Arab J Geosci (2015) 8:5475–5490 DOI 10.1007/s12517-014-1620-7
ORIGINAL PAPER
Integrated geologic and geophysical studies of north unstable shelf seismicity, Egypt Elsayed Fergany & Mahmoud Mekkawi & Maha Abdel Azeem & Ahmed Khalil
Received: 28 December 2013 / Accepted: 4 September 2014 / Published online: 17 September 2014 # Saudi Society for Geosciences 2014
Abstract The large impact of moderate earthquakes in unstable shelf of north Egypt on human society provides a strong motivation to study and understand the systematics of their occurrence. In this study, we examine the correlation of north Egypt unstable shelf earthquakes using five geologic and geophysical data sets: a newly compiled age-province map, Bouguer gravity data, aeromagnetic anomalies, tectonic stress field, and GPS velocity rate measurements. Based on the qualitative and quantitative interpretation of these five data sets, we inferred that (1) although surface features disappeared of unstable shelf crust, Phanerozoic crust shows clear correlation of crustal age and earthquake frequency. (2) The seismic ages are during Precambrian and Paleozoic unstable shelf crust. (3) Seismicity is correlated with the major tectonic events in the geologic history of Egypt. Unstable shelf seismicity mainly (1) follows the NW–SE lineaments and (2) forms clusters at what have been termed stress concentrators (e.g., intersecting faults and igneous intrusions) at EN–WS and E–W structural trends. The correlation of seismicity with NW–SE-oriented lineaments implies that the unstable shelf seismicity is related to the accretion and rifting processes that have formed the Red Sea and Gulf of Suez which still records active rifting. An analysis of hypocentral depths for unstable shelf earthquakes shows that the frequency and moment magnitude of events are nearly uniform for the entire 5–30 km E. Fergany (*) : M. Mekkawi : M. A. Azeem : A. Khalil National Research Institute of Astronomy and Geophysics (NRIAG), Helwan, Egypt e-mail:
[email protected] M. Mekkawi e-mail:
[email protected] M. A. Azeem e-mail:
[email protected] A. Khalil e-mail:
[email protected]
depths over which crustal earthquakes extend. We appreciate that the deep structure of the crust, in particular the existence of deeply penetrating faults, is the controlling parameter, rather than lateral variations in rheology or high pore pressure. We conclude that the distribution of the unstable shelf earthquakes in north Egypt is consistent with the existence of deeply penetrating crustal faults that have been reactivated in the present stress field. Future earthquakes may occur anywhere along the geophysical lineations that we have identified. This implies that seismic hazard is more widespread in the Nile Delta Basin and Cairo province of north Egypt than indicated by the distribution of limited and inaccurate historical seismicity. Keywords Unstable shelf earthquakes . Stress field . Seismic faults . Bouguer gravity . Aeromagnetics . North Egypt
Introduction The large impact of moderate earthquakes in unstable shelf of north Egypt on human society provides a strong motivation to study and understand the systematics of their occurrence. The origin of the earthquakes within the continental intraplate has been the subject of debate over a long time. Unfortunately, limited or inaccurate historical information and the infrequent occurrence of surface ruptures complicate efforts to better understanding these intraplate events (Stein et al. 1989; Newman et al. 1999; Li et al. 2007; McKenna et al. 2007; van Lanen and Mooney 2007). Several authors have presented models to explain the occurrence of the stable continental region earthquakes. These models require conditions, or a combination of conditions, that result in either a zone of weakness or a zone of stress concentration in the crust. Such conditions include intersecting fault systems, igneous intrusions, glacial unloading, crustal strength due to high pore
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pressure or high heat flow, and other factors that increase stress (Gangopadhyay and Talwani 2003; Campbell 1978; Liu and Zoback 1997; Kenner and Segall 2000; Long 1988; Stein et al. 1989; Talwani 1988, 1999; Talwani and Rajendran 1991; Vinnik 1989; Zoback and Richardson 1996; Stein and Newman 2004; Mazzotti et al. 2005). In principle, each of these models assumes that seismicity results from the reactivation of preexisting zones of weakness (Sykes 1978). Based on these models, a spatial correlation of unstable continental shelf earthquakes with extended and/or rifted regions is expected. Although historical documents of continental region earthquakes in Egypt cover a long time interval (∼4000 years), their location and intensity parameters have inaccurate information. This makes it difficult to obtain a true picture of where past moderate-to-large continental region earthquakes might have occurred. This is an important point, since most seismic hazard assessments have relied on the main assumption that future moderate and large earthquakes will occur in the same region as historical events. As a result, the seismic hazard in seismic continental intraplate regions could be underestimated (Kafka 2007; Leonard et al. 2007; Swafford and Stein 2007). Egypt lies in the northeast corner of the African content close to the northeastern margins of the African plate (Le Pichon et al. 1973). The northern boundary of the African plate in the eastern Mediterranean is poorly defined (McKenzie 1972). To the east, there is a convincing evidence that the Red Sea is a young proto-ocean (Girdler and Styles 1974), and in the northeast, the Red Sea bifurcates into the Gulf of Suez and Gulf of Aqaba around the small Sinai subplate (Le Pichon et al. 1973). Basement rocks crop out in southern Sinai, the Eastern Desert, and as isolated inliers in southern Egypt, such as near Aswan and at Gebel Uweinat at the extreme southwestern corner of Egypt, that is, within the Craton area of Egypt, associated with the Nubian–Arabian shield. The remainder of the surface of Egypt is covered by sedimentary rocks of both the stable and unstable shelf areas (Said 1962) as shown in Fig. 1. Stable and unstable shelves are parts of a trough that follows along the outer margin of the massif or Craton. In the unstable shelf asymmetric linear folds, overthrusts and diapirism are common. The basement complex has been encountered in several wells in northern Egypt (Said 1962, 1992), and there is no reason to believe that it is not continuous throughout Egypt below the sedimentary cover from the exposures to the south and east (Morgan 1990). The oldest dated rocks in the basement complex are Late Archen (2673±21 Ma) in age and are found in the Uweinat Mountains. Egypt entered the Phanerozoic during a period of rapid continental growth through Pan African arc accretion. Three distinct ages represented a growth of the Egyptian basement complex from an Archean continental nucleus in southwestern Egypt to Late Precambrian juvenile crust are indicated in Egypt: a Middle or Late Archean age with Early Proterozoic reactivation in the Uweinat massif, an Early
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Fig. 1 Sketch of the structural aspects of the Nubian Arabic Shield margin in northern Egypt (after Said 1962). Egypt historical (brown circles) and instrumental (blue circles) earthquake catalog. Box outlines unstable shelf earthquakes under the study area
Proterozoic event of major crustal formation and reactivation, and a Middle to Late Proterozoic age with Eocambrian to Cambrian reactivation and possible addition (Morgan 1990). Following rapid continental growth in the Late Precambrian– Early Paleozoic, Egypt and continental mass of which northeast Africa was a part entered a period of relative tectonic quiescence. Phanerozoic rocks are dominated by relatively undeformed and unmetamorphosed sedimentary strata, with the most intense tectonic activity concentrating and responsible for the modern continental margins of northeast Africa. Figure 2 summarizes the main global tectonic events that have directly or indirectly affected the geology structure of Egypt (Morgan 1990). In this paper, we focus on the unstable shelf earthquakes within north Egypt (Fig. 1). These earthquakes were located at Cenozoic age in the southeastern borders of the Nile Delta Basin and around the Cairo province of north Egypt. We examine the correlation of the north Egypt unstable continental shelf earthquakes using five geologic and geophysical data sets of a compiled age-province map, Bouguer gravity data, aeromagnetic anomalies, the tectonic stress field, and GPS velocity rate measurements to assess the spatial characteristics of these earthquakes and provide insights into the physical properties of weak zones that promote the unstable continental
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Ne CENOZOIC
Fig. 2 Major tectonic events in the geologic history of Egypt (Morgan 1990)
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Red Sea opening
Pa Voiconism and Syrian Arc deformation. Major elkaline Magmatism
J
PALEOZOIC
P
Drainage N
Volcanism and Tectonism: Rifting of Turkey from N.Egypt. Initial Atlantic rifting; Afro-Arabian strike-slipe
Tr 245
Colcoreous Sedimentation
Closure of southern Tethys
Rifting of S. Atlantic
K
Drainage S
Volconism and minor tectonism associated with Creation of Pangeo (Hercynian and closure of Telthys.
Clastic sedimentation eustatic sea-level changes and epeirogenic movements
C Drift north
D
Drainage N
S O
Close opproach to South PaleGlaciation
E
Drift south
570
Pan African island arc occretion PRECAMBRIAN
Clastic Sedimentation
Uplift and Gulf of Suez rifting
66.4
MESOZOIC
Epeirogenic uplift and subsidence
Subsidence of dakhla land Kufra basins
Pr Reworking and stablization of Nile cration
A
Formation of proto-crust for Nile cration
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shelf seismic activity. Expected hazard level and risk are inferred based on the orientations of present stress field and weak activated structure trends.
Methods To investigate the spatial distribution of unstable continental shelf earthquakes and their correlation with geophysical lineaments, we use standard data processing methods to enhance the expression of geophysical lineaments. Several researchers (Langenheim and Hildenbrand 1997; Wallach et al. 1998; McBride et al. 2002; Lamontagne et al. 2003) analyzed the geologic and the geophysical lineaments to investigate the occurrence of the continental region earthquakes and consider potential seismogenic sources that yielded an improved assessment of the seismic hazards. We distinct geophysical lineaments as clear geometric trends that can be observed in gravity and/or aeromagnetic
anomaly data. Geophysical anomalies, such as regional (short wavelength) gravity and magnetic anomalies, are caused by lateral variations in crustal density and magnetizations, and therefore provide insight into the crustal structure. In this study, we examine the correlation between seismicity and geophysical anomalies on a continental scale for north Egypt. We also consider present stress field and GPS velocity indicators that define tectonic stress orientation. Several studies and measurements are available for north Egypt, and these provide a clear picture of the tectonic stress orientation.
Data Earthquake catalog Egypt forms the northeastern corner of Africa and is bounded by three active tectonic margins: African–Eurasian plate, the Red Sea, and Levant-Dead Sea transform fault. Three main
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tectonic deformations affecting northern Egypt, which are the Jurassic–Early Cretaceous rifting, Late Cretaceous–Early Tertiary wrenching and Miocene, and post-Miocene extension. Egypt has a very long historical record of earthquakes going back to about four millennia. The seismicity of Egypt was studied by many authors (Sieberg 1932; Ismail 1960; Gergawi and El-Khashab 1968; Maamoum et al. 1984; Kebeasy 1990; Abou Elenean 1997 and Badawy 2005). Where seismic activities are reported in four narrow belts (Levant–Aqaba; Northern Red Sea-Gulf of Suez–Cairo– Alexandria; Eastern Mediterranean–Cairo–Fayum; and Mediterranean coastal dislocation), which represent the major tectonic trends in northern Egypt. We use here a recent compiled earthquake catalog of Fergany (2012), which contains a record of events with moment magnitudes ≥4.5 (Fig. 1). This catalog includes historical and instrumental earthquakes extended from 2200 BC to 2009 AD. The compiled seismicity catalog may be regarded as spanning four time periods of distinctly different quality: (1) historical seismicity, prior to 1900, and consisting mainly from macroseismic information; (2) the early instrumental period, from 1900 to 1963 inclusive, consisting of a combination of macroseismic information and relatively low quality instrumental data; (3) the instrumental period, from 1964 to 1998, consists of good quality instrumental data especially for large and moderate events; and (4) a very recent instrumental period 1998–2009, after the completion of the Egyptian National Seismic Network, where small earthquakes are also included correctly in the catalog. The uncertainties in location were improved over these periods with events reported prior to the instrumental period having lateral uncertainties in excess of 30 km. Depths over this period are much worse determined. Few inland historical moderate size earthquakes caused serious damages to the northern part of Egypt (Ambraseys et al. 1994). The most significant events located at the study area occurred to the west of the Gulf of Suez apex in 1754 (Mw=5.3) and the other occurred in 1847 (Mw=6) at a very close site to the recent Dahashour earthquake (1992), southwest Cairo. It is considered as the strongest shock in the last 150 years in Lower Egypt, before the occurrence of Dahashour Earthquake (Mw=5.9). With the establishment of the Egyptian National Seismic Network (ENSN), much inland and offshore seismic activity with more accurate parameters has been detected. Moderate earthquakes frequently occur in the Egyptian inland areas caused severe loss of human life as the 1992 Dahashour earthquake, southwest Cairo. Although Egypt has a very long historical record of earthquakes, going back to four millennia, detailed and reliable information are available only for few destructive events and the other events have a limited accuracy in location and size. The inaccuracy earthquake parameters almost extend from
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historical earthquakes up to 1927 due to the lack of the world seismic stations. Based in the accuracy of earthquake parameters, we divide the earthquake catalog of the study area into historical data cover the period from 2200 BC to 1927 AD and instrumental data cover the period from 1927 to 2009 (Fig. 3). Bouguer gravity and aeromagnetic anomaly Gravity data is taken from the project of establishing the gravity map of Egypt which accomplished by general Petroleum Company (1983). Gravity data is used to estimate the depths to the shallow objects by applying the concept of 3D Euler deconvolution without knowledge of the structural index (SI). Euler’s homogeneity equation has long been used to determine either the depth or the nature of a simple equivalent source (Thompson 1982). The method treats the field sources (gravity or magnetic) not as preconceived geological models but rather as consisting of elementary point sources with different falloff rate of the field with distance (e.g., single pole, point dipole, line of dipoles, point mass, line of masses, etc.…). The aeromagnetic anomaly data has two tasks in this work: The first objective is to confirm the existence and location of subsurface faults. The second objective is to estimate the depth of these faults. The second vertical derivative and analytical signal are applied to aeromagnetic data using Geosoft Software (2009). Tectonic stress field Many authors have studied tectonic stress field in Egypt using variety data of earthquake focal mechanisms, GPS, and borehole breakouts. The tectonic stress map for north Egypt is taken from the present-day stress field in Egypt map (Badawy 2001, 2005). The author used earthquake focal mechanisms and GPS observations in Egypt and compiled the stress field for Egypt.
Interpretation Crustal age A recent geologic province map of Egypt was provided by the Egyptian Geological Survey and Mining Authority (1981). The north Egypt continental unstable shelf seismicity shows a strong correlation with crustal age (Fig. 3) with exception of some historical earthquakes. It is noted that seismicity is associated with Early Cenozoic and Tertiary, as shown in Fig. 3. For Egypt, it is not easy to consider which age is an aseismic or seismic rocks where Egypt continent was formed after many episodes of uplifting, rifting, and magmatic activity at different ages as shown in Fig. 2.
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Fig. 3 Unstable continental shelf earthquakes. White circles are historical events from 2200 BC to 1927 AD. Black circles are accurate instrumental events from 1927 to 2009
Bouguer gravity The Bouguer map of the study area is shown in Fig. 4. To interpret the gravity anomalies of the study area, we have applied a pilot technique proposed by Stavrev (1997) and Geroveska and Bravo (2003) assuming a linear background with unprescribed structural index based on the properties of the differential similarity transformation (DST). The Euler
deconvolution is applied to the data after resampling to 90× 100 data points. The window chosen is 6×16 grid data points in the x and y directions, respectively. The partial horizontal derivatives of the field are used after approximating the field with bicubic splines. The vertical derivative is obtained in the frequency domain using standard filters in the frequency domain. To reduce the number of clusters, only the second cluster criteria are presented in this work to summarize the
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Fig. 4 Bouguer anomaly of the study area
final results. During the process of calculation, the analytic signal (AS) is calculated (Fig. 5). Many maxima can be observed on the 3D perspective view of the AS map. They represent possible contacts and or edges with sharp density contact. Moreover, minor peaks can be observed that may relate to some deeper geologic boundaries. Dashed lines are posted on the map to mark major features.
Figure 6 shows the 2D representation to the horizontal projections of the sources of surface gravity anomalies and cluster indices. Most of the effective sources are located to the west of the Nile River and Cairo; these are clusters marked as 1, 2, 3, 4, 5, 6, and 7. Other relatively minor clusters can be observed to the east of the Nile River; these are clusters 8, 9, 10, 11, and 12. Figure 7a shows a 3D distribution to the
Analytic signal (AS)
Analytic signal (AS)
2 250 2
1.8
1.8
2.5 200
2
1.6
1.6
1.4
1.4 1.5
150
1.2
1.2 1
1
1 100
0.5
0.8
0.8 0.6
0.6
0 300
50
300
200 200 100
100 0
0.4
0.4 0.2
0.2 0 0
50
100
150
200
0
Fig. 5 Analytic signal (total gradient) of gravity field, north Egypt. Top panel is a 3D perspective view; bottom panel is a plane view
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X, km
250
250
200
200
150
150
100
100
50
50
0
0
0
50
100
150
200
0
50
Y, km
100
150
200
Y, km
Fig. 6 Clusters, sources horizontal projections, and clusters indices as summarized for all groups from left to right, respectively
gravity anomaly sources. Close inspection of the spatial distribution of clusters shows that (1) it varies from shallow to deep (>16 for cluster 4; located west Rasheed branch of the Nile river, and 11 km for cluster 1 close to Wadi El Rayan, whereas, clusters 3 and 8 are relatively shallow (about 3 km). (2) Cluster 11, depth about 10 km, correlates well with the Nile Delta Hinge zone. (3) The lower boundary of the Katanya High in the western desert is clearly bounded from its western limit by cluster 2, depth 9.9 km and from its north eastern limit by cluster 5, depth 7.8 km. This is well confirmed when correlating with the second vertical derivative map of the area in Fig. 8. Figure 7b is the structural indices (SI) of the solutions as resulted from the deconvolution process; most of the SI are less that 0.5 indicate contact or fault as a dominant structural feature. Table 1 summarized the results of the present analysis. The depth and structure index of each cluster is given together with the x and y coordinates for easy tracking.
Aeromagnetic anomaly The aeromagnetic map of Egyptian General Petroleum Corportion (1983) in Fig. 9 is processed and analyzed using Oasis Montaj (Geosoft, 7.1). The resultant maps are reduce to pole (RTP), horizontal gradient vertical derivative (VD) and analytical signals (AS). The qualitative interpretation of the RTP magnetic map are the following: I. From analysis RTP magnetic anomaly map (Fig. 10), the northern part of the Nile Delta area is represented by geomagnetic anomalous negative closures, its elongated shape, low gradient, high relief, and negative polarities suggest a deep sedimentary basin (Delta Basin). II. The central part of the area has anomalies of different polarities, characterized by irregular shape, high sharpness, and high gradient. This illustrates a continuous tectonic activity in that area. In the southern part of the
Fig. 7 Depth to the sources of surface gravity anomalies (a), and corresponding structural indices (SI) (b)
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An important step in the interpretation of magnetic data is the study of the anomaly with a special shape; linear anomalies correspond to dikes and (or) faults, and circular anomalies correspond to intrusive bodies. The structural elements can be detected simply by inspecting the alignment of anomalies and by distinguishing the noticeable abrupt change between positive and negative anomalies, particularly at the location of sharp gradients. In this respect, the following five structural trends were recognized from the magnetic maps (Figs. 11 and 12). Northern trending structures This trend has a mean strike of about N 40° E. According to Camp and Roobol (1991), the northeast trending structures of the western central Arabian Shield is related to the Samran fold belt which appears to have formed in the late Proterozoic.
Fig. 8 Second derivative filtered data with zero contour and probable faults indicated. The cluster indices are posted. Blue and red circles are historical and instrumental earthquakes
area, the anomalies have an elongated shape, negative polarity, and low gradient. III. These observations explain the presence of sedimentary basin having thick sedimentary deposits; spherical shape anomalies of different closures, high gradient, and high sharpness are also appearing in the southwest part of the RTP maps. This indicates the uplifting of the basement rock surface in the area. These anomalies with the intervening magnetic gradients reflect basement of different lithological natures dissecting by structural elements of circular and linear features (Grant and West 1965). IV. Tectonic trend analysis of magnetic maps (HG and VG)
West–northwest trending structures (Najd structure) The peak of this trend has a mean strike of about N 45°W. This trend is considered as one of the most profound fault systems which affected the Precambrian basement (Said 1962) and has a good agreement with the tectonic structural works of Youssef (1968) and Orwig (1982). The age of faults is Precambrian. East–west trending structures (Tethyan structure) This trend is referred to the Tethyan trend. East–west structural trend is prominent features in of the Precambrian basement of southeastern Mediterranean Sea (El Shazlyl 1977; Ross and Uchupi 1977). They appear to have formed in the late Paleozoic to Early Jurassic. East–northeast trending structures ENE trend has a mean strike of N 60°E and of Cretaceous age. The trend is known
Table 1 Statistical analysis of clusters (second stage) Cluster index
NO. points
Xave (km)
Xcon
Yave (km)
Ycon
Zave (km)
Zcon
Nave
Ncon
1 2 3 4 5 6 7 8 9 10 11 12
37 52 6 18 43 124 76 13 9 65 31 20
42.32 83.52 8.98 190.03 111.73 41.94 84.17 147.67 9.26 98.87 233.51 18.43
13.42 9.03 1.99 4.73 6.58 17.81 21.4 14.82 7.31 9.36 9.1 2.3
16.59 30.65 40.19 59.23 79.61 89 115.82 126.55 142.29 153.19 188.36 204.44
9.1 9.64 3.81 4.97 2.94 8.14 1.65 14.67 3.5 2.07 11.47 11.14
11.05 9.98 3.46 16.16 7.87 6.66 6.46 3.55 7.94 9.34 10.09 8.37
5.13 2.91 1.22 7.03 2.95 4.42 4.1 3.49 3.5 6.18 4.67 2.47
0.49 0.38 0.07 0.32 0.14 0.48 0.33 0.24 0.13 0.87 0.36 0.5
0.61 0.42 0.12 0.48 0.15 0.66 0.32 0.41 0.19 1.11 0.57 0.43
ave and con are average value and confidence interval, respectively, for variables X, Y, and Z; Ncon confidence interval for estimated structural indices
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Fig. 9 Total aeromagnetic magnetic anomaly (Egptian General Petroleum Coroportion 1983)
as a Syrian Arc. The peaks of this trend show a strong intensity on all the rose diagrams maps (Fig. 13). North 35° west (Red Sea trend) Red Sea trend has a peak on the Nubian Shield ranges in direction between N 25° and 55° W. This trend shows relative strength in the Red Sea region and also stronger on the Arabian side as compared to the Nubian side. The peaks of this trend show only moderate to strong intensity on the rose diagrams (Fig. 13). These results are in a good agreement with El Shazly and Saleeb (1979). The north–northwest trend faults along the Red Sea. V-depth estimation The quantitative interpretation is important to estimate the subsurface structures and source of magnetic anomalies that affected in the study area. The fast method to estimate the magnetic sources is 3D Eular Deconvolution (after, Reid et al., Fig. 10 RTP of aeromagnetic map. Black and white circles are historical and instrumental earthquakes
1990). Salem and Ravat (2003) developed a method (ANEUL), which is based on the combination of the analytic signal and the Euler deconvolution methods, to estimate the depth and geometry of the magnetic sources using this equation: AASo zo ¼ ðN þ 1Þ ð1Þ AAS1 x ¼ xo ;y ¼ yo where x and y are the coordinates of the measurement points; |AAS0| and |AAS1| are the amplitudes of the analytical signal of the anomaly. This method is used by Ardestani (2009) and applied to Bouguer anomaly map. Using the structural index (N=0.5) for fault, contacts, and dykes in the sedimentary basin. The estimated depths for defined deep sedimentary basin of Delta and
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Fig. 11 Horizontal gradient magnetic map of the study area showing faults system. Black and white circles are historical and instrumental earthquakes
north Egypt are ranging from 1000 to 6000 m with average depth of 3500 m. Crustal stress The crustal stress field of north Egypt was investigated by Badawy (2001, 2005) based on earthquake focal mechanisms and GPS observations. The majority of inland earthquake focal mechanisms in Egypt are normal with strike-slip component or strike-slip faulting events. Focal mechanism solutions for earthquakes occurred in and around the area of study are shown in Fig. 14. The focal mechanisms deduced by Abou Elenean (2007) indicate a dominant tension stress along the NE African corner. The tension axis trends ENE–WSW along the Gulf of Suez, Gulf of Aqaba, and Red Sea rift take NNE– SSW trends towards the land. The major part of deformation takes place along the main rift faults; part of the rifting
Fig. 12 Vertical derivative magnetic map of the study area showing the faults. Black and white circles are historical and instrumental earthquakes
deformation is transferred towards the land to rejuvenate the preexisting WSW–ENE, EW, and WNW–ESE faults with predominant normal faulting and slight shear. Badawy (2001) applied the inversion method of Gephart and Forsyth (1984) to calculate the orientation of the principle stress axes and the shape of the stress tensor. The best fitting tensor in Egypt shows homogeneity stress field. The tension stress regime is dominant in northern Egypt. In north Egypt, the P-axis cluster in a vertical plane of WNW–ESE orientation and the T axis are almost horizontal and trend NNE–SSW. The inverse solution reflects these orientations, showing welldefined almost subhorizontal σ3 trending SSW, and gently plunging to ESE σ1. The orientations of the principle stresses suggest that the stress regime in this area is of general transtension (Guiraud et al. 1989). Data from Cairo GPS network, the magnitudes of the horizontal velocities vary between 4 and 8 mm/year with an
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Fig. 13 The frequency-azimuth rose diagram showing the dominant trends of the structural element
average of 5.5 mm/year in NNW–SSE direction. The implication is that the crustal stress field is favorably oriented to activate faulting on NNW–SSE and WWN–EES to oriented lineations mechanisms of normal with strike slip component, which are predominant source mechanisms in north Egypt. Figure 15 shows distribution of the dominant stress axes. It
Fig. 14 Focal mechanism of small and large earthquakes with lower hemisphere projection and the dark quadrant denotes compression. The size of beach ball is scaled relative to the magnitude. The box outlines the study earthquakes area (after Abou Elenean, 2007)
indicates numerous of stress orientations with dominant NW– SE orientation. These are likely due to responsible of local weakness zones to regional and local stress concentrations. The average GPS velocity field shown in Fig. 16 quantifies the northward motion of African plate at rate of 5.15±1.1 mm/ year with respect to the Eurasian plate (Badawy 2005).
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Fig. 15 Distribution of the stress axes derived from the focal mechanisms of moderate-to-large earthquakes. Symbol length is proportional to the dominant ones (after Badawy 2001). The rectangular box outlines the study area
Hypocentral depths and temporal distributions of moment magnitudes Earthquake hypocentral depths and magnitudes can provide additional insights into the conditions that give rise to seismicity of the unstable continental shelf. The hypocentral depths uncertainty depends on the available seismic network station coverage. Keeping these uncertainties in mind, we consider hypocentral depth estimates from continental north Egypt. The data show a maximum number of events within the 10–15 km depth and a moderate number of events within the 5–10 and 20–25 km depth. We also consider moment magnitude versus depth (Fig. 17). The peak moment magnitude is reached a peak of Mw=6 within the 20–25 km depth. A plausible interpretation of this observation is that the largest unstable continental shelf earthquakes initiate at ∼20 km depth and may rupture up to the surface. Figure 18 shows the time magnitude plot of the instrumental catalog entire period of 1900–2009. It is clear that the detection of the moderate and
small moment magnitudes increased with increasing the number of monitoring earthquake networks in the last 30 years. With increasing accuracy of events, small earthquake activity can be used in relocation of the historical earthquakes.
Discussion and conclusions Earthquakes in unstable continental shelf are commonly attributed to the reactivation of upper crustal zones of weakness that are favorably oriented with respect to the regional stress field. The most commonly cited examples of zones of weakness are faults associated with ancient continental interior rifts and paleorifted margins. However these two tectonic environments only provide an explanation of 48–62 % of stable continental earthquakes (Schulte and Mooney 2005). The north Egypt unstable continental shelf earthquakes are distributed spatially within Cenozoic Age that covered north Egypt. It is concentrated in the eastern part of the unstable
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Fig. 16 Average horizontal velocity vectors estimated from repeated GPS observations with its 95 % ellipses in ITRF96 reference frame (after Badawy 2005)
shelf located at different distances around Cairo province in additions to a few historical earthquakes located in/around the southern part of the Nile Delta Basin (Fig. 3). However, the basement in north Egypt is covered by thick sediments and no surface faults are detected; it is not adopted to join these earthquake sources to Cenozoic. Also, is the South Delta Basin an aseismic zone?
In this study, we try to (1) outline the faults in the study area and delineate the weak zones, (2) define the age of the faults and which tectonic event build it, (3) illuminate the reality of historical earthquakes in the South Delta Basin, and (4) understand the causes and hazard size of these seismic zones. Geophysical anomalies, as expressed in Bouguer gravity and aeromagnetic anomaly maps, and geologic tectonic events are
8
10 Moment magnitude Number of events
9
7
8 6 5
6
4
5 4
3
Moment magnitude
Number of events
7
3 2 2 1
1 0
0
10
20 Depth (km)
30
0 40
Fig. 17 Distribution of north unstable continental shelf (NSCR) seismic events as function of depth (x-axis) and the corresponding moment magnitudes and number of events (y-axis)
Fig. 18 Time magnitude plot of the instrumental catalog entire period of 1900–2009
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used to define the trend, age, and depth of the subsurface structures. The following main three structural trends are recognized and follow the geologic structure strike as follows: (1) NE–SW trend—according to Camp and Roobol (1991), the northeast trending structures of the western central Arabian Shield is related to the Samran fold belt which appears to have formed in the late Proterozoic. (2) NW–SE trend—this trend is considered as one of the most profound fault systems which affected the Precambrian basement (Said 1962) and has a good agreement with the tectonic structural works of Youssef (1968) and Orwig (1982), and the age of faults is Precambrian. (3) E–W trend—this trend is a prominent feature in of the Precambrian basement of southeastern Mediterranean Sea (El Shazlyl 1977; Ross and Uchupi 1977). They appear to have formed in the late Paleozoic to Early Jurassic. On the other hand, the analysis of focal mechanisms delineated that the northern Egypt is dominated by normal faulting with strike-slip component with broad possible variations of primary stress in a plane of WNW–ESE orientation (Badawy 2005). There is a remarkable correlation between the north unstable continental shelf instrumental earthquakes and the NE– SW/NW–SE trends defined by geophysical data (Figs. 8, 10, 11, and 12). Although the South Delta Basin has been nearly aseismic for the past 150+years, it is notable a correlation between the historical earthquakes and the E–W fault trends. This aseismic state can be attributed to the changing in the direction of the tectonic movements and the thick sediments of the basin. This means that the structure trends in the South Delta Basin can be activated with local stress accumulation at long recurrence period. It raises the possibility that the South Delta Basin has a significant seismic potential. On the other hand, we can say that the big uncertainties in the spatial distribution of the historical earthquakes moved the real location to the north where the population settled in the Nile Delta Basin. The hypocentral depths of north unstable continental shelf earthquakes are compared in this work with the depths of structural faults estimated by the potential field data and giving a good agreement in the depths
