The investigation of geodynamics parameters in the south of Egypt using geodetic and earthquake activities

ABSTRACT Egypt’s southern region has significant economic and historical importance. However, due to the lack of data, this area has not gotten sufficient geodynamic research. Minor, to moderate earthquake activity along tectonic structures originating in the region from several faults is felt in southern Egypt. The study’s geographical scope includes the Red Sea coast, the Eastern Desert, the Western Desert, Aswan, and Nasser Lake, and it lies between 22° and 27° 30 N and 25° to 34° E. In this paper, the crustal deformation and earthquake events in the southern part of Egypt have been studied. The Global Navigation Satellite System (GNSS) observations taken by Egyptian Permanent Geodetic Network (EPGN) have been processed and adjusted for the period 2010–2019 using Bernese v.5.2 software. In addition, we investigate the source mechanism of the significant earthquakes ≥3 magnitudes using the waveform data recorded by the Egyptian National Seismic Network (ENSN) during the period 2012–2019. The focal mechanisms of these earthquakes were constructed with high reliability based on the P-wave first motion polarities. The results show that the average residual velocity concerning Nubian fixed is about 0.5 ± 0.25 mm in the east direction and 1 ± 0.3 mm in the north direction.


Introduction
Seismic hazard and geodynamics assessment represent the first stage in the establishment of any major project.Earthquake activities and crustal deformation studies become very essential in hazard assessment.Southern Egypt represents a very large area that can be used in the sustainable development of Egypt.The study area contains major projects such as the High Dam, Aswan in the southernmost part of the area.Also, this area represents a promising region for industrial and agricultural investments.The earth's crust of Sudan and South Egypt were widely believed to be stable regions (Said 1962).It was revealed by Nagati (1986) that several Mesozoic basins may have existed in southern Egypt.However, during the Neocomian and Barremian, rifting of continents in Arabia and Africa was highly active (Guiraud and Maurin 1992).Bosworth (1992) concluded that during the Early Cretaceous, southern Egypt suffered from faulting and various rifts appeared in central and north of Sudan.In the last ten years, geophysical prospecting and 2D seismic observations have been collected in southern Egypt.The provided information suggests the existence of several rift basins with NW-SE to WNW-ESE orientations that surround central and southern Egypt's Nile Valley.From southeast to northwest, these basins make up the internal rift system, which also includes Beni Suef half grabens and the Khariet, Nuqura, Komombo, and Assiut basins (Figure 1).This basin system is comparable to the basin system in Central Africa in terms of age and direction Mohamed Said and Sakran 2017;Sakran et al. 2019).Due to insufficient or incomplete subsurface data, this basin system's structural and tectonic details are still poorly understood.
Important elements of the seismic features of crucial structures like bridges, dams, and infrastructures include earthquake activity and focal mechanisms.The fault plane solution, also known as the focal mechanism, is a diagram that illustrates the fault's slip direction and the kind of fault (normal, reverse, or strike-slip fault).Along with other details about the focal region, it also offers the P and T axes.The projection of the fault orientation and slip on the lower hemisphere surrounding the hypocentre is the 2-dimensional focal mechanism circle.
The southern part of Egypt is affected by minor, small to moderate seismic activity along tectonic structures within the area (Rashwan et al. 2021).In this study, we focus on the area extending from 22° to 27° 30 N and 25° to 34° E, which includes distinct regions (Red Sea coast, The Eastern desert, The Western Desert, the Awan area, and Nasser Lake).Nasser Lake, the second man-made reservoir in the world was created after the construction of the Aswan High Dam.Cross faults in the east-west and northsouth directions are present in the high Dam area, which is thought to be the most seismically active area in the study area (such as Kalabsha, Seiyal, and Khor El-Ramla faults (Radwan et al. 2015)).The epicentres of the earthquakes were centred on active faults around North Nasser Lake and West of the High Dam.The largest source of seismic activity in the Aswan region, which is characterised by a spread of hypocentres within the overall thickness of the crust, is the Kalabsha fault (Radwan et al. 2015).

Seismotectonic setting
The study area starts from the city of Assiut to the political borders between Egypt and Sudan (Figure 2).Before the major earthquake, which occurred on 14 November 1981, with a magnitude of 5.5, this region was regarded as having minimal seismic activity.Two historical events were only recorded in this region (Maamoun et al. 1984); the first installed instruments were farther to the north, creating a data gap for the study area; and the Aswan instruments, installed in 1975, did not record any earthquakes until the earthquake in November 1981.This earthquake hit 60 km southwest of the Aswan High Dam, which is situated along the Kalabsha fault and is a remote outpost of the Sinn El-Kaddab plateau that is retreating (Toppozada et al. 1984).Due to the epicentre's proximity to the High Dam, the earthquake increased the seismic risk in the area (Boulos et al. 1990).Because the high dam is vital to the region's socioeconomic prosperity by supplying hydroelectric power, flood control, and irrigation, a seismic evaluation of the area is required to reconsider the high dam's structural integrity.
The Beni-Suef area, the Abu Dabbab area, the south of Abu Dabbab, and the Barnes area are a few of the earthquake-prone areas found along the Eastern and Western Deserts.The Aswan region and Nasser Lake are defined by significant seismicity because of a group of local faults (Figure 2).The Aswan area is one of Egypt's largest seismic active zones.The Aswan region is a tectonically active region due to the numerous uplifts, folds, and faulted blocks that exhibit indications of previous movement in the area's geomorphology (Figure 2A).
In the Aswan region, two fault systems were found, one with an E-W tendency and the other with an N-S trend.According to these patterns, the seismic activity in the Aswan area is therefore concentrated in a few zones, though the rate of seismicity is concentrated along the Kalabsha fault, E-W trend, and its extension towards the east is consistent with the fault's surface trace (Hosny et al. 2014) (Figure 2B).Seismic activity is concentrated around the southernmost parts of the Khur El Ramla and Kurkur faults, along faults flowing from north to south.Low seismic activity was found at the Abu-Dirwa fault and where the Syial and Gabal El-Baraq faults join in the N-S direction.The core of Nasser Lake experienced many little earthquakes in the E-W direction.Several smaller occurrences were also observed along the northern end of the Dabud fault that faces High Dam (Figure 2C).
High seismic activity can be found in the 24 km to the western Red Sea coast Abou Dabbab area.According to Hosny et al. (2014), there are two main directions in which minor and major faults and lineaments tend to trend.The first is NNW-SSE, which is parallel to the Red Sea coast, and the second is ENE-WSW, which is perpendicular to the first direction and reported in the structural setting by the Egyptian Geological Survey and Mining Authority.
Between the Nubian and raised Arabian Shields, the Red Sea is enclosed by an extended escarpment.The Red Sea rift reportedly started in the Oligocene and developed in the Miocene, according to structural and stratigraphic investigations.People who reside close to the Nile Valley are at risk from seismic activity because of the moderate-sized earthquakes that occasionally occur in southern Egypt.Because of their large separation from the epicentre, the effect of these events exceeds the effects of earthquakes that occurred in the Red, Mediterranean Seas and Gulfs of Suez and Aqaba.Thirteen events that took place in the southern region of Egypt have been investigated for this study (Table 1).The focal mechanisms from the P-wave first motion polarity are shown in Figure 5.

Literature review
From a geodynamic point of view, many papers study the geodynamic behaviour around Nasser Lake (Kebeasy et al. 1987, Mohamed S.;Mohamed et al. 2012;Radwan et al. 2015).Crustal deformation of the southern corner of Egypt has been studied (Mohamed et al. 2012;Saleh et al. 2018) using GNSS data from 2007 to 2014.According to the study, the Abu-Dabbab region is the largest seismic cluster in the southern area of Egypt.However, there is a need for improvement in the spatial distribution of the GPS stations in the southeast of Egypt.Therefore, accurate ground motion estimations for this region will result from a well-distributed GNSS network.Rashwan et al. (2021) find that the GNSS velocity field demonstrates that Egypt is rigidly attached to the Nubian plate while the northeastern portion of the Sinai Peninsula exhibits differential motion regarding to Nubian.The authors recommend the necessity to increase the density of the current GNSS stations over some significant regions of Egypt, such as the northern and eastern ones to understand local and regional tectonic processes and to provide appropriate means to mitigate natural hazards in Egypt.Sawires et al. (2021) tried to study the seismic and geodynamic activities in Egypt.They show the presence of ten zones of shallow crustal seismic sources (EG-01 to EG-10; depth <35 km).The Nile River and its delta, the Gulf of Suez-Red Sea Rift, and the possibly shallow active seismic regions along the Gulf of Aqaba-Dead Sea Transform make up the majority of seismicity.These zones did not cover the southern part of Egypt because of lacking GNSS stations in this area.Mahrous et al. (2022) studied the crustal deformation along the Northern part of Egypt using data spanning the time period from 2009 to 2019.He concluded that over the study of ten years, the observed movements take one direction.This clears from the same way, direction, and overall shape of the geodetic point's motions.This same behaviour indicates that most of these movements are caused by the movement of the earth's regional tectonic plates and do not cause a significant effect on the area's stability.Mohamed et al. (2022) showed that the earthquake activities in the northern part of Aswan concentrated at the intersection between the east-west and northsouth direction of faults.
In the current study, depending on more accumulative GNSS measurements and well-disturbed GNSS stations in Aswan and south of Egypt, we tried to get a more accurate and representative study for the crustal deformation in the southern part of Egypt.

Material and methods
The  May stack normal equations (which are provided in binary format in files with the extension *.NQ0) and simultaneously allows for the best fitting of the combined solution to a prior coordinate set in terms of a similarity transformation.The baseline solution created the files for each baseline's coordinate and variance-covariance.After the observation files have been filtered up and most of the ambiguities are set to their values of integers, the last step is to estimate the final fixed solution of ambiguity.Based on the normal equations from daily solutions, ADDNEQ2 calculates the final coordinate solution.Three no-nettranslations and three no-nets are used to achieve the datum definition (Palano et al. 2020;Mahrous et al. 2022).
The shape of the Earth changes because of the relative movements of the tectonic plates Shen et al. (2015).Crustal deformation occurred particularly in the plate conversion areas.While a significant tectonic process is produced by crustal deformation, earthquakes occur by growth of tectonic stress.The degree of information precision of crustal deformation is crucial for realising the dynamics and tectonics processes of the relevant regions.Right today, crustal displacements can be precisely measured using geodetic methods.The ability of permanent GNSS station networks to precisely detect crustal velocity on a worldwide scale aid in the monitoring of ongoing crustal deformation.By modifying velocity fields in relation to spatial, strain rates can be calculated to examine the interior distortion of the crust of the earth.
An improved least-square inversion technique was put in by Shen et al. (2015) to investigate the geodynamic deformation in the active seismicity of Los Angeles fault systems.By using a bilinear matching with weighted inputs of data derived from a distance to an estimated point and supposing local homogeneity of a strain-rate field, one can simultaneously determine the velocity and strain-rate fields.The distance decaying constant, a hyperparameter, regulates how much Weight Depends on Distance (DDC).This approach has been frequently used to explain the properties of crustal deformation fields since it is simple to comprehend and put into practice.
The "Strain Tool" created by Shen et al. (2015) was used to determine the strain parameters for the current study.The software Strain Tool can estimate and compute the parameters of the strain tensor using a set of data points and tectonic velocities on the earth's crust.The primary executable (StrainTensor.py), a Python module called strain, and a collection of shell scripts for displaying StrainTensor.pymake up this utility's three main parts (Shen et al. 2015).A technique to quantify a velocity field and a strainrate field from spatially discrete velocity data was put by Shen et al. (2015).Using their approach and assuming the following formula: where (Δx i ,Δy i ) = (x i −x i y i −y) are the observation station's and the estimation point's respective relative positions.δu i and δv i are called errors in measurements.

Results and discussion
From seismic view, as indicated in Table 1, the fault plane solutions in the research region were utilising the P-wave initial motion of polarity.These solutions represent several fault types, including normal faults with strike-slip shown by red beachballs; strike-slip faults shown by green beachballs, and reverse (thrust) faults indicated by blue beachballs (Figure 5).The focal mechanism solutions in Nasser Lake point to four events along the Kalabsha fault that were characterised by strike-slip movement with nodal planes that oriented NE-SW as the dominant characteristic of the area.The other two events, one along the Abu-Dirwa fault and the other in the southwest of Nasser Lake were normal faults with minor strikeslip components.On the other hand, only one occurrence, which is west of Nasser Lake, represents the reversal fault (Figure 7).The findings of the focal mechanism are in good agreement with the complex faults near Nasser Lake, including the Kalabsha fault plane, Seiyal fault, Gabel El-Barqa fault, Kurkur fault, Khor El-Ramla fault, and Abu Dirwa fault.They are also consistent with earlier research by Hassib et al. (2010), Abou Elenean (2003), Hussein et al. (2013), Hosny et al. (2014) and Badreldin et al. (2019).These occurrences' focal depths are shallow (>10 km).However, the Aswan region has suffered from a cluster of shallow-depth low to moderate earthquakes followed by swarm activity that is localised along the complicated fault system that has been found (Hassib 1997;Abou Elenean 2003).Three earthquakes occurred in the Eastern Desert; the solutions from two of them, which occurred in the middle of the region, point to a normal fault with a minor strike-slip component that runs NE-SW and E-W, while the results of the third earthquake point to a reverse fault that runs NW-SE.The Abu-Dabbab area is one of the significant seismic zones in this region, characterised by high levels of seismic activity but no significant primary seismic shock.The Abu-Dabbab region has experienced compression and tension, per the results of the stress and strain analysis.The first primary strain is compressed in the NE -SW direction, and the second is an extension in the SE -NW direction.Normal faults with moderate strike-slip components ranging NE-SW to E-W are visible in the western Nasser Lake region of the Western Desert's southern region (Figure 6).These focal mechanism results are well in line with the direction of the strain.

Velocity parameters estimation
In our research, GNSS data were used to compute the horizontal deformation of the geodetic network stations.International Terrestrial Reference Frame (ITRF)(2014) has been used as a datum for GNSS measurements.Horizontal velocities can be divided into ITRF (2014) velocity and residual velocity.ITRF velocity illustrates the velocity including the plate motion.While residual velocity includes only the local movements of the points after reducing the effect of plate motion.First, there are considerable changes in the horizontal regional velocity field were recorded throughout the observation period.The ITRF horizontal movement magnitudes are uniformly distributed.It is clear, all geodetic stations are oriented northeast, which is their common direction of the Nubian plate.The average velocities including Nubian plate motion are 23 mm in Easting and 18.8 mm in Northing (refer to Table 2 and Figure 6).The obtained outcomes have a considerable agreement with the predicted outcomes of the main velocities of Egypt's horizontal motions (Mahrous et al. 2022).
In addition to ITRF2014 velocities, residual velocities have been calculated (Figure 7).Euler pole orientation parameters for the Nubian plate computed by Altamimi 2009 have been used.After applying the Euler pole orientation parameters, the residual velocities were calculated.Figure 6 shows the magnitude and direction of the residual

Strain parameters estimation
The strain rate tensors are crucial in tectonic and geodynamic investigations.The seismic hazard estimation offered by tensor analysis depends on the fault strain accumulation, the dilatation strain rate, and the maximum shear strain rate, including its direction (Goudarzi et al. 2014).The principal axes of the strain of the considered region are displayed in Figure 8 where the area shows extension in general which is affected by the extension in the Red Sea.So, we can say that extensional deformation is the major trend in the south of Egypt.In Assiut, we can find that the compression appears side by side with the extension forces.The maximum extension is displayed in the east along the Red Sea with values of about 10 Nano-strain.Figure 9 illustrates the values of maximum shear strain; the south of Egypt shows low strain values     varying between 1 and 8 Nan-strain per year.Moreover, it appeared that the maximum strain increased from the Nile Valley towards the Red Sea.Also, the Assiut area shows high strain values compared with the rest of the southern area.The dilation rates in the south of Egypt is presented in Figure 10.The studied area shows a very low level of dilation rate reaching 3 Nano-strain per year.Positive dilation values are displayed as the main extension in the area.Negative dilation values can be found in the north in Assiut area which represents compression forces.

Summary and conclusion
In this paper, we tried to evaluate the geodynamics of South Egypt depending on seismic and geodetic observations.The results show that the area has shallow earthquake activities with a magnitude less than 4. The seismic activities are concentrated in the Aswan area because of the presence of load variation in Aswan Lake.In addition, the presence of the Kalabsha and Seiyal fault systems affects the Aswan area.Moreover, the Red Sea fault system is a major source of seismic activity.
The fault plane solutions in the study area were obtained using the polarity of the P-wave first motion.These solutions indicate different types of faults, normal faults with strike-slip components, strike-slip faults, and reverse (thrust) faults.The focal mechanism solutions in Nasser Lake indicate four events located along the Kalabsha fault and characterised by strike-slip movement.While the other two events reflect normal faults with minor strike-slip components, one of them is located along the Abu-Dirwa fault, and the other event is located at SW Nasser Lake.On the other side, only one event reflects reverse fault and is located west of Nasser Lake.The Eastern Desert was occupied by three earthquakes, the solutions of two events located in the middle part of the Eastern Desert indicate a normal fault with a minor strike-slip.While the southern part of the Western Desert (western Nasser's Lake) exhibits normal faults with minor strike-slip components trending NE-SW to E-W directions.The results of the focal mechanism are good in agreement with complex faults around Nasser Lake, the Eastern Desert, and the Western Desert and consistent with previous studies suggested by many authors and are good in agreement with strain direction.
From the geodynamics point of view, the area has low deformation values and represents a stable area.The local movement rate varies between submillimetres to 3 mm.The dilation and shear strain show non-significant values of less than 8 Nanostrain per year.The strain values increase in the east towards the Red Sea direction and in the north of the Assiut area.
Generally, the main deformation source in the south of Egypt is the Red Sea extension.Red Sea extension represents normal faults and positive dilation values.But the Aswan region has identical behaviour with strike-slip and reverse faults.This behaviour is related to its complicated structure and environmental effects.The area is stable from a seismic and geodynamics view and represents a privileged area for mega projects.In addition, it is recommended to extend the GNSS stations to cover the western part of the southern part of Egypt which has a small number of stations.

Figure 1 .
Figure 1.The relation of Northeastern African tectonic features from the Early cretaceous rift (grey) to the Mesozoic-Early Cenozoic rift (yellow) (after Mohamed Said and Sakran 2017).

Figure 2 .
Figure 2. (A) tectonic setting of the south of Egypt.(B) surface geology of Aswan area after Woodward-Clyde Consultants (1985), (C) local seismic activity was recorded by the Egyptian National seismic Network (ENSN) during the period from November 1997 to December 2019.The data collected from the ENSN_bulletins http://ensn.nriag.sci.eg.

Figure 3 .
Figure 3.The distribution of GNSS and seismic stations.

Figure 5 .
Figure 5. Focal mechanism solutions of selected earthquakes used in the south of Egypt.Western Desert, Easter Desert, Nasser Lake, Aswan.Red beachballs indicate normal faults with the strike-slip component, green beachballs indicate strike-slip faults and blue beachballs indicate reverse (thrust) faults.

Figure 8 .
Figure 8. Principal strain axes in the of Egypt were calculated from GNSS velocity.

Figure 9 .
Figure 9. Maximum strain rate of south Egypt in Nano-strain.

Figure 10 .
Figure 10.Dilation rate of south Egypt in Nano-strain.

Table 1 .
The source parameters of the selected earthquakes occurred in the southern part of Egypt during the period (2012-2019) with local magnitudes ≥3.

Table 2 .
Horizontal and residual velocity values.It is noticed that all points take the north direction with variable values of velocities.The residual velocities in southern Egypt vary between submillimetres to 3 mm.Maximum values were noticed in MANM and REST around Aswan Lake.This may be related to the considerable source of seismic activity in the south of Egypt.Also, MANM and REST have campaign measurements with a low level of accuracy compared with permanent stations (refer to error values in Table2).