ABSTRACT
To evaluate Paleo- and Recent seismic activity for the Central and South Eastern Desert of Egypt; seismological data were collected through the period 1906- 2007 and several geological criteria were observed such as fractures, folds, dykes, linear valleys, uplift and subsidence features, pressure ridges, sag ponds, cracks and joints, fault scarps, landslide and features of liquefaction. The earthquake hazard of the Central and South Eastern Desert of Egypt has been clarified by identification of zones of earthquakes for the area between latitudes 22° N & 27° N and longitude 32° to the coast of Red Sea.
The interpretation of the entire set of data ensured the recognition of the relationship between the recorded earthquakes and zones of lowest strength along folds and Paleo-fractures, especially at the end or bend of the fractures where two segments of major fractures meet. The distribution of earthquakes during this period revealed a collection of earthquakes around active faults such as Aswan area or subsurface magmatic activity such as Abu Dabbab region; in addition some scattered earthquakes were recorded due to reactivation of certain regions. The occurrence of geological events in central and southern Egypt is associated with structural trends started in Precambrian but were repeatedly reactivated.
INTRODUCTION
The geodynamics and sedimentation history of the Red Sea was studied by many authors. Purser and Philobbos (1993) studied the outcrops of rocks in the NW Red Sea area covered by a sedimentary section extending from Late Oligocene to Post Miocene. They emphasized the relation between tectonics and sedimentation. Ahmed et al. (1993) studied the sedimentary facies and depositional environments for the Quaternary sediments in NW Red Sea coast. Purser et al. (1993) described a variety of deformational structures for Neogene sediments in the NW Red Sea coast as formed as a result of multiple earthquake shocks. They summarized the types of deformations in the forms as liquefaction of soft sands, plastic folding and fracturing of slightly consolidated mud, sand and gravel and at last brecciation of lithified sediments. According to Garson and Krs (1976) several transverse fractures perpendicular to the Red Sea (Fig. 1) and linear magnetic anomalies parallel to the Red Sea are related to deep-seated dikes.
They added that there are shear zones related to left lateral movement along the Red Sea and a series of N 60 E faults which pass through the Ras Banas as well as a swarm of dikes in an area 5-10 km wide zone follows the fault direction with addition to a chain of ring structures related to this series. The faults which pass through the Red Sea are weakness areas and it is easy to release energy through these regions and are reactivated a gain in recent age, in turn observing of geological criteria along these areas.
Geophysical and geological studies of Aswan region in co-operation between the Geological Survey of Egypt and the United Nations Development Program (UNDP) showed a clear relationship between deep-seated block faulting and postulated transverse structures in the Red Sea (Garson and Krs 1976). The same authors postulated from geophysical and geological investigations that there are blocks bounded by transverse fractures oriented perpendicular to the Red Sea and linear anomalies parallel to the Red Sea due to deep-seated tholeitic dikes and shear zones related to left-lateral movement along the Red Sea. Meshref (1990) concluded from magnetic interpretation for the northern part of the Red Sea that there are six transform faults. The azimuth of the northernmost transform faults is about 25º. Ras Banas is bounded by two transform faults with azimuth 35º, where the motion of Arabia relative to Nubia is changed. Meshref presented a model explaining this change as being due to evolution of Gulf of Suez, Red Sea and Gulf of Aden from Oligocene to Late Miocene, the extension along Red Sea from Oligocene to Early Miocene resulting in the separation of Arabia from Nubia, by the Middle Miocene Gulf of Aden started to open with another extension. The resultant extension made new direction of motion between Arabia and Nubia in Ras Banas area, so recorded earthquakes are large numbers at Red Sea next to Ras Banas area due to the resultant structures, field trips were carried out to prove geological evidence for active and potential active criteria at this area.
The greatest earthquake effects occur generally near the fault rupture (Vittori et al. 1991), Seilacher (1969) introduced the term seismite as the deformation of sediment induced by strong earthquake. The present study is the reconnaissance work of active tectonic geomorphological criteria and Paleo-seismic evidence in the Eastern Desert of Egypt. The selected area is located between latitude 22° N to latitude 27° N and from the Red Sea coast to the River Nile to the west. The aim of tectonic geomorphology is to obtain information about the surface evidence of active faults (the weakness zone). Several geomorphic features were created in Egypt due to structural events since the Pan African time. In the Early Eocene time the Tethys covered all Egypt, during the Oligocene the Red Sea hills started to rise in East Egypt. A major uplift of the Red Sea hills took place during the Early Miocene. The mountains are dissected by faults and some of their planes are occupied by later dikes, faults developed many horsts and grabens.
PALEO-TECTONICS AND RECENT RECORDED EARTHQUAKES
Earthquakes do not occur at random, but their hypocenters follow certain trends reflecting the tectonic history of the region (Figs 2 and 3). Epicenters were plotted for the Eastern Desert of Egypt on geologic maps; they were found to represent two categories; the first is a collection of earthquakes around active faults such as Aswan area and subsurface magmatic activity (Abu Dabbab area), the second follows Paleo-tectonic fractures at places characterized by the following:
1- Heterogeneous medium and at the tectonic contact.
2- Metamorphic rocks such as gneiss, schist, talc-carbonate, chlorite and serpentine.
3- Shear zones.
4- At the end of a fault or a fold and at the bend of faults.
5- Associated with deep-seated tectonic structures and dyke swarm.
El-Kazzaz (1996) stated that the mineral deposits are related to shear-zones where fluids are focused. The fluids are derived from the site of metamorphic reactions (Stel 1986; Sibson et al. 1988). Excess fluid pressures may activate or reactivate weaknesses (Sibson et al. 1988) and allow fluids to escape, also Kissin and Ruzajkin (1997) studied the effect of water on the development of natural and induced earthquakes sources explaining the metamorphic processes and release of fluids, in turn increasing seismic activity.
PALEO-TSUNAMI DEPOSITS ON RED SEA BEACH
Several wells were drilled to study the properties of core samples, an example of drilled wells is shown in table, 1 (Salem 2000) its location in Fig. 5, the results revealed some lithological characteristics of sedimentary rocks (Salem 2008 a) as following:-
1- Rapid variation of sedimentation cycles at the beach of the Red Sea.
2- The rocks are characterized by heterogeneous properties and ill-sorted, there are occurrences of fossils and shell fragments, low values of the thicknesses of sediments 3-The carbonate rocks contain clastic fragments and carbonate blocks are included within clastic rocks, with increasing the distance from the beach to the west the sediments are less heterogeneous.
The core samples contain Paleo-liquefaction phenomenon of Pliocene sediments at depth 48 meter from the top surface of the well as shown in 1 of Fig. 6. (western side of the map, Fig. 5). Paleo-landslides were observed at several sites (Fig. 5). The observation of Paleo-geological features shows that the area suffered from Paleo-seismic activity. The foregoing conditions of deposition were similar to the lithological characteristics of recent tsunami deposits
Salem (2008 a) suggested a model (Fig. 7) for Paleo-tsunami deposits on the Red Sea beach in Egypt. He stated there were two factors of deposition on the beach of the Red Sea, the first (flash flood) ran from the west (basement mountains) to the east through ancient wadis and the second (Tsunami waves) originated from bottom of the sea and directed to the west leaving tsunami deposits on the beach.
Active Landforms
The term active tectonic (process, structures and landforms) refers to those tectonic processes that produce deformation of earth crust on a time scale of significance to human life.
Most geologists would consider a fault to be active if it has moved during the past ten thousand years (Holocene Epoch). Any fault that moved during the Quaternary period (the past 1.65 m.y.) may be classified as potentially active. Faults that have not moved during the past 1.65 m.y. are generally classified as inactive
Tectonic geomorphology is defined as the study of landforms produced by tectonic processes or the application of geomorphic principles to solve the tectonic problems.
Paleoseismology is the study of prehistoric earthquakes as regards their location, timing and size. Paleoseismologists can only study earthquakes that produce
recognizable surface deformation; such earthquakes have been termed "morphogenic earthquakes".
The studied area includes several landforms as a result of potentially active faults which have moved during the past 1.65 m. y. These landforms can be distinguished as, Linear valleys, subsidence features, pressure ridges, off set features, cracks and joints, fault scarps and fault exposures, raised beaches and landslides.
GEOLOGICAL OBSERVATIONS
The detailed studies of the geological features (Fig. 1) are arranged as given below:-
1- Cracks Of Qena-Safaga Road (No. 1 in Fig. 1). The interpreted magnetic map (Fig. 4) indicates that the cracks are located at the ends of two faults trending NW-SE. During the period 1993-1994 five seismographs were installed to surround the cracks, while the author was watching the drum of the recorder (MEQ 800), the instrument recorded an earthquake (S-P was nearly equal zero) and at the same time he saw water column was rising as a result of damage of water pipe line (due to the recorded earthquake) which passes along the area (Fig. 8).
2- Abu Dabbab Area
This area (Fig. 2) is characterized by lateral variation of rocks starting with sedimentary rocks at the coast for a short distance, then basement rocks to the west. According to Morgan et al. (1983) the area is characterized by high heat flow (92 m W m-2). Some geological features (geological feature No. 2 in Fig. 1) were observed including (Fig. 9) fractures and landslides, related to the seismic activity of the area. The geological observations are limited near the coast where the occurrences of the outcrops of sedimentary rocks.
Ibrahim and Yokoyama (1998) stated that the focal depths of earthquakes for Abu Dabbab area range from 1 km to 10 km and earthquake epicenters do not follow any trend, so Abu Dabbab earthquakes are of swarm type. They added that the basement rocks are Precambrian that making the earthquakes have sounds. They mentioned that the swarms are of igneous origin. Badawy et al. (2008) stated that the seismic activity of Abu Dabbab without a large mainshock is typical of volcanic activity, no historical earthquakes stroke the area, there is no relation between the seismicity of the area and regional tectonics. Flying camp was carried out at the Abu Dabbab area, while the author was sleeping at the ground, he heard sounds such as sound of a factory especially when he put his ear on the ground, so these earthquakes originated from the motion of magma to intrude the fractures, in turn the sounds were produced.
3- Uplifting
Bull (1984) gave three cases of seismic activity and identification of uplifting, (Fig. 10) the first maximal uplift (1 in Fig. 10), where the dipping of Quaternary deposits is related to the basement rocks, this indicates the rate uplifting > the rate of channel down cutting with mountain and piedmont aggregation. The second is slow uplift (2 in Fig. 10), it's slower than the first, this case is located at the mouth of Wadi Ghadir (geological feature No. 3 in Fig. 1) near the Red Sea coast, where the Quaternary deposits overlies the basement rocks with dipping angle to the east and is related to the basement rocks (1 in Fig. 11). Field trips were carried out to note normal fault scarp (Quaternary deposits) at the same area, the angle of fault scarp is nearly vertical (1 in Fig. 12) to indicate the age of the fault is recent because the slope angle is inversely related to the age and the number of bevels is likely to give of the total number of seismic events (Vittori et al. 1991, 2 in Fig. 12), also mountain front of sedimentary rocks is nearly straight at Ghadir area near to Red Sea, it's good evidence to prove slowness of erosional forces and prediction of active forces which affect the area to produce a straight mountain front.
In general the distribution of microearthquakes during the period 1906-2007 covers all Eastern Desert of Egypt but the near sight on the area which extends from Wadi Ghadir to Hamata including geological features No. 3 to 5 (Fig. 1) indicated that some recorded earthquakes are distributed along a trend of parallel to the beach of Red Sea (near to the border of the Red Sea) at the edge of marginal underwater cliffs, this may be the fault plane where the slow motion of uplift are occurred. Morgan et al. 1983 studied regional geothermal exploration in Egypt using the thermal gradient/heat flow technique and ground water temperature, they suggested that Egypt is located in northeastern corner of the African plate, gave it the chance to possess geothermal resources along its eastern margin. They added that at Wadi Ghadir well, (at the coast of Red Sea) high gradient of 55 m K m-1 was measured to give a local thermal anomaly (the measured heat flow was 175 m W m-2), a rapid decrease of gradient from 55 to 30 m K m-1 over a distance of 5 km to the west of the first well then a gentle decrease in gradient to 24 m K m-1 at the distance of 18.5 km from the coast to the west. They stated that "the very high heat flow at the first site (at the beach) may in part be due to thermal refraction caused by the juxtaposition of lower conductivity sediments with higher conductivity crystalline basement by the fault to the east of the first site". Gravity interpretation at the mouth of Wadi Ghadir indicated a steeply dipping fault down-throwing to the east (at the Red Sea), approximately 0.5 km east of the first well (Morgan et al. 1980). The third case is inactive, there is no uplift, no motion as shown in 2 of Fig. 11 an example some portions of Wadi Hodeen where sand sheet is represented by large area, the rate uplifting << the rate of channel down cutting with mountain and piedmont aggregation (3 in Fig. 10), also the evidence of inactive case are represented by some Wadis of southern portions of Eastern Desert with U shape (Fig. 14).
4- Paleo-Liquefaction
Paleo-liquefaction (2 in Fig. 6) was observed at the beach of the Red Sea at Hamata area (geological feature No. 4 in Fig. 1). The sandstone of Pleistocene contains features of Paleo-liquefaction indicating strong Paleo-earthquake (near the Paleo-liquefaction area) with a magnitude > 5 stroke the area. There is a link between Paleo-liquefaction phenomenon and the distribution of Paleo-seismic activity. The deformed area includes the epicenter of the mainshock and its related aftershocks. The identification of Paleo-aftershocks area is very important. It attracts the fluid flux. According to Micklethwaite and Cox (2004) the aftershocks area is characterized by highly permeability zone, faults with small displacement, repeated events, less mature faults, highly fractures zone, breccias, vein networks and aftershock activity occurs over extended period. Fluid flux tends to intrude through this area to deposit minerals (Salem 2008 b), so the probability of occurrence of mineral deposits near the area of Paleo-liqufaction is relatively higher.
5- Subsidence Features
Localized uplift and subsidence are associated with tectonic events (1 in Fig. 13), hence associated with earthquakes. A subsidence feature at Hamata village was observed at Quaternary terraces (geological feature No. 5 in Fig. 1), it extends from the west to the sea with large extension, and affects the hard sedimentary rocks (coral reefs) at the beach.
6- Linear Valleys
It is one of landforms which may be produced by active strike slip fault. Linear valleys are troughs along main fault traces. They often develop because of continued movement along recent fault traces crushes the rocks and make it more vulnerable to erosion. Streams commonly follow these zones of weakness.
Linear valleys are observed between Ras Banas and Marsa Humayrah (No. 6 in Fig. 1). They extend from east to west such as Wadi Diban, Wadi kalalat, Wadi Khuda, Wadi Abu Hadd, Wadi Marafay and Wadi Rhabah while sinuous and embayed valleys with U shape are common in the southern portion of the area as shown in Fig. 14.
7- Cracks and Joints of Marsa Humayrah
They are located along the Red Sea coast at several locations where coral reefs and clastic rocks occur. They are represented at Marsa Humayrah (No. 7 in Fig. 1).
Cracks of Marsa Humayrah have three trends (Soliman 2005)
1. N-S trend is the main trend (1 in Fig. 15)
2. N-40° trend (2 in Fig. 15)
3. N-70° trend
The cracks could have been produced by tectonic or untectonic processes. If cracks have a certain trends they may be produced by tectonic movements. Cracks with enchelon pattern may be produced by active strike slip faults.
8- Pressure Ridges and Sag Ponds
The subsidence features may be represented by sag ponds, depressions, lagoons and small bays (No. 8 in Fig. 1). Localized uplift and subsidence are associated with bends and steps in strike slip faults (Soliman 2005). A right bend in a right lateral fault produces an area of subsidence and thus is called a releasing bend. If two or more approximately parallel fault traces in strike slip fault step these are called releasing steps and also can produce areas of subsidence. The sediments in sag pond areas are sabkha, silt, fine grained sand and salt. Generally sag ponds are found with bays and concave shore line. It is located with lagoon at Sharm El Madfa with trend E-W as shown in 2 of Fig. 13. It is associated with pressure ridges at north Sharm El- Madfa (Fig. 16). Two or more approximately parallel fault traces in strike slip fault can produce area of uplifting.
9- Paleo-Landslide of Humra Dom
It is secondary Paleo-seismic evidence as a result of earthquakes shaking and occurred away from fault trace and may be instantaneous and postseismic. landslide is recorded and appeared in Quaternary pit (Fig. 17), it shows the gravels channel change it's strike and overturned, this landslide is topples type caused by action of horizontal shaking (No. 9 in Fig. 1).
Fault Scarps
Fault scarps are located in the studied area at Wadi Ghadir, Berenice, south of Marsa Hufarat El Malh and Ras Abu Fatmah at the southern part of the area.
10- Fault Scarp at South Marsa Hufrat El Malh
It is lies near the Red Sea coast (No. 10 in Fig. 1) that was produced by active normal fault with trend N 55 W, it is the evidence of active uplifting in the investigation area (1 in Fig. 18)
11-Fault Scarp at Ras Abu Fatmah
This fault scarp is extending for some distance at south of the studied area (No. 11 in Fig. 1) with trend N 60 W (2 in Fig. 18), the fault scarp is a normal fault. Normal faults are dominant in the southern part of the area (Abu Ramad) with an echelon pattern. Abu Ramad is characterized by the occurrence of cracks, with trends N-S and N 40° and N 70° (Soliman 2005)
12- Recent Fractures of Wadi Allaqi
El-Kazzaz, 1996 studied Allaqi shear zone located in the South Eastern Desert, especially the central portion of that Wadi. The area is made up of ancient volcaniclastics and dismembered ophiolitic rocks intruded by granitoids, he added that the rocks have been affected by strong and complex deformation. Allaqi shear zone resulted from collision of the Arabian-Nubian Shield with Nile craton. It is characterized by the high distributed of veins which vary in thickness from few centimeters to over a meter. The fractures dilated sinistrally and were filled with quartz. The gold occurrences are restricted to quartz veins and alteration zones. Greiling et al. (1996) studied the regional features of the Eastern Desert of Egypt and noted the occurrence of sutures, major ophiolite nappes and major Late-orogenic granite they added a major displacement of NE block toward NW related with Wadi Kharite-Wadi Hodein. Helga de Wall et al. (2001) studied the southern boundary of Egypt and north Sudan (1 in Fig. 20), they stated that the Hamisana zone (HZ) is one of the major high strain zones of the Pan-African (Neoproterozoic) Arabian–Nubian shield (ANS).
It trends broadly N–S from northern Sudan into southeastern Egypt and meets the present Red Sea coast at latitude 23°N.
Epicenters were plotted at the map (2 in Fig. 20) to delineate the relationship among the distribution of microearthquakes, the Paleo-tectonic trends and the type of rocks. The epicenters are related with Paleo-tectonic trends such as Wadi Allaqi, the contact between Phanerozoic cover and Pan-African basement, Ophiolite complex, and Gneiss rocks. During field trips, recent fractures (Fig. 19) in Quaternary deposits were observed at Wadi Allaqi (geological feature No. 12 in Fig. 1) to note reactivation of Wadi Allaqi.
CONCLUSIONS
The response of the Earth's crust to stress differs from one place to another depending on the value of stress, the direction of the applied force and the physical properties of the medium. Seismic energy is easier to escape through Paleo-deformed areas. The distribution of earthquakes in the Eastern Desert of Egypt shows the majority of events are microearthquakes. Their locations follow active faults, subsurface magmatic activity and Paleo-tectonic trends.
It is the first trial to note Paleo-tsunami deposits at the Red Sea beach from comparison between lithological characteristics of the recent tsunami deposits and lithological characteristics of sedimentary rocks at the Red Sea beach.
There are several geological features related to ancient or recent earthquakes. When a geological feature is associated with Quaternary deposits it gives us information about active or reactivated areas, while Paleo-geological feature within Paleo-sediment means that a paleo-earthquake stroke the area and left a shadow. The geologist must take into consideration a circle around the signature of Paleo-seismic event. It's the area of different geological events, perhaps a mineralized zone (hydrothermal solution intruded the Paleo-fractures).
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