The Current State of Deformation Parameters in the Nile Delta, Egypt, Using GNSS and Seismological Data

Abstract


Introduction
The Nile Delta is located in the northern part of Egypt, where the Nile River flows into the Mediterranean Sea.The delta extends, covers for area 160 kilometers from north to south between (30°-31.8°N and 30°-32°E), whereas the shoreline stretches 240 kilometers from Alexandria to Port Said.Two promontories, Damietta and Rosetta as well as three brackish lakes, Burullus, Manzala and Idku, are connected to the sea.Six harbours found on the shoreline, El Gamil, Idku and New Burullus for fishing, while Port Said, Damietta, and Alexandria for trade.
The problem under investigatet alion related to evaluate the phenomenon of Nile Delta subsidence and uplifting by computing the horizontal and vertical movements of both local and horizontal crustaldeformation of the Nile Delta with regard to the rise of Mediterranean Sea level due to Global warming.Regular monitoring and accurate measurement of subsidence and coastline retreat in the Nile Delta isbecoming increasingly important for better understanding of these two phenomena and to provide decision-makers with useful information for the purpose of sustainable development.The Delta subsidence has been studied by many authors such as Basem et al. (2020) and Saleh et al. (2018).The research plan focuses on taking precise and details positional and temporal measurements of local and regional crustal deformation of the Nile Delta as well as determining and interpreting therates of crustal movements, including regional and local velocities.In addition to estimate the straintensor parameters, the rates of dilatation, shear and principal strain rates have been computed.Theaccurate and reliable geodetic data of Global Navigation Satellite System (GNSS) of about 14 geodetic permanent stations, for the period from 2013 to 2020, were analyzed utilizing the Bernese 5.2 software.

Tectonic Setting
Most recent sedimentary land mass occupying the Nile Delta is characterized by a thick sedimentary layer.In the Pleistocene Nile deposits, it was forming a thick alluvium layer (Said, 1990).And a faulty hinge line orientated WNW to ESW designated at Kafr El Sheikh Latitude city separates the Nile delta into two zones.The faulty flexure that divides the south delta zone from the north delta plain basin is known as this hinge line (Khaled et al., 2014 andMosconi et. al., 1996, ).Sestini (1995), Zaghloul et al., (2001) and Orwig (1982) recognized the structural trends as following; 1.The Tethyan trend, an east-west trend that may be associated with the early Mesozoic and probably earlier continental margin rifting of the southern Mediterranean.The Oligo-Miocene Hinge Zone, Mit Ghamr Fault, the northern and southern flexures of the Nile Delta onshore flexures are the most well-known instances of this trend.2. A northeast-southwest trend known as the Rosetta trend Age of the Late Cretaceous.The faults are supposed to have started in Alexandrina, which is found in the Mediterranean Sea's north-east corner.These faults have both vertical and horizontal motion, as well as strike slip displacement.3.During the Miocene, the northwest-southeast trend was active, Temsah or Bardawil Line in the eastern offshore Nile Delta.Structural of the Eastern Mediterranean is the consequence of a complex interplay between threeprimary fault trends (Abd El Aal et al., 2000) (Fig. 1).The faults that characterize the Messiniansalt basin include the NW-SE (Temsah) fault trend, the NE-SW (Rosetta) fault trend, and the E-W faults.These trends appear to be old hereditary basement faults that reactivate periodically throughout the history of the region, and they are parallel to the Mediterranean plate borders.

Seismic Activities
The Nile Delta is situated in the northern section of the active rift, which is thought to be linked to Red Sea tectonics.The majority of crustal deformation is predicted to occur in northern Africa active mountain belts, which have high seismicity and topography, due to the African plate northward convergence (Badawy, 2005).According to seismicity maps, focal mechanisms, and stress field maps, Egypt can be classified into eight seismic zones (Mahmoud, 2003).The delta itself has low seismic activity.It is affected by the most active zones in Egypt and adjacent areas such as seismic zones of the Eastern Mediterranean, Red Sea Northern side also the active part of the Gulf of Aqaba and Suez as well as recently Dahshour area.Fig. 2 shows the instrumental seismicity of Egypt.The Delta has minor seismicity based on instrumental seismicity, although it is substantially impacted by the above mentioned nearby seismic active zones (Marzouk et al., 2011).Egypt (1927Egypt ( -2020) ) taken from catalogue of the international seismological center, (Rashwan et al., 2021)

Materials and Methods
In our research, 14 GNSS stations have been selected to cover all the Nile Delta area and the nearest places, some of these stations permanent (Working continuously; SAID, DAMT, EDFN, DAMN, TANT, HAML, BORG, MNSR, MSLT, KATA, PHLW) and other campaign station (Not working continuously; PLBS ,ASHM and AGRM).The period of GNSS stations observation from 2013 to the end of 2020, Fig. 3 shows the GNSS location of the Nile Delta stations.The geodetic network established by the National Research Institute of Astronomy and Geophysics, Helwan, Egypt (NRIAG).Day by day, the data was analyzed, and normal equations for each campaign's days were saved.These normal equations were then integrated to generate the final solution for each campaign, with the processing stages carried out using Bernese software version 5.2 (Dach et al., 2007), and correct network coordinates estimated in ITRF ( 2014) as follows: The daily exact orbits and weekly Earth orientation parameters are acquired from the IGS data center.The data is prepared and preprocessed by downloading and preparing orbit files.The accurate orbits are translated into the celestial reference frame and saved in the Bernese standard orbit file after the clock information is extracted.The OBS-MAX system is used to create a series of independent baselines and all baseline files are inspected and adjusted for cycle slip in Phase preparation.A float solution is calculated using all of the session's baselines to achieve the daily solution and all baselines are modified using GPSEST.Session after session, the data was analyzed and normal equations for each campaign session were saved then the normal equations were integrated using ADDNEQ2 to generate final campaign solutions also the repeatability of station coordinates from several campaigns was computed to determine their accuracy.As a result of processing, crustal deformation rates were derived.
On the other side, Strain Tool has been used to calculate the strain parameters.Strain Tool is a software package allows estimating and calculating the parameters of the strain tensor from provided list of data points and tectonic velocities on the earth crust.There are three main parts to this tool; a python module (called Pystrain), the main executable (StrainTensor.py), a collection of shell scripts forplotting StrainTensor.pyresults (Shen, 2015).

Results and Discussion
The hinge zone, which controls the distribution of sedimentary facies in the Nile Delta, is one of the primary structures controlling the Nile Delta and extends from east to west through the center ofthe delta.Also, this hinge zone divides the Delta into two structural sedimentary territories North andSouth Delta territories.On the other hand, one of the other parameters that control the geodynamics ofNile Delta is the gas and oil production which affects the pressure stream in the delta.The obtained work is aim to examine the geodynamical characteristics of the Nile Delta and their relationship to the hinge zone by employing continuous GNSS observation along the Nile Delta.

Velocity Parameters Estimation
Firstly, the horizontal regional velocity field values for the processed stations in ITRF2014 showed variations across the observation period.Absolute horizontal movement magnitudes are uniformly distributed.It is clear that all of the monitored stations are oriented in the same direction, which is northeast.The obtained results are in a good agreement with the computed results of horizontal movements of Egypt (Pietrantonio et al., 2016).The regional trends along the Delta were estimatedusing the model of the Global Kinematic (DeMet et al., 1990) and it is necessary to remove the regional trend in order to detect any possible local velocity.
Secondly,Euler pole parameters have been used to estimate the local horizontal velocities.Therefore,we may deduce that the Delta is not evenly uniformed and is made up of smaller blocks based on the local horizontal velocities.The velocity parameters of the GPS network are summarized in Table 1.Vertical velocity is a tool for determining the rate of Nile Delta subsidence and uplift since it can be used to assess that.The northern part of the Nile Delta shows obvious subsidence, which is more clearin the northeastern part, as shown by DAMT, AGRM, HAML and SAID station (average value -4.53 mm/yr), and the northwestern part, as shown by BORG, DAMN and EDFN (average value -3.31 mm/yr) (Fig. 4).The central part of the delta is showing the highest rate of subsidence (average value -9.10 mm/yr) recorded at TANT and MNSR stations.However, little uplift is visible in the Deltasurrounding areas, as measured by MSLT and KATA stations (average value 0.32 mm/yr).
From Fig. 5, it is noticed that the hinge zone represents a transitional zone between thecontinental shelf which shows subsidence in the north and unstable shelf showing uplift in the south of the hinge zone.On the other side and due to increasing oil fields production in Nile Delta and surrounding areas such as Tuna, Denise, Temsah, Karous , Port Foad and Northeast delta wells, (Fig. 6),we can say that there is some relationship between increasing in oil production and subsidence rates in the crust together.So, we can conclude that the oil and gas production, especially at northern east andcenter part of the Nile Delta effect on the subsidence rate as in Mansoura , Port-said and Tanta.So, we recommend that the linked between these studies are needed for better understanding local and regional deformation in the Delta.

Strain Parameters Estimation
The strain rate tensors have great importance in tectonic and geodynamic studies, the fault strain accumulation, uplift or subsidence, the dilatation strain rate, or the maximum shear strain rate, including its direction, are all crucial factors for seismic hazard assessment provided by tensor analysis (Goudarzi et al., 2014).The dilatation rates of the considered region are displayed in Fig. 7, the extension force is influential in the northwestern and southern parts of the research region, shows positive values varying from 0.1 to 38.9 Nano-Strain, while the compression force is influential in the northeastern section and shows negative values varying from -0.6 to -72.3 Nano-Strain.The studied region is divided between three zones of dilatation, one negative dilatation zone and two positive zones, which are separated by tectonic characteristics and structure (Fig. 8), the area of study is situated between three moderately active areas (high in the northwest, medium in the south, and low in the middle), display shear strain values varying from 0.1 to 34 Nano -Strain on average, indicating that the majority of the investigation area is characterized by a moderate shear strain and the risk of occurrence an earthquake is may low.(Fig. 9) depicts the distribution of the major axes of strains for the region, the majority of compression and extension forces are limited in the Nile Delta area, with extension forces increasing towards the northwest and decreasing in the south, while compression forces are concentrated in the northeast.While the center part of the Delta shows little deformation.

Conclusions
The main finding of this study is to look at the geodynamical aspects of the Nile Delta by calculating horizontal and vertical movement components as well as strain variables factors like principal axis strain, dilatation, and maximum shear strain.The horizontal absolute movements are evenly distributed.It is visible that all of the observed stations are moving in the same direction, which is northeast, in agreement with the Nubian plate motion.Regarding the horizontal local movements in Northing, it ranges from -0.7 mm/yr to 2.26 mm/yr, while in Easting, it ranges from -2.3 mm/yr to -0.90 mm/yr.About vertical movements, the northern section of the Nile Delta is subsiding, while the northeastern part has a faster rate of subsidence than the northwestern part as shown by SAID, DAMT, HAML and AGRM stations (approximately -2.1 to -6.7 mm/yr).The northern west part has a subsidence rate of approximately -1.5 to -4.9 mm/yr as shown by EDFN, DAMN and BORG.Also, the center section is subsiding with a rate -9.1 mm/yr recorded at TANT and MNSR stations.However, low uplifting values are present in the surrounding areas of Nile Delta as measured at MSLT and KATA stations (about 0.23 to 0.42 mm/yr).The subsidence rate at Mansoura point shows high values relative to the other points which may relate to the gas production in this area.Furthermore, according to the strain values, the most compression and extension forces are confined and characterized by medium levels of shear strain, meaning that the possibility of an earthquake occurrence is minimal.This work may be considered as critical in understanding the current status of deformation parameters in the Nile Delta, as well as the details of horizontal and vertical movements.Also, give helpful information to decision-makers in supporting of sustainable development and new projects.

Fig. 3 .
Fig. 3. Locations of the Nile Delta GNSS network

Table 1 .
The horizontal and vertical velocities factors of GPS network