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Implications of terrain movements in Egypt
JOURNAL OF GEODYNAMICS10, 73-83 (1988)
73
IMPLICATIONS OF TERRAIN M O V E M E N T S IN EGYPT
MOHAMED M. NASSAR
Faculty of Engineering, Ain Shams University, Cairo, Egypt (Accepted July 18, 1988)
ABSTRACT Nassar, M. M., 1988. Implications of terrain movements in Egypt. Journal of Geodynamics, I0: 73-83. The purpose of this paper is to study the causes of localized terrain movements in Egypt, The motivation behind this research has been the vast progress in constructing huge engineering structures (dams, bridges, tall buildings, etc.) as well as extending the urban activities in many new cities. These must be properly studied to ensure their safety versus their cost and other economic factors. In addition, the recent tendency is towards building nuclear power stations whose locations must be carefully investigated against the hazard and danger of inevitable atomic leakage, especially in the case of seismically active regions. Also the discovery of new oil wells and mines and the effects of future depletion require considerable attention from qualified investigators. The relative tectonic movements of North Africa and Southern Europe, the seismic activities around the Alexandria region, the presence of faults related to the region of the High Dam and its reservoir in Aswan, the erosion of the banks of the River Nile and its islands as well as coastal lines along the Mediterranean and the Red Sea, and the deformation and damage to large buildings in the Cairo area are examined here as a few examples of the implications of the earth's deformations within Egyptian territory. Strong recommendations are made concerning the necessity of studying and monitoring the terrain movements in the areas where new cities, large engineering constructions and power plants are planned to be erected.
1. CAUSES OF LOCAL TERRAIN MOVEMENTS
There are many causes of local terrain movements that can be classified into two groups, natural and artificial (man made). Natural causes include tidal effects, atmospheric effects and erosion and deposition caused by water action. The artificial causes include every addition or removal of heavy loads on or within the earth's crust.
0264-3707/88/$3.00
© 1988 Pergamon Press plc.
74
NASSAR
1.1. Natural Causes (1) Tidal efjbcts. By tidal effects we mean the deformations caused by both earth tides and water tides. In the case of earth tides, the deformation phenomenon is due to variations in the gravitational force exerted by celestial bodies; where the corresponding force is known as the tidal force. At any point within, or on the surface, of the earth, the gravitational force exerted by a celestial body (say the moon) can be split into two components [Vanicek and Krakiwsky, 1982]: first that equalling the gravitational force acting at the centre of gravity of the earth, and second, that equalling the remainder, Fig. (1). As a whole, it can be seen from Fig. tl ) that the tidal force tries to deform the equipotential surfaces of the earth's gravity field so that its shape prolates in the direction of the celestial body. More precisely, the shapes will be elongated in the direction of the resultant force exerted by the configuration of all the celestial bodies (sun and other bodies). From astronomical observations it was found that, on average, the solar potential is about 46% of the lunar potential. Contributions from other celestial bodies are much smaller. Although the actual deformations due to the earth tides are very small, they are essential for precise geodetic measurements and must be considered depending on the local point of interest.
MOON J
\
I
Fig. I.
Tidal force due to the moon.
T E R R A I N M O V E M E N T S IN E G Y P T
75
Water (disregarding for the moment its inhomogeneity) is constantly adjusting its level to coincide with an equipotential surface and therefore follows the movements of the equipotential surface in response to the above mentioned tidal force. Because the bottom slopes toward the shore, the incoming tidal water is trapped and pitches up along the shoreline. Only small islands, rising steeply from the sea floor, are free from this effect. This piling effect evidently is more pronounced in confined and shallow areas, like bays and straits (for instance Suez and A1-Akaba bays in our case) where the water tide can be magnified several times. Sea tides may amount to 10 m in some cases which, along with its variations, will certainly cause the earth to deform especially along shore lines I-Dahlen, 1976; Van Hglchama, 1965]. (2) Deposition and erosion. Deposition occurs in some areas along large rivers (e.g. River Nile) which will be combined with erosion in other areas. Deposits of solid particles in sedimentary basins make an additional load on the surface of the earth. Any such load produces local vertical deformation of the crust. It should be clear that a load at one point on the surface of the earth will cause the crust to yield not only immediately underneath the load but also in the surrounding area because of the lithosphere. The subsidence will be the largest immediately under the load and will gradually diminish with distance from the load [Bradshaw et al., 1979]. Natural removal (e.g. erosion) of any significant load produces an uplift. A well known example is the rebound observed around the salt lakes along the north coast of Egypt after the evaporation of water. (3) Atmospheric effects. These are due to variations in metereological parameters. However the principal factor is the prevailing wind which affects the water currents, causing deposits and erosions along waterways and also affecting the sandstone layers covering the earth's surface. In the latter case, the wind currents erode and remove sand grains from some places and deposit them as sand dunes in other places depending on the direction and power of the wind. The height of such dunes may go up over 30 m as is the case in Egypt, which causes many practical problems in addition to its influence as a huge deforming load on the earth's crust [Stolz and Larden, 1979]. 1.2. Artificial (man-made) causes There are generally three man-made causes: Depletion of liquids and minerals; building huge engineering constructions, and experimental nuclear explosions: (1) Depletion of liquids and minerals. This includes the depletion of the
76
NASSAR
ground water (e.g. in some provinces surrounding Cairo and Alexandria), and oil and minerals (e.g., in Sinai and along the Red Sea), leading to mass changes. The observed gravity changes do indicate mass changes and associated deformations. (2) Building huge engineering constructions. This includes heavy buildings and bridges (e.g. in Cairo), dams with large reservoirs (e.g. Aswan Dam and the High Dam), urban developments in new cities (e.g. October 6 and Ramadan 10), new harbours (e.g. Domyat harbour), long pipe lines (e.g. Somed pipe line between Swiss and Alexandria), tunnels (e.g. underground Metro in Cairo and traffic tunnels undrneath the Suez Canal), artificial salt basins (e.g. Alexandria and Port Said), etc. The terrain movement here depends on the amount of the load and the properties of the soil which these works are based on. The terrain movement depends on the type of soil, loose or dense, cohesive (silt and clay) or uncohesive (aggregate, sand), and organic or inorganic. (3) Nuclear explosions. [Davis, 1972] The aftershocks of nuclear explosion are phenomena similar to large shallow earthquakes. In the case of an earthquake, the readjustment of rocks in the vicinity of a fault is unlikely to completely release the stored elastic energy with the first catastrophic break. However, once a new rupture has been opened it is likely that successive earthquakes will complete the process. Generally the first shock has to be of magnitude of 6 or more for after-shock to occur. The magnitude of the largest after-shock is at least one unit less than the main shock and the activity dies away within weeks. Rather similar rules seem to apply after very large underground nuclear explosions. About five tests with magnitudes greater than 6 have led to some activity, but no earthquake clearly associated with a test has been recorded farther than 40 km from the test site. A megaton test in the Aleutians also triggered some relatively small activity. The energy released was probably local and stored by the explosion itself rather than tectonic on the large scale.
2. EVIDENCE O F T E R R A I N D E F O R M A T I O N S IN E G Y P T
2.1. Earthquake activity in Alexandria and the area surrounding it It has been found [Kebeasy et al., 1981] that the Alexandria region in the northwest of Egypt, has shown earthquake activity for a long period of time. All the available information about historical, recent and microearthquakes (about 130) that took place until 1978 was collected and investigated. The epicentre distribution pattern is depicted in Fig. 2 from which it can be seen that the seismotectonic trend is mainly NW and SE.
TERRAIN MOVEMENTS IN EGYPT
77
.o
6
Abu Hamme,
;0 o
Kin 218E
31
219 LEGEND 4.5 - 4.9
5-5.4
5.5-5.9
/>6
<4.5
historical 2200B.C.-1908 recent 1909-1978 Fig. 2.
o
o
0
0
micro earthquake 1958-78
Epicenter distribution of all earthquakes around Alexandria.
Also there is a seismic active area in the neighbourhood of Egypt in Libya that has a significant effect on the northeast region of Africa in which we are living [Kebeasy, 1982-1. For an area of relatively rapid development, like Alexandria, consideration of possible earthquake effects becomes important. It is well known that the northwest extensions of the Egyptian horizontal and vertical control networks of the first order pass through the Alexandria region up to the western border. If geodesists wish to maintain the required accuracy of their leveling networks in this region they have to acquire a knowledge of
78
NASSAR
the terrain movements of the area so that they can adopt a suitable policy on releveling. The same holds true for the case of the geographical coordinates of the horizontal control nets. On the other hand, through the investigation of horizontal and vertical movements, the three-dimensional representation of terrestrial points can be achieved and the dynamic treatment of geodetic networks can be adopted [Nassar, 1981 ]. 2.2. Aswan fault and cracks around Aswan Lake Aswan (or Nasser's) Lake is the second largest man-made lake in the world, extending from the southern part of Egypt (South of the High Dam) to the northern part of Sudan. Filling started in 1964 and reached a maximum water level of about 177 m in November 1978. Since then the water level has fluctuated between 171 m and 177 m. On November 14, 1981 an earthquake took place in Aswan lake region [Kebeasy et al., 1982]. This earthquake was preceded by three main preshocks and followed by a large number of post-shocks. Several cracks on the west bank of the lake and several rock falls and minor cracks on the west bank were found. The hatched area in Fig. (3) shows the location of these effects. The largest of these cracks is about 1 m wide and 20 km long. The earthquake was felt to a distance of about 400 km from Aswan, and there was minor damage to old buildings in Aswan itself. The cause of this earthquake may be attributed to tectonic activity and the fluctuations in the lake water level. Periodical geodetics measurements for detecting the movements in the area around the lake are essential to predict the future implications of this event. In fact joint research between the Egyptian Academy of Science and Helwan Institute of Astronomy and Geophysic with some participation from interested university members is now underway. Hence, it goes without saying that the maintenance of geodetic networks in the region of Aswan has become a necessity for safety, economy and related considerations. 2.3. Deformations of tall buildings in Cairo Over the last two years or so, specially in Cairo, tall buildings (or towers) have been erected in different locations. Significant deformations in such buildings (e.g. Dokky and Zamalek towers), monitored by the author and his associates, were found to be due to terrain movement in the immediate vicinity. If the rate of uplift or subsidence in an area is known, engineers can adopt means for preventing damage. In the same sense, if the sinking of
TERRAIN MOVEMENTS IN EGYPT
•1
79
°I
N31E
32
-I
~
~3
N m
25
25 Kum 0 m b o
High Dam
2g
24
j' I r
...
-.
:;-
.
.,,-.
Abu
J[1.°l
:']E
32"1
33"1
Fig. 3. Location of Aswan Lake. The hatched area is that of pronounced effect of the earthquake of 14 November 1981.
cities can be detected, must be done before, collected information taken into account in
corrective measures can be adopted. This monitoring during and after executing the construction, and the about the associated terrain movements must be the design and protection of the construction.
2.4. Erosion of shorelines of the River Nile and its islands It has been recognized that the erosion of sea shores and the banks of rivers is an important factor which must be considered when planning development in shore zones. Recently, the Egyptian Ministry of Irrigation has established a research institute for shore protection [All et al., 1983]. Its main duties are to measure the erosion of the Mediterranean Sea and the River Nile shores and to implement measures to minimize it. JOG
10/1-6
80
NASSAR
For Sabha island and the shores of the River Nile near the City of Edfo in Southern Egypt Fig. (4), aerial photography shows that the shape and the size of this island has changed significantly. The island lost 17% of its area over a period of 22 years. Erosion took place at the east side of the island. At some places more than 200 m of land width were lost. Sedimentation took place at the west side, especially near the bridge. Significant change in the shape of the island also occurred in the north. Several erosion and sedimentation zones along the banks of the river itself are also visible. Many studies have been made for the north shores of the country, especially for the province of Domyat where a new harbour is being constructed. Similar studies of erosion of sea shore are required at Port Said for the harbour which will be constructed there. It is worthwhile mentioning that the erosion of the Mediterranean shore line of Egypt is very significant (in the order of 50 m over a few tens of years). The location of the Domyat to Port Said road was changed in the last few years.
EDFO CENTER
it"" J Fig. 4.
J
Geographic location of Sabha island.
TERRAIN MOVEMENTS IN EGYPT
81
2.5. Hazard o f atomic power stations Local terrain movements must be monitored over a reasonably long period of time before proceeding to construct atomic power stations. It is very essential to determine the rate of movement and to take it into consideration in the design of such important structures to prevent the hazard associated with atomic radiation or leakage due to unexpected deformations. An atomic power station is planned in the area of Sydi Kirair near Marsa Matrouh west of Alexandria. This area is seismically active, and a preliminary study suggested that this location should be changed. Another place in the western desert of Egypt is being considered [Habib, 1984]. It is strongly recommended that a complete study for terrain movements in the new area must be performed before taking the final decision concerning this power station.
3. SUMMARY AND RECOMMENDATIONS
The improvements in and discoveries concerning sensitive seismic instruments allow us to know most of the positions of active seismic zones. The main reasons for regional terrain movements are plate tectonics, volcanoes and earthquakes. The natural causes of local terrain movements are earth tides, atmospheric effects, and deposition and erosion. The manmade causes are depletion of liquids and minerals, construction of huge buildings and nuclear explosions. The most important evidence in Egypt includes earthquake activity in Alexandria, faults and cracks around Aswan lake, damage to engineering constructions, and erosion and deposition along shorelines. The following recommendations are made: Egypt should participate in the tectonics studies proposed by NASA, Germany, and the competent international and African organizations. See Egyptian Academy of Science, 1982; Wassef, 1981. The study of seismic zones must be enlarged by extending the seismic station networks as well as installing up-to-date instruments, especially in the apparently active seimic areas, for instance in the Alexandria and Aswan regions. Seismic data must be considered in the process of designing large engineering constructions in the area of the existing fault south of Aswan, and the associated implications must be studied by a geodesy group of one or more of the Egyptian universities using precise geodetic terrestrial techniques after the current preliminary investigation are completed.
82
NASSAR
A complete study and analysis of terrain movements must precede the construction of dangerous engineering structures like the planned atomic power station in the western desert of Egypt. Egyptian geologists, geophysists and geodesists must be encouraged to form a working team or group supervising research projects connected with the detection of regional and local terrain movements. This can be easily organized in Egypt, for instance through the Egyptian Academy of Scientific Research and Technology. All Egyptian engineering faculties and institutions offering advanced geodetic surveying courses within their curriculum must update these courses to include the fundamentals of geophysics and advanced construction surveying techniques and instrumentation in order to qualify their graduates to engage in terrain movement studies and deformation measurement. ACK NOWLEDGM ENT
The author would like to express his thanks and appreciation to his assistant Eng. M . F . E1-Maghraby for his help in proof reading the manuscript of this paper as well as drafting the illustrations. Thanks also go to Mr. M. Abd-Allah for his efficient typing of this paper. REFERENCES Ali, M. E. and Soliman, S. A., 1983. Use of Aerial photography for the measurement of erosion of the sides of the River Nile. Presented at the fifth regional cartographic conference for Africa, 28 Feb.-7 March, 1983 held in Cairo, Egypt. Bradshaw, M.J., Abbott, A.J. and Gelsthorpe, A. P., 1979. The earth's changing surface. English Language Book Service, England. Dahten, F. A., 1976. The passive influence of the oceans upon the rotation of the earth. Geophys. J. R. Astron. Soc., 46. Davis, D., 1972. Nuclear Explosions. In Understanding the Earth, MIT Press, U.S.A. Egyptian Academy of Science, 1982. Resolutions of the joint meeting between the geodesy and seismology sub~zommittees of the Earth physics and measures, concerning the NASA project of tectonic studies between North Africa and South Europe. Helded in Cairo on March 28, 1982. Habib, A. A., 1984. Personal communication with the author. Kebeasy, R. M., 1981. Seismicity and seismotectonics of Libya. Dept. of seismology, Helwan Institute of Geophysics and Astronomy, Bulletin of II SEE, Vol. 19 (1981). Kebeasy, R. M., Maamoun, M., Albert, R. N. H. and Megahed, M., 1981. Earthquake activity and earthquake risk around Alexandria, Egypt. Dept. of seismology, Helwan Institute of Geophysics and Astronomy, Bulletin of If SEE, Vol. 19 (1981) Kebeasy, R. M., Maamoun, M. and Ibrahim, E. M., 1982. Aswan lake induced earthquakes. Dept. of seismology, Helwan Institute of Geophysics and Astronomy, Cairo, Egypl. Nassar, M.M., 1981. Merits of artificial satellites for the Egyptian geodetic community. Scientific Bulletin of Civil Eng., Faculty of Eng., El-Azhar University, October 1981, Cairo, Egypt.
TERRAIN MOVEMENTS IN EGYPT
83
Stolz, A., and Larden, D.R., 1979. Seasonal displacement and deformation of the earth by the atmosphere. Journal Geophysics Research 84. Van Hglchama, T. E. A., 1965. The water balance of the earth. Published in Climatol 9-59-110 Drexl Institute of Technology, Philadelphia, PA. Vanicek, P. and Krakiwsky, E. J., 1982. Geodesy the concepts. North Holland Publishing Company the Northerland. Wassef, A. M., 1981. First international symposium on crustal movements in Africa 6-16 May 1981, UNECA, ADDIS ABABA. Chairman, Commission of recent crustal movement (AFRICA).
73
IMPLICATIONS OF TERRAIN M O V E M E N T S IN EGYPT
MOHAMED M. NASSAR
Faculty of Engineering, Ain Shams University, Cairo, Egypt (Accepted July 18, 1988)
ABSTRACT Nassar, M. M., 1988. Implications of terrain movements in Egypt. Journal of Geodynamics, I0: 73-83. The purpose of this paper is to study the causes of localized terrain movements in Egypt, The motivation behind this research has been the vast progress in constructing huge engineering structures (dams, bridges, tall buildings, etc.) as well as extending the urban activities in many new cities. These must be properly studied to ensure their safety versus their cost and other economic factors. In addition, the recent tendency is towards building nuclear power stations whose locations must be carefully investigated against the hazard and danger of inevitable atomic leakage, especially in the case of seismically active regions. Also the discovery of new oil wells and mines and the effects of future depletion require considerable attention from qualified investigators. The relative tectonic movements of North Africa and Southern Europe, the seismic activities around the Alexandria region, the presence of faults related to the region of the High Dam and its reservoir in Aswan, the erosion of the banks of the River Nile and its islands as well as coastal lines along the Mediterranean and the Red Sea, and the deformation and damage to large buildings in the Cairo area are examined here as a few examples of the implications of the earth's deformations within Egyptian territory. Strong recommendations are made concerning the necessity of studying and monitoring the terrain movements in the areas where new cities, large engineering constructions and power plants are planned to be erected.
1. CAUSES OF LOCAL TERRAIN MOVEMENTS
There are many causes of local terrain movements that can be classified into two groups, natural and artificial (man made). Natural causes include tidal effects, atmospheric effects and erosion and deposition caused by water action. The artificial causes include every addition or removal of heavy loads on or within the earth's crust.
0264-3707/88/$3.00
© 1988 Pergamon Press plc.
74
NASSAR
1.1. Natural Causes (1) Tidal efjbcts. By tidal effects we mean the deformations caused by both earth tides and water tides. In the case of earth tides, the deformation phenomenon is due to variations in the gravitational force exerted by celestial bodies; where the corresponding force is known as the tidal force. At any point within, or on the surface, of the earth, the gravitational force exerted by a celestial body (say the moon) can be split into two components [Vanicek and Krakiwsky, 1982]: first that equalling the gravitational force acting at the centre of gravity of the earth, and second, that equalling the remainder, Fig. (1). As a whole, it can be seen from Fig. tl ) that the tidal force tries to deform the equipotential surfaces of the earth's gravity field so that its shape prolates in the direction of the celestial body. More precisely, the shapes will be elongated in the direction of the resultant force exerted by the configuration of all the celestial bodies (sun and other bodies). From astronomical observations it was found that, on average, the solar potential is about 46% of the lunar potential. Contributions from other celestial bodies are much smaller. Although the actual deformations due to the earth tides are very small, they are essential for precise geodetic measurements and must be considered depending on the local point of interest.
MOON J
\
I
Fig. I.
Tidal force due to the moon.
T E R R A I N M O V E M E N T S IN E G Y P T
75
Water (disregarding for the moment its inhomogeneity) is constantly adjusting its level to coincide with an equipotential surface and therefore follows the movements of the equipotential surface in response to the above mentioned tidal force. Because the bottom slopes toward the shore, the incoming tidal water is trapped and pitches up along the shoreline. Only small islands, rising steeply from the sea floor, are free from this effect. This piling effect evidently is more pronounced in confined and shallow areas, like bays and straits (for instance Suez and A1-Akaba bays in our case) where the water tide can be magnified several times. Sea tides may amount to 10 m in some cases which, along with its variations, will certainly cause the earth to deform especially along shore lines I-Dahlen, 1976; Van Hglchama, 1965]. (2) Deposition and erosion. Deposition occurs in some areas along large rivers (e.g. River Nile) which will be combined with erosion in other areas. Deposits of solid particles in sedimentary basins make an additional load on the surface of the earth. Any such load produces local vertical deformation of the crust. It should be clear that a load at one point on the surface of the earth will cause the crust to yield not only immediately underneath the load but also in the surrounding area because of the lithosphere. The subsidence will be the largest immediately under the load and will gradually diminish with distance from the load [Bradshaw et al., 1979]. Natural removal (e.g. erosion) of any significant load produces an uplift. A well known example is the rebound observed around the salt lakes along the north coast of Egypt after the evaporation of water. (3) Atmospheric effects. These are due to variations in metereological parameters. However the principal factor is the prevailing wind which affects the water currents, causing deposits and erosions along waterways and also affecting the sandstone layers covering the earth's surface. In the latter case, the wind currents erode and remove sand grains from some places and deposit them as sand dunes in other places depending on the direction and power of the wind. The height of such dunes may go up over 30 m as is the case in Egypt, which causes many practical problems in addition to its influence as a huge deforming load on the earth's crust [Stolz and Larden, 1979]. 1.2. Artificial (man-made) causes There are generally three man-made causes: Depletion of liquids and minerals; building huge engineering constructions, and experimental nuclear explosions: (1) Depletion of liquids and minerals. This includes the depletion of the
76
NASSAR
ground water (e.g. in some provinces surrounding Cairo and Alexandria), and oil and minerals (e.g., in Sinai and along the Red Sea), leading to mass changes. The observed gravity changes do indicate mass changes and associated deformations. (2) Building huge engineering constructions. This includes heavy buildings and bridges (e.g. in Cairo), dams with large reservoirs (e.g. Aswan Dam and the High Dam), urban developments in new cities (e.g. October 6 and Ramadan 10), new harbours (e.g. Domyat harbour), long pipe lines (e.g. Somed pipe line between Swiss and Alexandria), tunnels (e.g. underground Metro in Cairo and traffic tunnels undrneath the Suez Canal), artificial salt basins (e.g. Alexandria and Port Said), etc. The terrain movement here depends on the amount of the load and the properties of the soil which these works are based on. The terrain movement depends on the type of soil, loose or dense, cohesive (silt and clay) or uncohesive (aggregate, sand), and organic or inorganic. (3) Nuclear explosions. [Davis, 1972] The aftershocks of nuclear explosion are phenomena similar to large shallow earthquakes. In the case of an earthquake, the readjustment of rocks in the vicinity of a fault is unlikely to completely release the stored elastic energy with the first catastrophic break. However, once a new rupture has been opened it is likely that successive earthquakes will complete the process. Generally the first shock has to be of magnitude of 6 or more for after-shock to occur. The magnitude of the largest after-shock is at least one unit less than the main shock and the activity dies away within weeks. Rather similar rules seem to apply after very large underground nuclear explosions. About five tests with magnitudes greater than 6 have led to some activity, but no earthquake clearly associated with a test has been recorded farther than 40 km from the test site. A megaton test in the Aleutians also triggered some relatively small activity. The energy released was probably local and stored by the explosion itself rather than tectonic on the large scale.
2. EVIDENCE O F T E R R A I N D E F O R M A T I O N S IN E G Y P T
2.1. Earthquake activity in Alexandria and the area surrounding it It has been found [Kebeasy et al., 1981] that the Alexandria region in the northwest of Egypt, has shown earthquake activity for a long period of time. All the available information about historical, recent and microearthquakes (about 130) that took place until 1978 was collected and investigated. The epicentre distribution pattern is depicted in Fig. 2 from which it can be seen that the seismotectonic trend is mainly NW and SE.
TERRAIN MOVEMENTS IN EGYPT
77
.o
6
Abu Hamme,
;0 o
Kin 218E
31
219 LEGEND 4.5 - 4.9
5-5.4
5.5-5.9
/>6
<4.5
historical 2200B.C.-1908 recent 1909-1978 Fig. 2.
o
o
0
0
micro earthquake 1958-78
Epicenter distribution of all earthquakes around Alexandria.
Also there is a seismic active area in the neighbourhood of Egypt in Libya that has a significant effect on the northeast region of Africa in which we are living [Kebeasy, 1982-1. For an area of relatively rapid development, like Alexandria, consideration of possible earthquake effects becomes important. It is well known that the northwest extensions of the Egyptian horizontal and vertical control networks of the first order pass through the Alexandria region up to the western border. If geodesists wish to maintain the required accuracy of their leveling networks in this region they have to acquire a knowledge of
78
NASSAR
the terrain movements of the area so that they can adopt a suitable policy on releveling. The same holds true for the case of the geographical coordinates of the horizontal control nets. On the other hand, through the investigation of horizontal and vertical movements, the three-dimensional representation of terrestrial points can be achieved and the dynamic treatment of geodetic networks can be adopted [Nassar, 1981 ]. 2.2. Aswan fault and cracks around Aswan Lake Aswan (or Nasser's) Lake is the second largest man-made lake in the world, extending from the southern part of Egypt (South of the High Dam) to the northern part of Sudan. Filling started in 1964 and reached a maximum water level of about 177 m in November 1978. Since then the water level has fluctuated between 171 m and 177 m. On November 14, 1981 an earthquake took place in Aswan lake region [Kebeasy et al., 1982]. This earthquake was preceded by three main preshocks and followed by a large number of post-shocks. Several cracks on the west bank of the lake and several rock falls and minor cracks on the west bank were found. The hatched area in Fig. (3) shows the location of these effects. The largest of these cracks is about 1 m wide and 20 km long. The earthquake was felt to a distance of about 400 km from Aswan, and there was minor damage to old buildings in Aswan itself. The cause of this earthquake may be attributed to tectonic activity and the fluctuations in the lake water level. Periodical geodetics measurements for detecting the movements in the area around the lake are essential to predict the future implications of this event. In fact joint research between the Egyptian Academy of Science and Helwan Institute of Astronomy and Geophysic with some participation from interested university members is now underway. Hence, it goes without saying that the maintenance of geodetic networks in the region of Aswan has become a necessity for safety, economy and related considerations. 2.3. Deformations of tall buildings in Cairo Over the last two years or so, specially in Cairo, tall buildings (or towers) have been erected in different locations. Significant deformations in such buildings (e.g. Dokky and Zamalek towers), monitored by the author and his associates, were found to be due to terrain movement in the immediate vicinity. If the rate of uplift or subsidence in an area is known, engineers can adopt means for preventing damage. In the same sense, if the sinking of
TERRAIN MOVEMENTS IN EGYPT
•1
79
°I
N31E
32
-I
~
~3
N m
25
25 Kum 0 m b o
High Dam
2g
24
j' I r
...
-.
:;-
.
.,,-.
Abu
J[1.°l
:']E
32"1
33"1
Fig. 3. Location of Aswan Lake. The hatched area is that of pronounced effect of the earthquake of 14 November 1981.
cities can be detected, must be done before, collected information taken into account in
corrective measures can be adopted. This monitoring during and after executing the construction, and the about the associated terrain movements must be the design and protection of the construction.
2.4. Erosion of shorelines of the River Nile and its islands It has been recognized that the erosion of sea shores and the banks of rivers is an important factor which must be considered when planning development in shore zones. Recently, the Egyptian Ministry of Irrigation has established a research institute for shore protection [All et al., 1983]. Its main duties are to measure the erosion of the Mediterranean Sea and the River Nile shores and to implement measures to minimize it. JOG
10/1-6
80
NASSAR
For Sabha island and the shores of the River Nile near the City of Edfo in Southern Egypt Fig. (4), aerial photography shows that the shape and the size of this island has changed significantly. The island lost 17% of its area over a period of 22 years. Erosion took place at the east side of the island. At some places more than 200 m of land width were lost. Sedimentation took place at the west side, especially near the bridge. Significant change in the shape of the island also occurred in the north. Several erosion and sedimentation zones along the banks of the river itself are also visible. Many studies have been made for the north shores of the country, especially for the province of Domyat where a new harbour is being constructed. Similar studies of erosion of sea shore are required at Port Said for the harbour which will be constructed there. It is worthwhile mentioning that the erosion of the Mediterranean shore line of Egypt is very significant (in the order of 50 m over a few tens of years). The location of the Domyat to Port Said road was changed in the last few years.
EDFO CENTER
it"" J Fig. 4.
J
Geographic location of Sabha island.
TERRAIN MOVEMENTS IN EGYPT
81
2.5. Hazard o f atomic power stations Local terrain movements must be monitored over a reasonably long period of time before proceeding to construct atomic power stations. It is very essential to determine the rate of movement and to take it into consideration in the design of such important structures to prevent the hazard associated with atomic radiation or leakage due to unexpected deformations. An atomic power station is planned in the area of Sydi Kirair near Marsa Matrouh west of Alexandria. This area is seismically active, and a preliminary study suggested that this location should be changed. Another place in the western desert of Egypt is being considered [Habib, 1984]. It is strongly recommended that a complete study for terrain movements in the new area must be performed before taking the final decision concerning this power station.
3. SUMMARY AND RECOMMENDATIONS
The improvements in and discoveries concerning sensitive seismic instruments allow us to know most of the positions of active seismic zones. The main reasons for regional terrain movements are plate tectonics, volcanoes and earthquakes. The natural causes of local terrain movements are earth tides, atmospheric effects, and deposition and erosion. The manmade causes are depletion of liquids and minerals, construction of huge buildings and nuclear explosions. The most important evidence in Egypt includes earthquake activity in Alexandria, faults and cracks around Aswan lake, damage to engineering constructions, and erosion and deposition along shorelines. The following recommendations are made: Egypt should participate in the tectonics studies proposed by NASA, Germany, and the competent international and African organizations. See Egyptian Academy of Science, 1982; Wassef, 1981. The study of seismic zones must be enlarged by extending the seismic station networks as well as installing up-to-date instruments, especially in the apparently active seimic areas, for instance in the Alexandria and Aswan regions. Seismic data must be considered in the process of designing large engineering constructions in the area of the existing fault south of Aswan, and the associated implications must be studied by a geodesy group of one or more of the Egyptian universities using precise geodetic terrestrial techniques after the current preliminary investigation are completed.
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NASSAR
A complete study and analysis of terrain movements must precede the construction of dangerous engineering structures like the planned atomic power station in the western desert of Egypt. Egyptian geologists, geophysists and geodesists must be encouraged to form a working team or group supervising research projects connected with the detection of regional and local terrain movements. This can be easily organized in Egypt, for instance through the Egyptian Academy of Scientific Research and Technology. All Egyptian engineering faculties and institutions offering advanced geodetic surveying courses within their curriculum must update these courses to include the fundamentals of geophysics and advanced construction surveying techniques and instrumentation in order to qualify their graduates to engage in terrain movement studies and deformation measurement. ACK NOWLEDGM ENT
The author would like to express his thanks and appreciation to his assistant Eng. M . F . E1-Maghraby for his help in proof reading the manuscript of this paper as well as drafting the illustrations. Thanks also go to Mr. M. Abd-Allah for his efficient typing of this paper. REFERENCES Ali, M. E. and Soliman, S. A., 1983. Use of Aerial photography for the measurement of erosion of the sides of the River Nile. Presented at the fifth regional cartographic conference for Africa, 28 Feb.-7 March, 1983 held in Cairo, Egypt. Bradshaw, M.J., Abbott, A.J. and Gelsthorpe, A. P., 1979. The earth's changing surface. English Language Book Service, England. Dahten, F. A., 1976. The passive influence of the oceans upon the rotation of the earth. Geophys. J. R. Astron. Soc., 46. Davis, D., 1972. Nuclear Explosions. In Understanding the Earth, MIT Press, U.S.A. Egyptian Academy of Science, 1982. Resolutions of the joint meeting between the geodesy and seismology sub~zommittees of the Earth physics and measures, concerning the NASA project of tectonic studies between North Africa and South Europe. Helded in Cairo on March 28, 1982. Habib, A. A., 1984. Personal communication with the author. Kebeasy, R. M., 1981. Seismicity and seismotectonics of Libya. Dept. of seismology, Helwan Institute of Geophysics and Astronomy, Bulletin of II SEE, Vol. 19 (1981). Kebeasy, R. M., Maamoun, M., Albert, R. N. H. and Megahed, M., 1981. Earthquake activity and earthquake risk around Alexandria, Egypt. Dept. of seismology, Helwan Institute of Geophysics and Astronomy, Bulletin of If SEE, Vol. 19 (1981) Kebeasy, R. M., Maamoun, M. and Ibrahim, E. M., 1982. Aswan lake induced earthquakes. Dept. of seismology, Helwan Institute of Geophysics and Astronomy, Cairo, Egypl. Nassar, M.M., 1981. Merits of artificial satellites for the Egyptian geodetic community. Scientific Bulletin of Civil Eng., Faculty of Eng., El-Azhar University, October 1981, Cairo, Egypt.
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Stolz, A., and Larden, D.R., 1979. Seasonal displacement and deformation of the earth by the atmosphere. Journal Geophysics Research 84. Van Hglchama, T. E. A., 1965. The water balance of the earth. Published in Climatol 9-59-110 Drexl Institute of Technology, Philadelphia, PA. Vanicek, P. and Krakiwsky, E. J., 1982. Geodesy the concepts. North Holland Publishing Company the Northerland. Wassef, A. M., 1981. First international symposium on crustal movements in Africa 6-16 May 1981, UNECA, ADDIS ABABA. Chairman, Commission of recent crustal movement (AFRICA).