Journal of African Earth Sciences 101 (2015) 177–185

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Journal of African Earth Sciences

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The northern Egyptian continental margin

http://dx.doi.org/10.1016/j.jafrearsci.2014.09.0091464-343X/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +20 (2) 2558 3887, +20 (2) 3388 5126; fax: +20 (2)2554 8020.

E-mail address: ahmedbadawy@Yahoo.com (A. Badawy).

Ahmed Badawy ⇑, Gad Mohamed, Khaled Omar, Walid FaridNational Research Institute of Astronomy and Geophysics (NRIAG), 11421 Helwan, Cairo, Egypt

a r t i c l e i n f o a b s t r a c t

Article history:Received 9 February 2014Received in revised form 9 September 2014Accepted 11 September 2014Available online 22 September 2014

Keywords:Source mechanismSource parametersContinental marginEgypt

Africa displays a variety of continental margin structures, tectonics and sedimentary records. The north-ern Egyptian continental margin represents the NE portion of the North African passive continental mar-gin. Economically, this region is of great importance as a very rich and productive hydrocarbon zone inEgypt. Moreover, it is characterized by remarkable tectonic setting accompanied by active tectonic pro-cesses from the old Tethys to recent Mediterranean. In this article, seismicity of the northern Egyptiancontinental margin has been re-evaluated for more than 100-years and the source parameters of threerecent earthquakes (October 2012, January 2013 and July 2013) have been estimated. Moment tensorinversions of 19th October 2012 and 17th January 2013 earthquakes reveal normal faulting mechanismwith strike–slip component having seismic moment of 3.5E16 N m and 4.3E15 N m respectively.

The operation of the Egyptian National Seismic Network (ENSN) since the end of 1997 has significantlyenhanced the old picture of earthquake activity across northern Egyptian continental margin whereas;the record-ability (annual rate) has changed from 2-events/year to 54-event/year before and after ENSNrespectively. The spatial distribution of earthquakes foci indicated that the activity tends to cluster atthree zones: Mediterranean Ridge (MR), Nile Cone (NC) and Eratosthenes Seamount (ERS). However,two seismic gaps are reported along Levant Basin (LEV) and Herodotus Basin (HER).

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The eastern Mediterranean Sea is characterized by high earth-quake activity (Fig. 1) and a complex tectonic setting which isnot fully understood yet. Several geodynamic models have beendeveloped to explain the processes in this region (Makris, 1976;Le Pichon et al., 1982; McKenzie, 1970, 1972; Badawy, 1996;Badawy and Horváth, 1999a,b). Still the subduction process southof Crete, crustal structure below the Mediterranean Ridge (MR)and deep structure of the North African passive margin remainpoorly understood. The geophysical data especially for the Africanmargin are limited to potential fields and some industrial reflectionseismic lines. From reflection seismic experiments (Chaumillonet al., 1996) at the southern edge of the Mediterranean Ridge(MR) a back-thrusting tectonic structure in the sediments wasidentified in the southward direction. This confirms the MR as anaccretionary complex (Le Pichon et al., 1982; Ryan et al., 1982;Mascle et al., 1995) but it is not yet clear if and how far thisback-thrusting reaches onto the African continental margin. Fur-

thermore, the question arises how far the extant compressionaffects a tectonization of the African continental margin.

To investigate these questions and better understanding thegeodynamic processes of the North Egyptian continental margin,the earthquake activities for more than 100-years has been re-evaluated. Source mechanism solutions for eight earthquakes havebeen investigated to shed some lights on the present-day stressregime affecting the north Egyptian continental margin. Sourceparameters and moment tensor of the recent felt and recordedearthquakes (October 2012, January 2013 and July 2013) have beenestimated.

The northern Egyptian continental margin has suffered fromhistorical and instrumental earthquakes (Maamoun et al., 1984;Ambraseys et al., 1994). Two historical earthquakes were widelynotable in 320 and 956 A.D. (Badawy, 1999). These events weredamaged numerous houses in Alexandria with maximum intensityVI on MSK scale (Ambraseys et al., 1994). Before the operation ofthe Egyptian National Seismological Network (ENSN) in 1997, themargin appears to be characterized by sparse activity and the onlyknown and significant earthquake to have occurred offshore ofAlexandria was 12 September, 1955 with surface magnitude of6.7 (Fig. 2). The total reported earthquakes along the northernEgyptian margin (31–34N, 25–35E) throughout 98-years were only225 (3-events with magnitude more than 5; 37-events from 3 to 5

178 A. Badawy et al. / Journal of African Earth Sciences 101 (2015) 177–185

and 185-events less than 3) events with an annual rate of 2-events/year.

By the installation of ENSN the old picture of the northern Egyp-tian margin has totally enhanced, whereas, the annual activity ratehas been jumped to 54-event/year with total recorded 864 (7-events with magnitude more than 5; 154-events from 3 to 5 and703-events less than 3) events within 16-years period. Almost ofearthquake activities tend to clustered at three zones (Fig. 2) theMediterranean Ridge (MR), Nile Cone (NC) and the EratosthenesSeamount (ERS). Along Levant Basin (LEV) and Herodotus Basin(HER) two seismic gaps are notable observed. Korrat et al. (2005)reported that although this region appears to be characterized bya sparse activity, there is a marked concentration of seismic activ-ity in the Nile Cone (NC) before and after ENSN’s installation.

Source mechanism solutions have proven to be of great value indefining the nature of earthquake faulting and its causative stres-ses. Instrumental recordings provide an ever expanding source ofdata for understanding earthquakes and source characterizations.The insufficient coverage of seismic stations and low seismicityuntil ENSN limit the number of source mechanism solutions thatcan be obtained for the northern Egyptian continental margin. Onlyeight earthquakes have sufficient data for source mechanism solu-tions (Fig. 2). For these events we gathered all available recordsand information from ISC and ENSN bulletins.

The source mechanism solutions in the northern Egyptian con-tinental margin (Fig. 2) reflect the variety of mechanisms from siteto site between normal and reverse with strike–slip componentmechanisms. All solutions demonstrate that the present-day com-

Fig. 1. Seismicity of the Eastern Mediterranean Region. Acronyms: AEG = Aegean SeaIB = Ionian Basin; MR = Mediterranean Ridge; LEV = Levantine Basin; LF = Levant Fault; RElenin and Hussein, 2007).

pressional stress related to the movement between African andEurasian plates and represented by normal faulting mechanism(63%) along NNW trend and reverse faulting mechanism (37%)along ENE–WSW trend (Fig. 2). The northern Egyptian continentalmargin represents the transition zone between the continental–Oceanic crusts where the stress field changes from dominated ten-sion on the Egyptian land to compression along the Hellenic Arczone. The present-day stress regime affecting the northern Egyp-tian continental margin, where maximum compressional stress isoriented N to N30�W, comprise normal faulting trending nearlyin the same direction and re-activated reverse faults in the perpen-dicular direction (Sofratom Group, 1984).

2. Geological and tectonics setting

The regional geology of the Egyptian continental margin wassubject of numerous investigations since it forms a significantstructural unit in tectonic framework of north Africa and easternMediterranean. The Egyptian continental margin considered as aremnant the Mesozoic Neo–Tethys Ocean and opened several rif-ting stages in the Triassic (Garfunkel, 1998, 2004; Robertson,1998). It is characterized by nine identified geomorphological landtypes: beach and coastal flat, coastal dunes, agricultural deltaicland, sabkhas, fish farms, Manzala lagoon, saltpans, marshes andurban centers (EL-Banna and Frihy, 2009). The Egyptian continen-tal margin is located to the south of the folded arc (Fig. 3) formingthe Mediterranean Ridge (MR), where the sea floor is occupied by

; Al = Alexandria City; CY = Cyprus; ERA = Eratosthenes Seamount; FL = Florence;A = Ras El Hikma Village; MA = Marsa Matruh City; JAK = Jebel Al Akhdar (from Abo

Fig. 2. Seismicity of the northern Egyptian continental margin. (A) Before ENSN from 1900 to 1997, with total reported 225-events. (B) From 1998 to 2013, with totalrecorded 864-events. (C) Source mechanism solutions of 8-earthquakes along the Egyptian margin. Green beach balls illustrate normal faulting mechanisms and red forreverse faulting mechanisms. Acronyms: Dead Sea Fault zone (DSF) Mediterranean Ridge (MR), Nile Cone (NC) and Eratosthenes seamount (ERS) Levant Basin (LEV) andHerodotus Basin (HER). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

A. Badawy et al. / Journal of African Earth Sciences 101 (2015) 177–185 179

the Nile Cone (NC), Levant Basin (LEV), Eratosthenes Seamount(ERS) and Herodotus Basin (HER).

It represents a very complex tectonic environment, whichincludes the northward moving African plate and the postulatedSinai sub-plate (Almagor, 1993; Badawy and Horváth, 1999a,b;Mascle et al., 2000), the NNW moving Arabian plate and the west-ward moving Anatolian plate. The rotation of the Anatolian plate isaccommodated by dextral strike–slip movement along the North-ern Anatolian transform fault and the corresponding sinisterlystrike–slip on the East Anatolian transform fault (Hall et al.,2005). The convergence rate of the African and the Eurasian platesis approximately 1 cm/year (Kempler and Garfunkel, 1994).According to Vidal et al. (2000), the Cyprian arc represents the cur-rent plate boundary between the African and Anatolian plates. Thelack of strong earthquakes of intermediate depth at the Cyprus Arcindicated that the subduction had ceased and the plate movementunderwent a large transverse component (Papazachos andPapaioannou, 1999). Woodside (1977) and Hall et al. (2005) sug-

gested that the subduction at the Cyprus Arc has stopped relativelyrecently and has been replaced by another deformation process,because all the oceanic crust has been subducted and the continen-tal part of the African plate now meets the Anatolian plate.

Abdel Aal and Egypt (1994) showed that there were six majorstructural trends that delineate the present Nile Delta and northernEgyptian continental margin. These trends are E–W Neogene hingeline, NE–SW Rosetta fault trend, NW–SE Temsah structural trend,Pelusium shear zone, NW–SE Red Sea–Gulf of Suez fault trendand the minor N–S Baltim fault trend. The structural movementsalong these trends through the Miocene influenced the distributionof sediments in the Nile Delta from its initial development to thepresent day, with some significance on the distribution of the Mio-cene reservoirs in the Nile Delta. Frihy et al. (2010) concluded thatthe study area was characterized by sea-level fluctuations between1.8 and 4.9 mm/year; the smaller rate occurs at the Alexandria har-bor, while the higher one at the Rosetta promontory. These unevenspatial and temporal trends of the estimated relative sea-level rise

Fig. 3. Regional tectonic map of the eastern Mediterranean including the northern Egyptian continental margin (faults and tectonics lines compiled from different source andreferences cited in the manuscript).

Fig. 4. Moment tensor inversion of the 19th October 2012 earthquakes along the Egyptian margin. The observed and synthetic seismograms at 3 selected BB ENSN stations(KOT, NKL, SLM) are also shown.

180 A. Badawy et al. / Journal of African Earth Sciences 101 (2015) 177–185

Fig. 5. Moment tensor inversion of the 17th January 2013 earthquakes along the Egyptian margin. The observed and synthetic seismograms at 3 selected BB ENSN stations(KOT, NKL, SLM) are also shown.

A. Badawy et al. / Journal of African Earth Sciences 101 (2015) 177–185 181

(RSLR) are interpreted with reference to local geological factors. Inparticular, Holocene sediment thickness, subsidence rate and tec-tonism are correlated with the estimated rates of relative sea levelchange.

3. Moment tensor inversions

The moment tensor solution is one of the most important infor-mation may getting for earthquakes. From the moment tensorinversion; we can obtain reasonable earthquake size (moment)and faulting type of an earthquake. It is also necessary for investi-gation of detailed earthquake source process. For relatively largeearthquakes (M > 6) the moment tensor solutions have been esti-mated using global seismic network by different agencies (USGS;EMSC).

Since the occurrence rate of small and moderate size earth-quakes are higher than the large earthquakes it is become essentialto estimate moment tensor solution for small and moderate earth-quakes. So far, a broadband seismological network has been estab-lished in Egypt consequently we can estimate the moment tensorsolutions of recent earthquakes along the Egyptian continentalmargin.

The Egyptian National Seismic Network (ENSN) stations at dis-tances 73–300 km have been used to retrieve the moment tensorfor the two recent felt earthquakes (19th October 2012 and 17thJanuary 2013) along the Egyptian continental margin. The com-puter software zSacWin EQ Processing (Quick Earthquake Analysis

and Archiving System) is used, (Yilmazer, 2012), the program readsSAC format. It makes use of the inverse-problem formulation ofKikuchi and Kanamori (1991), based on six elementary MTs. TheGreen’s functions are calculated by the discrete-wave numbermethod (Coutant, 1990; Bouchon, 2003). To generate the Green’sfunctions a velocity model derived by EL Hadidy (1995). The num-ber of traces able to be examined and the sampling rate of the dis-played data are variable and also changeable using some optionsfor working on the data:

� Filtering with recursive or non-recursive filters with possiblyincluding taper window is possible. The program offers Butter-worth, Bessel and Chebychev I and II filters for highpass, low-pass, bandpass and bandreject filtering.� Editing the header.� Cutting special segments of data while the selection works both.� Merging data.� Integrating, rotating, differentiating, FFT, removal of instrumen-

tal response and re-sampling the data is possible.

For location the program uses the code ‘‘Hypo71 (Lee and Lahr,1975). These tasks are of major importance since in many cases thedata contain various spurious signals, e.g., trends that can affectthe inversion, while at the same time these are not easily recogniz-able in the time series data. The waveform correlation fit amongthe observed and synthetic waveforms for the used stations.

We approximate the horizontal location of centroid to be theepicenter determined by the Egyptian National Seismic Network

182 A. Badawy et al. / Journal of African Earth Sciences 101 (2015) 177–185

(ENSN) broadband (BB) seismic stations (KOT, NKL, and SLM).Figs. 4 and 5 show the source mechanisms and the comparisonbetween observed and synthetic waveforms from 19 October2012 and 17 January 2013 earthquakes respectively. As can beenseen the two earthquakes are typical normal faulting mechanismswith strike–slip component.

4. Source parameters estimations

Assuming a simplified circular source model and using displace-ment spectra, it is possible to determine the dynamic sourceparameters of a given earthquake. In the following we use themodel of Brune (1970, 1971) for simple waveform data, and theanalysis was restricted to the P-waves. We selected P-wave signalwindows that avoid contamination from other phases and main-tain the resolution and stability of the spectra. A cosine taperwas applied to the selected data. The Fourier amplitude spectrumwas calculated using an FFT routine applied to vertical compo-nents. The antialias filter for ENSN instruments has a cut of fre-quency of 40 Hz. To avoid problems associated with the visualdetermination of spectral parameters we applied a least square fit-ting technique based on Brune’s model.

We estimated the source parameters based on the Brune’ssource model (Brune, 1970, 1971), extended by Boatwright(1978), Hanks and Wyss (1972), Madariaga (1977), Randall(1973), and others. The far-field seismic displacement spectrumAd(f) can be modeled by (Boatwright, 1978; Lindley andArchuleta, 1992),

Fig. 6. Example of fitted displacement spectra of the 19th October 2012 earthquakes

Adðf Þ ¼Xo � R�1 � expð�pft�Þffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

1þ ff c

� �2cr ð1Þ

where R is hypocentral distance, f is the frequency, Xo is the low fre-quency spectral amplitude, fc is the corner frequency, c is the sourcespectral fall off and t⁄ is the average attenuation path that defined as

t� ¼Z

path

1Q � Vs

dr ð2Þ

where Vs is the seismic shear wave velocity, Q is the quality factorand the integral is taken over the ray path. Velocity spectrumAv(f) used to fit our spectrum data is defined as,

Avðf Þ ¼2pf �Xo � R�1 � expð�pft�Þffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

1þ ff o

� �2cr ð3Þ

To estimate the Fourier spectra of the respective events, thevelocity records of every station were corrected to zero baselineand instrument response. We selected P-wave signal windows thatavoid contamination from other phases and maintain the resolu-tion and stability of the spectra. A cosine taper was applied tothe selected data. The Fourier amplitude spectrum was calculatedusing an FFT routine applied to vertical components. The antialiasfilter for ENSN instruments has a cutoff frequency of 40 Hz. Oncethe corrected velocity amplitude spectrum is determined, theparameters t⁄, Xo, fo and c were varied to converge the best fitsto the Fourier spectrum using a least square nonlinear technique.

along the Egyptian margin at 4 selected ENSN stations (BRNS, KRG, DK1, NWKL).

Fig. 7. Example of fitted displacement spectra of the 17th January 2013 earthquakes along the Egyptian margin at 4 selected ENSN stations (BRNS, KRG, DK1, NWKL).

A. Badawy et al. / Journal of African Earth Sciences 101 (2015) 177–185 183

By assuming Brune’s model (c = 2), the number of free parametersdecreases from four to three and the fit is more robust. The asymp-totic high frequency spectral decay is bounded between 1.5 and 3(Hough, 1996).

Figs. 6–8 display examples of P-wave spectra that fitted by thesmoothed spectral model at selected ENSN stations. According toBrune’s model, the source dimension is related to the corner fre-quency fo by:

r ¼ 0:37mf o

ð4Þ

In this equation r is the radius of the circular rupture area and v isthe P-wave velocity (6.5 km/s) near the source. In velocity spec-trum, the asymptotic low frequency spectral amplitude Xo is uti-lized to estimate the seismic moment Mo following Brune (1970):

Mo ¼4pqm3HXo

RðH;/Þ ð5Þ

where q is the density (2.7 g/cm3), H is the hypocentral distanceand R(H, /) is the radiation pattern coefficient. In the present studythe RMS average value over the focal sphere of 0.51 for the P-wavehas been used (Fletcher, 1982).

In order to evaluate stress drop from seismic data, a time his-tory of slip must also be prescribed from a model. For example(Brune, 1970), if we take a circular rupture as source model forthe earthquake we obtain the relation:

Dr ¼ 0:44Mo

r3 ð6Þ

and the average displacement U is given by:

U ¼ Mo

lpr2 ð7Þ

where l is the shear modulus.The obtained dynamic source parameters of the studied earth-

quakes are: average seismic moments of 3.55E16 N m,4.32E15 N m and 1.96E14 N m. The stress drops are: 0.13 MPa;0.12 MPa and 0.01 MPa for 19 October, 2012; 17 January 2013and 6 July 2013 events respectively.

5. Discussion and conclusions

The northern Egyptian continental margin is of great impor-tance from economic point of view as a hydrocarbon productivezone in Egypt which quite significant to recognize off-shore sedi-mentary basins and near surface structures. It is characterized byremarkable tectonic setting and mainly controlled by three majorthree geological factors (Gaullier et al., 2000). These are the NileRiver as sediments source; a thin layer of weak mobile Messinianevaporates flooring these sediments and the subduction regimeaffecting the eastern Mediterranean basin that led to shorteningin the Mediterranean Ridge (MR) and subsidence of Herodotusbasin (HER). This tectonics has been interesting for many authorsto study rifting mechanisms in relation to earthquake activities(e.g. El-Sayed et al., 2004; Korrat et al., 2005).

The northern Egyptian margin is constructed in a complex geo-dynamic setting. Eastward, it is bounded by the Dead Sea Faultzone (DSF), and to the north, by the Cyprus convergent zone andthe Mediterranean Ridge (MR). Furthermore, a Sinai sub-plate,

Fig. 8. Example of fitted displacement spectra of the 6th July 2013 earthquakes along the Egyptian margin at 4 selected ENSN stations (BRNS, KRG, DK1, NWKL).

184 A. Badawy et al. / Journal of African Earth Sciences 101 (2015) 177–185

locked between Arabia and Africa has been suspected either fromplate-tectonics considerations (e.g., McKenzie, 1970; Le Pichonand Francheteau, 1978; Courtillot et al., 1987), and earthquake dis-tribution (Salamon et al., 1996; Badawy and Horváth, 1999a,b). Thewestern boundary of this sub-plate is believed to prolong thenorthwest-trending Suez-Cairo-Alexandria fault zone (Kebeasy,1990) offshore Egypt and should therefore be covered by the east-ern Nile Cone (NC). This hypothesis is in good agreement withobservations collected during PRISMED II cruise (1998) that evi-denced a 150 km long NNW–SSW narrow fault system, bisectingthe eastern part of the northern Egyptian margin (Bellaiche et al.,1999; Mascle et al., 2000; Gaullier et al., 2000). The surface expres-sion of these probably deep-rooted tectonic structures is compli-cated by salt tectonics.

Recent earthquake observations after the operation of ENSNhave indicate that outside of the Dead Sea Fault zone (DSF) thereis a marked concentration of activity (Fig. 2) along MediterraneanRidge (MR), Nile Cone (NC) and Eratosthenes seamount (ERS) inaddition to two seismic gaps along Levant Basin (LEV) and Herod-otus Basin (HER) are notable observed as well. We have interpretedthe earthquake activities in Nile cone area (Fig. 2) as a possiblenorthern submerged extension of the gulf of Suez Rift zone (Said,1962), delineating the north-western boundary of a Sinai sub-plate(Joffe and Garfunkel, 1987; Salamon et al., 1996) whose othersboundaries are relatively well constrain (Badawy and Horváth,1999a,b; Mascle et al., 2000). If such an hypothesis is correct, itwould indicate that parts of the Mesozoic margin of Egypt has beenre-activated, during early to middle Miocene, as a consequence ofthe Gulf of Suez rifting episode and following an east–west direc-ted extension. The resulting structural pattern is enhanced by

underlying thick Messinian evaporates that are likely progressivelygliding towards north (Gaullier et al., 2000) and generating eitherspecific salt deformations, and/or salt reactivations, in an activetrans-tensional tectonic belt constrain (Badawy and Horváth,1999c). In most of the Nile cone the presence of underlying saltis at the base of widespread and important sedimentary collapsesassociated to numerous growth faults rooting in the evaporate lay-ers (Gaullier et al., 2000).

Eratosthenes seamount (ERS) is a prominent, continental crust-rooted, feature, about half way between Egypt and Cyprus. It hasbeen demonstrated that the seamount, which shows as a flat-topped, about 80 km long, and relatively shallow (900 m) relief,is progressively breaking as a result from its bending due to colli-sion with Cyprus arc. Geophysical data illustrate how the sea-mount, made of a thick pile of Mesozoic and partly Cenozoicsediments, is cut by a dense set of normal east–west trending,faults (with some strike slip components) and tilting towardsnorth. Facing the thought that bounds the Cyprus margin to thesouth, and as already proposed by Robertson (1998), some of thenormal faults seem to be progressively re-activated into southwardverging thrusts, bounding previous horsts, now on the process tobe incorporated to Cyprus collision zone. The progressive tiltingtowards north of the seamount is probably at the origin of numer-ous slumps well seen along its southern slope (Gaullier et al.,2000). The obtained focal mechanism solutions (Fig. 2) andmoment tensor inversion (Figs. 4 and 5) show normal faultingmechanism with strike–slip component. We have preferred theNNW-trending fault to be the fracture plane in consistence withprevious studies and present-day stress regime (Badawy andHorváth, 1999c; El-Sayed et al., 2004; Korrat et al., 2005).

A. Badawy et al. / Journal of African Earth Sciences 101 (2015) 177–185 185

The Mediterranean Ridge represents a physiographic collisionof African and Crete and is chiefly characterized by the N145-trending, more than 200 km long, northern submarine extensionof the onshore Rosetta branch of the Nile Delta. Earthquake activ-ities tend to clustered along the Mediterranean Ridge and threereverse earthquake source mechanisms are reported (Fig. 2). There,active compressive tectonics leads to a series of folds, reverse fault-ing, and piggy-back basins. These folds are generally salt-cored(Gaullier et al., 2000).

Acknowledgments

The authors are grateful to the Associate Editor Prof. Jean-PaulLiégeois and the anonymous reviewers for their critical reviewswhich have greatly helped to improve the paper. This work hasbeen carried out at Earthquake Division of the National ResearchInstitute of Astronomy and Geophysics (NRIAG), the authors arealso grateful to the all staff members of the ENSN.

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