SEISMIC HAZARD ASSESSMENT AND SEISMIC SAFTY FOR THE PROPOSED ASTRONOMICAL OBSERVATORIES AT ABU TARTUR AND FAYED SITES, EGYPT

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1 Fourth International Conference of Earthquake Engineering and Seismology May 2003 Tehran, Islamic Republic of Iran SEISMIC HAZARD ASSESSMENT AND SEISMIC SAFTY FOR THE PROPOSED ASTRONOMICAL OBSERVATORIES AT ABU TARTUR AND FAYED SITES, EGYPT El-Hadidy, S.; Tealeb, A. A.; Mohamed,A. A.; Kamal Abdel-Rahman, and Atiya, K. National Research Institute of Astronomy and Geophysics, Helwan, Cairo, Egypt. ABSTRACT The assessment of seismic hazard for the selected sites was done by delineating the seismic sources in the northern part of Egypt, selecting the controlling earthquake, determining the attenuation models of peak ground acceleration with distance, and finally defining the seismic hazard at the selected site. The soil effect at the site of interest is calculated and introduced to the estimated ground motion. Both of deterministic and probabilistic approaches are used to evaluate the maximum ground motion at both of Abu-Tartur and Fayed sites. 2. SEISMICITY OF NORTHERN EGYPT AND ITS VICINITY The seismicity of Egypt was studied by many authors, e.g., Sieberg, (1932); Ismail (1960); Gergawi and El-Khashab (1968); Maamoun and Ibrahim (1978); Maamoun et al. (1984); Albert (1987); Kebeasy (1990) and Abu El-Enean (1997), and Deif (1998) to delineate the seismotectonic sources. In their studies, the seismicity maps are approached based on the regional geological structures and, sometimes, to the dominant tectonic stress. In brief, the seismic activities are reported to be concentrated in the following belts: 1- Gulf of Aqaba-Dead Sea trend. 2- Gulf of Suez trend. 3- Cairo-Suez road trend. 4- East Mediterranean-Cairo-Fayum trend. 5- Mediterranean coastal dislocation trend. 6- Southwest Cairo seismogenic zone. Historical and recent seismicity as well as its related phenomena show that the Eastern Mediterranean region is affected by the seismic activity of the Hellenic and Cyprean Arcs. Among the geological hazards originated in these arcs, are the large earthquakes of intermediate focal depths. They are the most significant for the Eastern Mediterranean (Papadopoulos, 1987). Shocks of this type could produce damage, even at great distances towards the convex side of the arcs (e.g., South Italy, Libya and Egypt). Therefore, the earthquakes of these two arcs are taken into account while conducting the seismic hazard study of the two sites under consideration. 3. DETERMINISTIC SEISMIC HAZARD ANALYSIS Based upon the spatial distribution of large and micro earthquakes, mapping of the maximum observed intensity, maximum possible or expected earthquake, maximum expected acceleration utilizing the

2 extreme values model, present day stress using the focal mechanism solutions, and correlation of earthquake epicenters with the tectonic and the available geophysical data. For a circular fault rupture, the seismic moment can be defined as: 16 3 Mo = (r ) σ (1) 7 Where M o ; is the seismic moment; (r) is the radius of the circular fault and σ; is the stress drop associated with the earthquake. The stress drop is related to the strength of the fault zone or the intensity of rupture needed to overcome that strength. For a constant fault radius, the stress drop is proportional to the seismic moment. Therefore, it is proportional to the rigidity, as well as to the fault displacement. Some of the regression relationships are reviewed regarding certain issues on their use as follows; 1- Source rupture length: It is the most frequently employed relationship. Deif (2000) found such relation to be represented by the following equation. Log (M o ) = log (L) (2) Where M o ; is the seismic moment and L; is the rupture length 2- Another approach has been adopted to use the historical record and to increase the maximum historical earthquake by arbitrary certain amount and assumes that this value is the appropriate maximum earthquake. This increasing increment is usually taken to be 0.5. This technique is used for areas for which the fault information is absent. Each of the techniques used to estimate the maximum magnitude is associated with some uncertainty. The source rupture length and the maximum historic earthquake on the fault with some increment above it are used to determine the maximum earthquake in each seismic source. The results of the above methods are expressed in terms of maximum expected seismic moment. The resultant maximum seismic moment could be transferred to the surface wave magnitude values using the following relationship: Log(Mo ) = 1.44( ± 0.051)Ms ( ± 0.293) (3) where M o ; is the seismic moment and M s ; is the surface wave magnitude Maximum earthquake magnitude calculations Using the same techniques for the Southern part of the Gulf of Suez and the activity of the Gulf of Aqaba, the maximum expected earthquakes are determined. While the historical earthquake record analysis is used for the rest of seismic sources. Tables (1and 2) summarize the results of the maximum expected magnitude maximum expected acceleration for each seismic source at the bedrock and the corresponding distance from the source to each of the interested site. Table 1. Maximum expected magnitude and acceleration at the bedrock for Abu-Tartur site. Seismic Source Maximum Acceleration (gal) Maximum expected Magnitude Distance to Abu- Tartur (Km) Southern Gulf of Suez Middle Gulf of Suez North eastern Desert Southwestern Cairo Gulf of Aqaba Table 2. Maximum expected magnitude and acceleration at the bedrock for Fayed site. Source Max. acc. Gals Max. magnitude Distance to Fayed site (Km) Southern Gulf of Suez Middle Gulf of Suez North eastern Desert Southwestern Cairo Gulf of Aqaba

3 3.5. Travel Path Effects The effects of the travel path on the earthquake ground motion are primarily related to the attenuation of the propagating seismic waves. The results of seismic hazard evaluation are greatly influenced by the regional attenuation formula, which is derived using the relation among earthquake size, distance between the site and the source, and the amplitude of ground motion. Using 12 isoseismal maps, Fat-helbary and Yutaka, (1994) proposed two relationships of intensity attenuation. Also, they converted the velocity by differentiating the records of one station of Aswan seismic network into acceleration and derived the attenuation relation for Aswan area as a function of a strong ground motion Acceleration-based attenuation models This model can be used in areas equipped with ground motion accelerographs. Ground motion acceleration usually plays the main role with regard to the structural damage in earthquakes and one of the basic parameters to define earthquake ground motion (Garcia and Canas, 1991). The model proposed by Joyner and Boore (1981) is adopted in this study. This model is selected because it is the most widely used attenuation law in Europe where the majority of the data set was recorded. The basic form of the attenuation model can be expressed as: log( a) = α + βm γ log R + br + σ (6) Where: a; is the peak ground acceleration in g and M; is its magnitude; R; is the hypocentral distance assuming an average depth of 20 km of the used earthquakes and α, β, γ, and b; are the model parameters. σ; is the standard deviation of log(a). a- Horizontal acceleration: The attenuation relation in terms of surface wave magnitude is found to be: log( a h ) = Ms 1.128log R R (11) Where a h ; is the peak horizontal acceleration in g and M; is its magnitude; R; is the hypocentral distance assuming an average depth of 20 km of the used earthquakes Deterministic Seismic Hazard Evaluation at the Bedrock Accumulating the above results together, the deterministic seismic hazard is assessed at the rocky areas in the sites of interest. The maximum expected acceleration from each seismic source at the bedrock of each site is given in tables 1and 2. From table (1), it is clear that the seismic source of the Southern Gulf of Suez has the greatest effect on the site of interest at Abu-Tartur although it does not have the greatest magnitude. Therefore the maximum magnitude of this seismic source is the controlling earthquake for the site. It is very important to mention here that these values are the peak ground accelerations regardless of its frequencies. It is thus necessary to simulate the shear wave motion at the site of interest and then to fourier transform it to determine the frequency at which the ground motion is maximum. From table (2), it is clear that the seismic source of the Gulf of Aqaba has the greatest effect on the site of interest at Fayed although it does not have the shortest distance. Therefore, the maximum magnitude of this seismic source is the controlling earthquake for the site. 4. SOIL EFFECT AT THE INVESTIGATED SITES Takahashi and Hirano (1941), derived the following relationship for a vertically propagating harmonic wave of unit amplitude. U(ω) = 2{cos 2 (ωh/β L ) + (ρ L β L /ρ R β R ) 2 sin 2 (ωh/β L )} -1/2 (6) where U(ω); is the frequency domain surface displacement and the subscripts L; and R indicate the properties of the overlying layer of thickness H and underlying bed rock respectively. An addition to the maximum amplification factor at a frequency β L /4H the above equation also predicts maximum amplitudes at frequencies of 3β L /4H, 5β L /4H etc Site effect determination: Shallow seismic refraction experiments at Abu-Tartur site were carried out along 6 profiles to calculate v p, v s and the characteristics. The soil effect calculations are done at the sites of the shallow seismic profiles for Abu-Tartour site. The thickness of the soil layer is determined from the results of the shallow

4 seismic refraction survey. These thickens are found to be very small and variable from a profile to another. Applying equation (6), the amplification curves for the shear waves are as shown in figure (4). These curves are drowning from the results of the shallow seismic profiles 1, 2, 3, 4, 5, and 6 (Fig. 5). From the figure, it is clear that the amplification of the ground motion due to the soil effect is very small and can be neglected A m plification P2 P P3 P5 P Frequency Figure 2. Amplification curves of the selected profiles at Abu Tartur site. For Fayed site, shallow seismic refraction experiments were carried out along some profiles at Fayed site to determine v p, v s that to be used in the calculation of soil effect. The soil effect calculations are done at the sites of the shallow seismic profiles. The thickness of the soil layer is determined from the results of the shallow seismic refraction survey. The thickness is found to be very small and variable from a profile to another. Applying equation (6), the amplification curves for the shear waves. These curves are belonging to profiles1and 2 (Fig. 6). From the figure, it is clear that the amplification of the ground motion due to the soil effect is found to be very small due to the thinning of the soil layer.

5 1.40 Am lification Factor 1.20 Amplification curves Profile 1 Profile Frequency Figure 3. Amplification curves of the two profiles at Fayed site. 5. SIMULATION OF THE CONTROLLING EARTHQUAKE AT THE BEDROCK OF THE SITES The most powerful hybrid approach to estimate ground motion is called the stochastic method. These methods are grown out of the observations that large part of strong shaking on the accelerograms usually appeared incoherent and essentially random in nature.of equal importance was the fact that the Fourier amplitude spectrum of earthquake ground motion acceleration predicted by the Brune (1970 & 1971) source model and it is always observed by the actual earthquakes was flat, or approximately constant in amplitude in frequencies greater than the corner frequency and less than the maximum frequency. Based on Boore (1983) development, the time history of the acceleration of the maximum earthquake initiated from the southern Gulf of Suez seismic source, is calculated and is shown with its amplitude spectrum in figure (4). The amplitude spectrum indicates that, the flat band is in the frequency range between 0.7 Hz and 1.6 Hz. Therefore, the peak ground acceleration could be in this frequency range. Amplitudes with frequencies higher than 1.6 Hz decreases dramatically as showed on figure (4). The peak ground acceleration at the bedrock of the investigated site, which could be initiated by the maximum

6 earthquake with surface wave magnitude 7.34, is 8.0 gal as shown on the figure. This is highly matched the results of the deterministic seismic hazard approach at the bedrock. Figure 4. Simulated time history of the shear ground motion on the bedrock at Abu Tartur site. 6.DETERMINISTIC SEISMIC HAZARD AT THE AREAS COVERED BY THE SOIL COVER: For Abu-Tartur site, the maximum ground motion at the bedrock is found to be 9.25 gal. From the amplification curves at different profiles the maximum amplification factors is 1.2. From the spectrum of the simulated shear waves, the frequency range from 0.7 to 1.7 shows the flat part of the acceleration spectrum. In this range the maximum amplification of the soil layer is negligible. Based on this fact, the maximum expected ground acceleration initiated from an earthquake with magnitude 7.34 at 382 km distance from the southern Gulf of Suez seismic zone will be equal to 10 gal. This indicates that the site of interest is quite safe from the seismic hazard point of view. Due to the very low level of seismicity, the probabilistic hazard at certain times could not be estimated. For Fayed site, the maximum ground motion at the bedrock is found to be 17.4 gal. From the amplification curves at different profiles the maximum amplification factors is Based on this fact, the maximum expected ground acceleration initiated from an earthquake with magnitude 7.63 at 292-km distance from the Gulf of Aqaba seismic zone would be equal to 23.5 gal. These results indicate that, the site of interest is quite safe from the seismic hazard point of view. 7. Probabilistic approach for seismic hazard assessment at Fayed site: 7.1. Earthquake Occurrence Model The recurrence relationship indicates the chance of an earthquake of a given size occurring anywhere inside the source during a specified time. Recurrence relationships are the core of the probabilistic seismic hazard analysis. These relationships are largely data dependent recurrence relationships for the individual sources have classically been represented by a straight line plots through the data. Log (N) = a - bm (7) where a and b are the equation parameters and n ; is the number of earthquakes and m; is magnitude. The accurate determinations of these parameters using the seismological observations are very important from

7 the seismic hazard point of view. The main difficulty is the lack of a representative sample of the earthquake activity for a long time and a wide range of magnitudes. This is so because the seismic data is optimistically considered complete in this century only, which is not enough to represent a complete seismic cycle. Historical information is available only for destructive earthquakes. An effective way to overcome this problem is the application of the so-called - mean method of Milne and Davenport (1969). Papazachos (1990) outlined the application of this method. The seismic zones which might affect the site under consideration are previously defined as; Middle of the Gulf of Suez (MGS); southwest Cairo (SWC) and the Egyptian Mediterranean coastal zone (EEC). The other seismic zones are distant enough from the site of interest. Table (3), shows the cumulative frequency magnitude relations as determined by the mean value method for each seismic zone. The straight lines were determined by the least square method. Table 3. The cumulative frequency magnitude relation parameters for the effective sources. Seismic Source B value a value Middle Gulf of Suez South-west Cairo Mediterranean coastal zone Because the earthquake acceleration in these sources affects large areas, the ratio of the affected area to the rupture length is large, and a point source approximation can be justified (Bender, 1982 & 1984) Seismic Hazard Model: Cornell (1968) introduced the idea of probabilistic seismic hazard into seismology and most of the recent approaches are just a modification of the original idea. The model of Bender and Perkins (1987) is adopted herein due to its capability for dealing with many uncertainties associated with the earthquake locations, source boundaries and attenuation variability. The program SEISRISK III (Bender, 1987) is used for the evaluation of the probabilistic seismic hazards parameters Ground Motion Hazard Values: The magnification coefficient calculated at the site is taken into consideration yielding the probabilistic seismic hazard in terms of ground acceleration at the area with probability 90% of not being exceeded in the next 50 years. The predicted ground motion acceleration values likely to occur at Fayed site is found to be 36 cm/sec 2 with a probability 90% of not being exceeded in the next 50 year. 8. SUMMARY AND CONCLUSIONS For Abu-Tartur Site, the seismic sources regionalization, it could be concluded that the seismic sources are concentrated in the Gulf of Aqaba-Dead Sea trend, the southern part of the Gulf of Suez, the northern part of eastern Desert, the offshore Mediterranean Sea coast and the southwest of Cairo zone. The comparison of the activity, the earthquake potential and the distance between the seismic sources and the investigated site indicates that the credible earthquake could be initiated from the southern Gulf of Suez seismic source zone. This seismic source is just at 382-km. distance from the Abu-Tartur site. Many damaging earthquakes were released from this seismic zone. The maximum earthquake hazard at the rocky areas at the site is erected from a peak ground acceleration of 9.25 cm/sec 2. It is resulting from the occurrence of an earthquake with magnitude 7.34, from southern Gulf of Suez seismic zone, at a distance of 382 kilometers. The time history of the maximum expected earthquake, initiated from the southwest Cairo seismic source, is calculated. The amplitude spectrum of this time history indicates that, the flat band is in the frequency range between 0.7 Hz and 1.7 Hz. Therefore, the peak ground acceleration could be in this frequency range. The magnification studies should also concern with this particular range. A soft soil layer covers a considerable part of the site under investigation. Such layer could considerably amplify the seismic ground motion if it is with considerable thickness. Therefore, the soil amplification is calculated and the deterministic seismic hazard is then evaluated. Since the amplification of the seismic wave is frequency dependent, it is important to determine the frequency at which the maximum amplitude at the rock is expected. The soil effect at the site of interest is found to be very small and could neglected. For Fayed Site, from the seismic sources regionalization, it could be concluded that the seismic sources are concentrated in the Gulf of Aqaba-Dead Sea Trend, the Southern part of the Gulf of Suez, the north eastern Desert, the offshore Mediterranean coast and the southwest of Cairo zones. The comparison of the activity, the earthquake potential and the distance between the seismic sources and the investigated site indicates that the credible earthquake could be initiated from the southern Gulf of Aqaba seismic source zone. This seismic source is Just 292-km. distance from the Fayed site. Many damaging

8 earthquakes were released from this seismic zone. The maximum earthquake hazard at the rocky areas at the site is erected from a peak ground acceleration of 17.4 cm/sec 2. It is resulting from the occurrence of an earthquake with magnitude 7.63, from Gulf of Aqaba seismic zone, at a distance of 292 kilometers. A soft soil layer covers a considerable part of the site under investigation. Such layer is could considerably amplifying the seismic ground motion if it is with considerable thickness. Therefore, the soil amplification is calculated and the deterministic seismic hazard is then evaluated. The soil effect at the site of interest is found to be very small. From the above mentioned results it can be concluded that both sites of interest are quite and safe from the seismic hazard point of view if the normal engineering precautions are taken into consideration. The two sites are far from any seismic noise (artificial or natural). 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