الفهرس 
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Abstract
The importance of deformation studies using geodetic methods for earthquake prediction has been realize for along time. Different geodetic techniques have been used to measure crustal deformation. A recent precise geodetic tool used in crustal deformation is the global positioning system GPS. There are other auxiliary data which could be relevant to the network`s behaviour such as gravity and seismic activity and temporal gravity variations could be lead to earthquake predictionNorth-eastern egypt, including the greater cairo has been the site of a number of earthquakes. The greater cairo region has been suffering from disastrous earthquake since B.S 2200 and might again be subjected to some earthquakes shaking in the future.
For this reason, after the accurrence of october 12.1992 earthquake with magnitude 5.9 on richter scale at dahshour area, 30km southwest of the center greater cairo city, the national research institute of astronomy and geophysics in egypt proposed geophysical and geodetic investigations programs to study the area at the epicenter in that time and around greater cairo region after that. In 1995 an optimal GPS geodetic network consisting of eleven geodetic stations was established in and around greater cairo for monitoring local crustal deformation to investigate the possible association between the crustal deformation and earthquake activity within the area under study.
The intial measurements were carried out in april-may 1996 and the measurements were repeated on october, 1997; july 1998; june 1999; january 2000respectively.The field works were carried out by using trimble receivers 4000SSI. The accuracy of cairo GPS geodetic network was investigated by using precise ephemerides and tropospheric model were applied in the final calculation of the baselines. Also, the data were processed by using Iono free fixed solution.These solutions contain no ionospheric biases, and fixed ambiguities give the best overall results.Longer occupation times increase the amount of data, this can help reduce multipath and effect od cycles slips.
The base station of the network is helwan station, its WGS-84 coordinates are determined from the coordinates of the satellite laser ranging station and nearest global GPS permanent stations.
For more complete study, relative gravity measurements for all geodetic stations have been made from 1996 to 2000 using G-type lacosta and romberg gravimeters every two years.We used the closing loop method for the gravity surveys. Gravity values of the geodetic points were determined relative to the first order gravity station in helwan.Also, the changes of the height of the GPS stations are investigated.
The average amplitude of the gravity changes between the epochs 1996 and 2000 Gal.
is approximately 22 ±3.5 μ
Gravity measurements proved the existence of considerable temporal variations of gravity.These non-tidal changes could be explained by dynamic processess with in the upper crust related to local and regional crustal deformations.
The annual rates of the displacement vectors at each GPS station are obtained during the all observation period.The magnitudes of the horizontal velocity vary between 2 and 6.4 mm/ year, with an average value of 3.4 ±1.5 mm/year towards northweat - southeast direction.
The network was divided into five blocks in which we assume homogenous plane strain. The principal strain rates, and also annual rate of maximum shear strain and dilatation in the five blocks throughout the periods from april,1996 to june, 1999 and january, 2000 are determined.