Plot of n value versus average lapse time/depth. Thestandard deviations are also shown in each plot.

94 H.H. Mohamed et al. / Tectonophysics 492 (2010) 88–98

Fig. 5 (continued).

95H.H. Mohamed et al. / Tectonophysics 492 (2010) 88–98

96 H.H. Mohamed et al. / Tectonophysics 492 (2010) 88–98

of multiple scattering becomes insignificant for local events with asmaller lapse time. However, the exact value of lapse time for whichmultiple scattering becomes significant depends on medium hetero-geneity and level of intrinsic attenuation. Sato (1988) showed thateven for large lapse times single scattering dominates over multiplescattering when the fractal dimension of number of scatterers in agiven region is less than 2.

The Q0 (i.e. Qc at 1 Hz) and n values (Tables 3a and 3b) for AswanReservoir are similar to those observed for Koyna reservoir in India atsimilar lapse times (Gupta et al., 1998). However, the variation of Q0

and n with depth for our study region is not linear. At certain averagelapse time ranges Q0 remains more or less constant or becomes lowerthan adjacent values (Fig. 5a). Such variations are also observed atother frequencies, as discussed in the next section. This could beindicative of presence of partial melts and/or fluid filled fractures atthese depth levels. The n values vary within a range of 1 to 1.2(Fig. 5k). Hence, its variation is not very strong. The average n value isaround 1.1, which is similar to that observed for the Koyna reservoirarea in India (Gupta et al., 1998) and also to those observed intectonically active and complex regions. The reason for similarity ofattenuation parameters for reservoir regions and tectonically activeregions is not understood.

4.1. Variation of Qc with depth

Sato (1978) and Pulli (1984) have shown that the scatterersresponsible for the generation of coda waves are generally assumedto be distributed over the surface area of an ellipsoid which can becalculated using the following formula:

x2

vtc2

� �2 +y2

vtc2

� �2− R2

4

! = 1 ð3Þ

where x and y are the surface co-ordinates, R is the hypocentraldistance, v is the velocity of S-wave and tc is the average lapse time,respectively. We have used S-wave velocity of 3.5 km/s and averagelapse time duration tc= tstart+W/2. The average depth of volume ofmedium from which coda wave generation would occur for differentlapse times is given by the formula h=√{(vtc/2)2−(R/2)2}+hav,where hav is the average hypocentral depth (Pulli, 1984; Havskovet al., 1989; Canas et al., 1995; Mukhopadhyay et al., 2008; Rahimi andHamzehloo, 2008). The values of ‘h’ for different tstart values are givenin Tables 3a and 3b. This could be considered as the average depth ofinvestigation related to a particular coda lapse time. However, itshould be remembered that this depth estimates are approximates asthe velocity in the real earth varies with depth. The average depth ofinvestigation increases with increasing average lapse time. Theaverage depth of investigation “h” versus Qc for different frequenciesare plotted in Fig. 5 to understand how attenuation of coda of differentfrequencies varies with average depth. It is observed that at allfrequencies Qc does not increase systematically with depth/lapse timeas is the case for Koyna reservoir region or many other regions of theworld. Between ∼70 km and ∼90 km Qc remains more or less con-stant at frequencies lower than about 4 Hz and at higher frequencies itbecomes lower than those at surrounding depths. Below 100 kmaverage depth Qc becomesmore or less constant at frequencies higherthan 8 Hz. At lower frequencies Qc is lower at ∼120 km average depththan that at surrounding depths. The reason for such variations couldbe that the media at ∼70 km to ∼90 km and ∼120 km average depthsare more attenuative compared to those immediately above andbelow them. Although Haggag et al. (2009) carried out travel-timetomography for this region using local earthquake data; they couldreveal 3-D variation in P- and S-wave velocity (Vp and Vsrespectively) and Vp/Vs structure only for the top 20 km. Presence

of fluid filled fractures at those depths could be a possible reason forsuch variation in attenuation characteristics with lapse time/depth.However, at such depths it does not seem possible that fluid filledfractures would be due to the presence of the reservoir. Alternatively,lateral variation in attenuation properties or presence of partiallymolten material at some depth could also explain such variation.Presence of molten rocks in Abu Dabbab, ∼300 km NE of the presentstudy area was proposed by Abdel-Fattah et al. (2008). This could alsoexplain the observed variation in Qc with lapse time/depth in theAswan Reservoir area, especially presence of partially molten regionsat some depths could be possible. However, they obtained very lowvalues of Qc for their study area as mentioned above, which is muchlower than those obtained by us.

Although most workers interpret variation of Qc with lapse time interms of its variation with depth, one has to remember that withincreasing lapse time coda from a larger volume of material reach astation. This could mean that coda from regions at a larger distancelaterally would also affect our observation. Hence, an alternateexplanation for above observation could be presence of more atten-uative medium at around 70 to 90 km and around 120 km distancefrom the region of study causes such variation of Qc with lapse time. Atpresent, we do not have any information about crustal structure ofregions at such distances from our network that can be comparedwith our results.

4.2. Comparison of result with global observations

We compare the Q0 and n values of our study region with that ofKoyna region in India, where reservoir induced seismicity is welldocumented (Gupta et al., 1998). Gupta et al. (1998) obtained coda Qfor 13 earthquakes of magnitude b3.0 occurring within epicentraldistance b60 km and focal depth b10 km. As these parameters aresimilar to those for our data set, wemake a comparison of the Q0 and nvalues of Koyna region (Table 3b) with that of ours (Table 3a). Weobserve that the Q0 values for Koyna region are slightly higher thanthat of Aswan region. This could be because Koyna is located in atectonically stable region, whereas Aswan is located in a moderatelyseismically active area. It is also noted that Q0 values increasessystematically for Koyna region but does not increase systematicallyfor Aswan region.

Abdel-Fattah et al. (2008) analyzed codaofmicroearthquakes of AbuDabbab region of eastern Sahara in Egypt, which lies approximately300 km NE of our study region. They obtained frequency relations forthat region as follows: Qc=(9±1)f(1.1±0.03), Qc=(16±1)f(1.0±0.03),Qc=(22±1)f(0.9±0.03) and Qc=(29±1)f(0.9±0.04) at 10, 20, 30, and40 s, respectively. It is observed that the n values are comparable tothose observed for Aswan Reservoir region, but the Q0 values are verylow compared to those obtained by us. However, it should be noted thatsuch low Q0 values are not observed for the lapse times mentionedabove for any other region of the world. However, it is to be noted thatMorsy and El Hefnawy (2004) obtained Q0 value varying between 18and 62 over an ∼150×200 km2 area around the Gulf of Suez which is∼300 to 500 km NE of our study area. This area encompasses the AbuDabbab region and the values of Q0 given in the two studies arecomparable. Our Q0 values are similar to those obtained for reservoirinduced seismicity of Koyna region (Gupta et al., 1998) and those fortectonically active regions as mentioned before, whereas those for AbuDabbab region are similar to those for volcanic regions. In the AbuDabbab region presence of magma is proposed based on heat flow dataand swarm like occurrence of microearthquakes and very low value ofQ0 as reported before (Abdel-Fattah et al., 2008).

A method of comparing Qc values from different regions of theworld was proposed by (Gusev, 1995). He plotted Qc versus lapse timefor central frequency 1.5 Hz, as well as frequency relation coefficient nversus lapse time for world wide data. He derived formulation for Qc

and n variationwith lapse time for an earthmodelwhere a turbid crust

97H.H. Mohamed et al. / Tectonophysics 492 (2010) 88–98

overlies a transparent mantle. The turbidity in the crust was assumedto decrease with depth by him. Turbidity represents randomness of aheterogeneous medium (Sato and Fehler, 1998). He considered caseswhere intrinsic attenuation were either zero or had some fixed valuefor a given frequency for the entire crust. According to his theoreticalmodel when intrinsic attenuation is zero then

Qc =2πftcN

ð4Þ

and when it is not zero then

1Qc

=N

2πftc+

1Qi

ð5Þ

where N is the power in the power-law decay of turbidity with depth,f is the frequency and Qi is the intrinsic Q. Gusev (1995) plotted theresultant theoretical curves along with the world wide data. We haveused his plots as base and superimposed the results of our analysis onthem.

In Fig. 6a and b variation of Qc with lapse time for central frequencyof 1.5 Hz and n versus lapse time for Aswan Reservoir area (plus sign)is superimposed on that for various other regions of the world.Average lapse time is taken to be tstart+W/2, as specified by Gusev

Fig. 6. Plot of a) Qc versus average lapse time for 1.5 Hz frequency and b) n value versusfrequency for Aswan Reservoir region superposed on that for a number of regionsaround the world. Data for all other regions are from Gusev (1995). Theoretical valuesfor these parameters obtained by Gusev (1995) are shown by dashed lines.

(1995). Qc versus lapse time for the present data set shows goodmatchwith the theoretically predicted values of Gusev (1995). Similarresults were also obtained by Mukhopadhyay et al. (2008) forChamoli region in Garwhal Himalayas, India. This shows that in theAswan Reservoir area turbidity decays fast, i.e. its decay is propor-tional to between 2nd and 3rd power of depth. On the other hand, thefrequency relation coefficient ‘n’ in the relation Qc=Q0f

n are sys-tematically too high compared to values theoretically estimated byGusev (1995). As proposed by Gusev (1995) from theoretical model-ling, this fact also indicates that turbidity decays rapidly with depth inthe study region.

5. Conclusions

The estimated Qc values for the Aswan Reservoir area show non-linear increase with lapse time. At some lapse times they decrease orremain more or less constant with increasing lapse time. This mightindicate thatQc does not increase uniformlywith depth, as is observedin most other places. However, with increasing lapse time the volumeof material from coda waves reach different stations also increases.The lateral extent of this volume also increases. This means that theobserved lapse time variation could also be due to lateral changes inthe crust. Based on the available results it is concluded that materialsat 70 to 90 km and around 120 km depth/distance from the study areaare more attenuative. This may indicate presence of fluid filled frac-tures and/or partially molten material at those places.

A comparison of Q0 and n values for the study area and that forKoyna reservoir region in India shows that Q0 values for AswanReservoir region for different lapse times are slightly lower than thosefor the Koyna reservoir region. This could be because Aswan lies in amoderately seismically active region, whereas Koyna lies in a tec-tonically stable region. The n values for both the region are com-parable. It is interesting to note that Q0 and n values for both theregions are similar to those for tectonically active regions of the world,although the reason for such similarity is not understood. Comparedto the more seismically active Al Dabbab region of Egypt Qc values inAswan region are higher. Qc versus lapse time plot for Aswan regionshows good match with the theoretically predicted values of Gusev(1995) but n value versus lapse time plots are systematically higherthan his theoretical predictions. This shows that turbidity decreaseswith depth in the study region.

Acknowledgements

We are thankful to the Director of NRIAG and the staff of Aswannetwork for their kind help and support. We are also thankful to Prof.J.R. Kayal for his help and support. Constructive suggestions by twounknown reviewers and Prof. Hans Thybo, Editor-in-Chief, helpedgreatly in improving the article.

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