Accepted Manuscript The Northern Egyptian Continental Margin Ahmed Badawy, Gad Mohamed, Khaled Omar, Walid Farid PII: DOI: Reference:
S1464-343X(14)00317-3 http://dx.doi.org/10.1016/j.jafrearsci.2014.09.009 AES 2141
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African Earth Sciences
Received Date: Revised Date: Accepted Date:
9 February 2014 9 September 2014 11 September 2014
Please cite this article as: Badawy, A., Mohamed, G., Omar, K., Farid, W., The Northern Egyptian Continental Margin, African Earth Sciences (2014), doi: http://dx.doi.org/10.1016/j.jafrearsci.2014.09.009
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THE NORTHERN EGYPTIAN CONTINENTAL MARGIN
Ahmed Badawy♣, Gad Mohamed, Khaled Omar and Walid Farid National Research Institute of Astronomy and Geophysics (NRIAG) 11421- Helwan, Cairo, Egypt.
Abstract Africa displays a variety of continental margin structures, tectonics and sedimentary records. The northern Egyptian Continental margin represents the NE portion of the North African passive continental margin. Economically, this region is of great importance as a very rich and productive hydrocarbon zone in Egypt. Moreover, it is characterized by remarkable tectonic setting accompanied by active tectonic processes from the old Tethys to recent Mediterranean. In this article, seismicity of the northern Egyptian continental margin has been re-evaluated for more than 100-years and the source parameters of three recent earthquakes (October 2012, January 2013 and July 2013) have been estimated. Moment tensor inversions of 19th October 2012 and 17th January 2013 earthquakes reveal normal faulting mechanism with strike-slip component having seismic moment of 3.5E16 N-m and 4.3E15 N-m respectively.
Corresponding author: Tel.: (202) 2558 3887, (202) 3388 5126 Fax: (202) 2554 8020, e-mail: [email protected]
The operation of the Egyptian National Seismic Network (ENSN) since the end of 1997 has significantly enhanced 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 ENSN respectively. The spatial distribution of earthquakes foci indicated that the activity tends to cluster at three 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). Key words Source mechanism, source parameters, continental margin, Egypt.
1- Introduction The eastern Mediterranean Sea is characterized by high earthquake activity (Figure 1) and a complex tectonic setting which is not fully understood yet. Several geodynamic models have been developed to explain the processes in this region (Makris, 1976; LePichon et al., 1982; Mckenize, 1970, 1972: Badawy, 1996; Badawy and Horvath, 1999a, b). Still the subduction process south of Crete, crustal structure below the Mediterranean Ridge (MR) and deep structure of the North African passive margin remain poorly understood. The geophysical data especially for the African margin are limited to potential fields and some industrial reflection seismic lines. From reflection seismic experiments (Chaumillon et al., 1996) at the southern edge of the Mediterranean Ridge (MR) a back-thrusting tectonic structure in the sediments was identified in the southward direction. This confirms the MR as an accretionary 2
complex (LePichon et al., 1982; Ryan et al., 1982; Mascle et al., 1995) but it is not yet clear if and how far this back-thrusting reaches onto the African continental margin. Furthermore, the question arises how far the extant compression affects a tectonization of the African continental margin. To investigate these questions and better understanding the geodynamic 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 have been investigated to shed some lights on the present-day stress regime affecting the north Egyptian continental margin. Source parameters and moment tensor of the recent felt and recorded earthquakes (October 2012, January 2013 and July 2013) have been estimated. The Northern Egyptian continental margin has suffered from historical and instrumental earthquakes (Maamoun et al., 1984; Ambraseys et al, 1994). Two historical earthquakes were widely notable in 320 and 956A.D. (Badawy, 1999). These events were damaged numerous houses in Alexandria with maximum intensity VI on MSK scale (Ambraseys et al, 1994). Before the operation of the Egyptian National Seismological Network (ENSN) in 1997, the margin appears to be characterized by sparse activity and the only known and significant earthquake to have occurred offshore of Alexandria was 12 Sept., 1955 with surface magnitude of 6.7(Fig. 2). The total reported earthquakes along the northern Egyptian margin (3134N, 25-35E) throughout 98-years were only 225 (3-events with magnitude more than 5; 37-events from 3 to 5 and 185-events less than 3) events with an annual rate of 2events/year.
By the installation of ENSN the old picture of the northern Egyptian margin has totally enhanced, whereas, the annual activity rate has been jumped to 54event/year with total recorded 864 (7-events with magnitude more than 5; 154-events from 3 to 5 and 703-events less than 3) events within 16-years period. Almost of earthquake activities tend to clustered at three zones (Figure 2) the Mediterranean Ridge (MR), Nile Cone (NC) and the Eratosthenes Seamount (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 by a sparse activity, there is a marked concentration of seismic activity in the Nile Cone (NC) before and after ENSN's installation. Source mechanism solutions have proven to be of great value in defining the nature of earthquake faulting and its causative stresses. Instrumental recordings provide an ever expanding source of data for understanding earthquakes and source characterizations. The insufficient coverage of seismic stations and low seismicity until ENSN limit the number of source mechanism solutions that can be obtained for the northern Egyptian continental margin. Only eight earthquakes have sufficient data for source mechanism solutions (Figure 2). For these events we gathered all available records and information from ISC and ENSN bulletins. The source mechanism solutions in the Northern Egyptian continental margin (Fig. 2) reflect the variety of mechanisms from site to site between normal and reverse with strike-slip component mechanisms. All solutions demonstrate that the presentday compressional stress related to the movement between African and Eurasian plates and represented by normal faulting mechanism (63%) along NNW trend and reverse faulting mechanism (37%) along ENE-WSW trend (Figure 2). The Northern 4
Egyptian continental margin represents the transition zone between the continentalOceanic crusts where the stress field changes from dominated tension on the Egyptian land to compression along the Hellenic Arc zone. The present-day stress regime affecting the Northern Egyptian continental margin, where maximum compressional stress is oriented N to N30oW, comprise normal faulting trending nearly in the same direction and re-activated reverse faults in the perpendicular direction (Sofratome, 1984).
2- Geological and Tectonics Setting The regional geology of the Egyptian continental margin was subject of numerous investigations since it forms a significant structural unit in tectonic framework of north Africa and eastern Mediterranean. The Egyptian continental margin considered as a remnant the Mesozoic Neo-Tethys Ocean and opened several rifting stages in the Triassic (Garfunkel 1998, 2004; Robertson 1998). It is characterized by nine identified geomorphological land types: beach and coastal flat, coastal dunes, agricultural deltaic land, sabkhas, fish farms, Manzala lagoon, saltpans, marshes and urban centers (El Banna and Frihy 2009). The Egyptian continental margin is located to the south of the folded arc (Figure 3) forming the Mediterranean Ridge (MR), where the sea floor is occupied by the Nile Cone (NC), Levant Basin (LEV), Eratosthenes Seamount (ERS) and Herodotus Basin (HER). It represents a very complex tectonic environment, which includes the northward moving African plate and the postulated Sinai sub-plate (Almagor 1993; Badawy and Horvath, 1999a,b; Mascle et al. 2000), the NNW moving Arabian plate 5
and the westward moving Anatolian plate. The rotation of the Anatolian plate is accommodated by dextral strike–slip movement along the Northern Anatolian transform fault and the corresponding sinisterly strike–slip on the East Anatolian transform fault (Hall et al. 2005). The convergence rate of the African and the Eurasian plates is approximately 1 cm/year (Kempler and Garfunkel 1994). According to Vidal et al. (2000), the Cyprian arc represents the current plate boundary between the African and Anatolian plates. The lack of strong earthquakes of intermediate depth at the Cyprus Arc indicated that the subduction had ceased and the plate movement underwent a large transverse component (Papazachos and Papaioannou 1999). Woodside (1977) and Hall et al. (2005) suggested that the subduction at the Cyprus Arc has stopped relatively recently and has been replaced by another deformation process, because all the oceanic crust has been subducted and the continental part of the African plate now meets the Anatolian plate. Abdel Aal and Lelek (1994) showed that there were six major structural trends that delineate the present Nile Delta and Northern Egyptian Continental margin. These trends are E–W Neogene hinge line, NE–SW Rosetta fault trend, NW–SE Temsah structural trend, Pelusium shear zone, NW–SE Red Sea–Gulf of Suez fault trend and the minor N–S Baltim fault trend. The structural movements along these trends through the Miocene influenced the distribution of sediments in the Nile Delta from its initial development to the present day, with some significance on the distribution of the Miocene reservoirs in the Nile Delta. Frihy et al. (2010) concluded that the study area was characterized by sea-level fluctuations between 1.8 and 4.9 mm/year; the smaller rate occurs at the Alexandria harbor, while the higher one at the Rosetta promontory. These uneven spatial and temporal trends of the estimated 6
relative sea-level rise (RSLR) are interpreted with reference to local geological factors. In particular, Holocene sediment thickness, subsidence rate and tectonism are correlated with the estimated rates of relative sea level change.
3-Moment Tensor Inversions The moment tensor solution is one of the most important information may getting for earthquakes. From the moment tensor inversion; we can obtain reasonable earthquake size (moment) and faulting type of an earthquake. It is also necessary for investigation of detailed earthquake source process. For relatively large earthquakes (M>6) the moment tensor solutions have been estimated using global seismic network by different agencies (USGS; EMSC). Since the occurrence rate of small and moderate size earthquakes are higher than the large earthquakes it is become essential to estimate moment tensor solution for small and moderate earthquakes. So far, a broadband seismological network has been established in Egypt consequently we can estimate the moment tensor solutions of recent earthquakes along the Egyptian continental margin. The Egyptian National Seismic Network (ENSN) stations at distances 73-300 km have been used to retrieve the moment tensor for the two recent felt earthquakes (19th October 2012 and 17 th January 2013) along the Egyptian continental margin. The computer software zSacWin EQ Processing (Quick Earthquake Analysis and Archiving System) is used, (Yilmazer, 2012), the program reads SAC format. It makes use of the inverse-problem formulation of Kikuchi and Kanamori (1991), based on six elementary MTs.. The Green’s functions are calculated by the discrete7
wave number method (Coutant 1990; Bouchon 2003). To generate the Green’s functions a velocity model derived by EL Hadidy (1995). The number of traces able to be examined and the sampling rate of the displayed data are variable and also changeable using some options for working on the data: • Filtering with recursive or non-recursive filters with possibly including taper window is possible. The program offers Butterworth, Bessel and Chebychev I and II filters for highpass, lowpass, 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 instrumental response and resampling 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 the data contain various spurious signals, e.g., trends that can affect the inversion, while at the same time these are not easily recognizable in the time series data. The waveform correlation fit among the observed and synthetic waveforms for the used stations. We approximate the horizontal location of centroid to be the epicenter determined by the Egyptian National Seismic Network (ENSN) broadband (BB) seismic stations (KOT, NKL, and SLM). Figures 4, 5 show the source mechanisms and the comparison between observed and synthetic waveforms from 19 October
2012 and 17 January 2013 earthquakes respectively. As can been seen the two earthquakes are typical normal faulting mechanisms with strike-slip component.
4-Source Parameters Estimations Assuming a simplified circular source model and using displacement spectra, it is possible to determine the dynamic source parameters of a given earthquake. In the following we use the model of Brune (1970, 1971) for simple waveform data, and the analysis was restricted to the P-waves. We selected P-wave signal windows that avoid contamination from other phases and maintain the resolution and stability of the spectra. A cosine taper was applied to the selected data. The Fourier amplitude spectrum was calculated using an FFT routine applied to vertical components. The antialias filter for ENSN instruments has a cut of frequency of 40 Hz. To avoid problems associated with the visual determination of spectral parameters we applied a least square fitting technique based on Brune's model. We estimated the source parameters based on the Brune's source model (Brune, 1970, 1971), extended by Boatwright (1978), Hanks and Wyss (1972), Madariaga (1977), Randall (1973), and others. The far-field seismic displacement spectrum Ad(f) can be modeled by (Boatwright, 1978; Lindley and Archuleta, 1992),
Ad ( f ) =
Ω o ⋅ R −1 ⋅ exp − πft * f 1 + fc
Where R is hypocentral distance, f is the frequency, Ω o is the low frequency spectral amplitude,
is the corner frequency, γ is the source spectral fall off and t* is the
average attenuation path that defined as
1 dr Q ⋅Vs path
Where Vs is the seismic shear wave velocity, Q is the quality factor and the integral is taken over the ray path. Velocity spectrum Av(f) used to fit our spectrum data is defined as,
Av ( f ) =
2πf ⋅ Ω o ⋅ R −1 ⋅ exp − πft * f 1 + fo
To estimate the Fourier spectra of the respective events, the velocity records of every station were corrected to zero baseline and instrument response. We selected Pwave signal windows that avoid contamination from other phases and maintain the resolution and stability of the spectra. A cosine taper was applied to the selected data. The Fourier amplitude spectrum was calculated using an FFT routine applied to vertical components. The antialias filter for ENSN instruments has a cutoff frequency of 40 Hz. Once the corrected velocity amplitude spectrum is determined, the parameters t*, Ωo, fo and γ were varied to converge the best fits to the Fourier spectrum using a least square nonlinear technique. By assuming Brune's model (γγ=2), the number of free parameters decreases from four to three and the fit is more robust. The asymptotic high frequency spectral decay is bounded between 1.5 and 3 (Hough, 1996). 10
Figures (6,7,8) displays examples of P-wave spectra that fitted by the smoothed spectral model at selected ENSN stations. According to Brune’s model, the source dimension is related to the corner frequency fo by:
In this equation r is the radius of the circular rupture area and v is the P-wave velocity (6.5 km/sec) near the source. In velocity spectrum, the asymptotic low frequency spectral amplitude Ωo is utilized to estimate the seismic moment Mo following Brune (1970): 5
4πρν 3 HΩ o Mo = R ( Θ, φ )
Where ρ is the density (2.7 g/cm3), H is the hypocentral distance and R(Θ,φ) is the radiation pattern coefficient. In the present study the RMS average value over the focal sphere of 0.51 for the P-wave has been used (Fletcher, 1980). In order to evaluate stress drop from seismic data, a time history of slip must also be prescribed from a model. For example (Brune, 1970), if we take a circular rupture as source model for the earthquake we obtain the relation:
∆σ = 0.44
and the average displacement U is given by:
Mo µπr 2
Where µ is the shear modulus. The obtained dynamic source parameters of the studied earthquakes are: average seismic moments of 3.55E16 N-m, 4.32E15N-m and 1.96E14N-m. The stress drops are: 0.13 MPa; 0.12 MPa and 0.01 MPa for 19 October, 2012; 17 January 2013 and 6 July 2013 events respectively.
5-Discussion and Conclusions
The northern Egyptian continental margin is of great importance from economic point of view as a hydrocarbon productive zone in Egypt which quite significant to recognize off-shore sedimentary basins and near surface structures. It is characterized by remarkable tectonic setting and mainly controlled by three major three geological factors (Gaullier, et al., 2000). These are the Nile River as sediments source; a thin layer of weak mobile Messinian evaporates flooring these sediments and the subduction regime affecting the eastern Mediterranean basin that led to shortening in the Mediterranean Ridge (MR) and subsidence of Herodotus basin (HER). This tectonics has been interesting for many authors to 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 geodynamic setting. Eastward, it is bounded by the Dead Sea Fault zone (DSF), and to the north, by the Cyprus convergent zone and the Mediterranean Ridge (MR). Furthermore, a Sinai sub-plate, locked between Arabia and Africa has been suspected either from plate-tectonics considerations (e.g., McKenzie, 1970; Le Pichon and Francheteau, 12
1978, Courtillot et al., 1987), and earthquake distribution (Salamon et al., 1996; Badawy and Horváth, 1999a, b). The western boundary of this sub-plate is believed to prolong the northwest-trending Suez-Cairo-Alexandria fault zone (Kebeasy, 19990) offshore Egypt and should therefore be covered by the eastern Nile Cone (NC). This hypothesis is in good agreement with observations collected during PRISMED II cruise (1998) that evidenced a 150 km long NNW-SSW narrow fault system, bisecting the eastern part of the northern Egyptian margin (Bellaiche et al., 1999; Mascle et al., 2000, Gaullier et al., 2000). The surface expression of these probably deep-rooted tectonic structures is complicated by salt tectonics. Recent earthquake observations after the operation of ENSN have indicate that outside of the Dead Sea Fault zone (DSF) there is a marked concentration of activity (Fig.2) along Mediterranean Ridge (MR), Nile Cone (NC) and Eratosthenes seamount (ERS) in addition to two seismic gaps along Levant Basin (LEV) and Herodotus Basin (HER) are notable observed as well. We have interpreted the earthquake activities in Nile cone area (Fig.2) as a possible northern submerged extension of the gulf of Suez Rift zone (Said, 1962), delineating the north-western boundary of a Sinai sub-plate (Joffe et al., 1987; Salomon et al., 1996) whose others boundaries are relatively well constrain (Badawy and Horvath, 1999a,b; Mascle et al., 2000). If such an hypothesis is correct, it would indicate that parts of the Mesozoic margin of Egypt has been re-activated, during early to middle Miocene, as a consequence of the Gulf of Suez rifting episode and following an east-west directed extension. The resulting structural pattern is enhanced by underlying thick Messinian evaporates that are likely progressively gliding towards north (Gaullier et al., 2000) and generating either specific salt deformations, and/or salt reactivations, in an active trans-tensional 13
tectonic belt constrain (Badawy and Horvath, 1999). In most of the Nile cone the presence of underlying salt is at the base of widespread and important sedimentary collapses associated to numerous growth faults rooting in the evaporate layers (Gaullier et al., 2000). Eratosthenes seamount (ERS) is a prominent, continental crust-rooted, feature, about half way between Egypt and Cyprus. It has been demonstrated that the seamount, which shows as a flat-topped, about 80 km long, and relatively shallow (900 meters) relief, is progressively breaking as a result from its bending due to collision with Cyprus arc. Geophysical data illustrate how the seamount, made of a thick pile of Mesozoic and partly Cenozoic sediments, is cut by a dense set of normal east-west trending, faults (with some strike slip components) and tilting towards north. Facing the thought that bounds the Cyprus margin to the south, and as already proposed by Robertson (1998), some of the normal faults seem to be progressively reactivated into southward verging thrusts, bounding previous horsts, now on the process to be incorporated to Cyprus collision zone. The progressive tilting towards north of the seamount is probably at the origin of numerous slumps well seen along its southern slope (Gaullier et al., 2000). The obtained focal mechanism solutions (Fig.2) and moment tensor inversion (Figs. 4, 5) show normal faulting mechanism with strike-slip component. We have preferred the NNW-trending fault to be the fracture plane in consistence with previous studies and present-day stress regime (Badawy and Horvath, 1999; El-Sayed et al., 2004; korrat et al., 2005). The Mediterranean Ridge represents a physiographic collision of African and Crete and is chiefly characterized by the N145-trending, more than 200km long, northern submarine extension of the onshore Rosetta branch of the Nile Delta. 14
Earthquake activities tend to clustered along the Mediterranean Ridge and three reverse earthquake source mechanisms are reported (Fig.2). There, active compressive tectonics leads to a series of folds, reverse faulting, 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-Paul Liégeois and the anonymous reviewers for their critical reviews which have greatly helped to improve the paper. This work has been carried out at Earthquake Division of the National Research Institute of Astronomy and Geophysics (NRIAG), the authors are also grateful to the all staff members of the ENSN. References
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FIGURE C APTAIN Figure (1): Seismicity of the Eastern Mediterranean Region. Acronyms: AEG =
Aegean Sea; Al = Alexandria City; CY = Cyprus; ERA = Eratosthenes Seamount; FL = Florence; IB = Ionian Basin; MR = Mediterranean Ridge; LEV = Levantine Basin; LF = Levant Fault; RA = Ras El Hikma Village; MA = Marsa Matruh City; JAK = Jebel Al Akhdar. (From Abo Elenin and Hussein, 2007). Figure (2): Seismicity of the northern Egyptian continental margin. (a) Before ENSN
from 1900-1997, with total reported 225-events. (b) From 1998-2013, with total recorded 864-events. (c) Source mechanism solutions of 8-earthquakes along the Egyptian margin. Green beach balls illustrate normal faulting mechanisms and red for reverse faulting mechanisms. Acronyms: Dead Sea Fault zone (DSF) Mediterranean Ridge (MR), Nile Cone (NC) and Eratosthenes seamount (ERS) Levant Basin (LEV) and Herodotus Basin (HER). Figure (3):
Regional tectonic map of the eastern Mediterranean including the
northern Egyptian Continental margin (faults and tectonics lines compiled from different source and references cited in the manuscript) Figure (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. Figure (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.
Example of fitted displacement spectra of the 19th October 2012
earthquakes along the Egyptian margin at 4 selected ENSN stations (BRNS, KRG, DK1, NWKL). Figure (7):
Example of fitted displacement spectra of the 17th January
2013earthquakes along the Egyptian margin at 4 selected ENSN stations (BRNS, KRG, DK1, NWKL). Figure (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).
We have studied seismicity and seismotectonics of northern Egyptian margin. We have estimated dynamic source parameters of three felt earthquakes. Earthquakes are tending to clustered along three zones, with two seismic gaps. Source mechanisms reveal both normal and reverse faulting mechanisms. Moment tensor inversions for recent events show normal faulting rupture.