abou dabbab asia

Seismological Research Letters Volume 82, Number 1 January/February 2011 81doi: 10.1785/gssrl.82.1.81

Seismological Aspects of the Abou Dabbab Region, Eastern Desert, EgyptH. M. Hussein, S. S. R. Moustafa, E. Elawadi, N. S. Al-Arifi, and N. Hurukawa

H. M. Hussein,1,2,3 S. S. R. Moustafa,1,2 E. Elawadi,2,4 N. S. Al-Arifi,2,5 and N. Hurukawa6

INTRODUCTION

Located about 24 km from the Red Sea, the Abou Dabbab region is characterized by significant microseismic activity. The seismic history of this region is marked by two moderate mag-nitude earthquakes, the 12 November 1955 Mb 6.1 and the 2 June 1984 Mb 5.1. The 1955 Mb 6.1 earthquake was the larg-est recorded event to occur within Abou Dabbab. Woodward Clyde Consultants (1982) computed the first-motion focal mechanism for the 1955 event. The mechanism has a strike-slip faulting solution with a normal dip-slip component. The T-axis has a NNW trend. The two nodal planes that were determined for this event give NNW to NW and ENE to ESE strike direc-tions. Earthquakes in the Abou Dabbab region have shown a tendency to cluster in space and time (Morgan et al. 1981).

This pattern suggests that the activity seems to have been mainly controlled by local sources (e.g., magmatic intrusion) triggered in turn by regional tectonics (Badawy et al. 2008). The heat flow value in Abou Dabbab is twice the average heat flow value in the Eastern Desert of Egypt (Morgan et al. 1985). El Hady (1993) reported from the distribution of earthquake’s focal depths and rheological studies that the brittle-ductile transition in the Abou Dabbab area occurs at a relatively shal-low depth range (~9–10 km). The existence of the brittle-ductile transition in this region can be attributed to shallow astheno-spheric intrusion, which causes an increase in the temperature.

Recordings of microearthquakes in Abou Dabbab from 1 August to 20 August 2004 from 11 portable instruments show that this region is distinguished by intense spatial clus-tering (Figure 1A). The error estimated in both horizontal dis-tance and depth is less than 0.3 km (Hussein et al. 2008). The cross-section perpendicular to the activity reflects two main

1. National Research Institute of Astronomy and Geophysics, Seismology Department, Helwan, Egypt

2. Shaqra University, Community College, Alquwayiyah, Saudi Arabia3. North Africa Seismological Group, Trieste, Italy4. Nuclear Material Authority, Airborn Exploration Division, Egypt5. Faculty of Science, King Saud University, Geology Department,

Riyad, Saudi Arabia6. Director, International Institute of Seismology and Earthquake

Engineering (IISEE), Building Research Institute, Tsukuba, Japan

Figure 1. A) Seismic activity in the Abou Dabbab region dur-ing August 2004. B) NW-SE oriented cross-section of the hypo-centers of the August 2004 earthquakes. This section is perpen-dicular to the strike of the composite fault plane solution of the shallower depth earthquakes. The vertical bar represents the magnitude scaling.

(A)

(B)

82 Seismological Research Letters Volume 82, Number 1 January/February 2011

spatial clusters with a variable depth range of 8–10 and 11–15 km, respectively (Figure1B). The upper cluster shows a nearly vertical plane about 600 m wide, while the lower one clearly indicates a northward-dipping plane. The individual fault plane solutions for the upper cluster indicate a thrust-faulting mechanism with variable fault trends (Badawy et al. 2008). The P-axis trends also indicate a heterogeneous stress regime in this cluster. The majority of the events used for construct-ing the focal mechanism solutions in the upper crust produced insufficient P-wave first motion for the individual solutions. However, it is not so obvious as to identify the active stress regime in the Abou Dabbab area, which has undergone several deformation episodes in the past. Therefore, we construct the composite fault plane solution using data from six earthquakes with local magnitudes ranging from 0.8 to 3. The obtained solution is correlated with the cross-section for the earthquakes to obtain the direction of the possible fault and the dip of the fault plane. In this study, we have also computed the focal mechanism solution of the 2 June 1984 Mb 5.1 Abou Dabbab earthquake using the polarities of the P-wave first motion data from the International Seismological Centre (ISC) bulletin in addition to data from local seismological stations in Egypt. We also compared this solution with both the composite solu-tion and the 1955 Mb 6.1 mechanism in order to throw some light on the seismotectonics of this zone, and we calculated the source parameters of the 2 June 1984 Mb 5.1 Abou Dabbab

earthquake including the seismic moment, fault radius, and stress drop using the spectral analysis technique. These param-eters are useful not only for understanding the physics of earth-quakes but for predicting the potential hazard associated with the fault related to it. In this work, we also estimate the base of the magnetic source in the Abou Dabbab area, which is assumed to be the depth where magmatic intrusion took place.

GEOLOGICAL SETTING

Egypt’s central Eastern Desert (CED), where the Abou Dabbab region is located, is almost exclusive built up of ophi-olitic mélange and associated rocks together with subordinate molasses-type sediments and late-tectonic volcanic and gran-itoid intrusive (El Ramly et al. 1993). CED is dominated by low-angle thrusts that were formed during the Neoproterozoic extensional tectonic phase of the Eastern Desert that began ~600 Ma and followed arc collision and northwestward ejec-tion of nappes (Fowler and Osman 2001). A NW-oriented sinistral strike-slip Najd fault system (NFS) overprinted the low-angle thrusts. This NFS is regarded as the last significant structural event affecting the Precambrian rocks in Egypt and Saudi Arabia (Abd El Waheed 2008; Abdeen et al. 2008).

The Abou Dabbab region is intruded by a series of gran-itoid bodies that intrude or are crossed by the shear zones or thrusts (Figure 2). There are two types of granite, the northern

34°20′ 34°30′ 34°39′

34°20′ 34°30′ 34°39′

25°20′25°20′

25°13′

Abou Dabbab Metavolcanics

Figure 2. Simplified geological map of the Abou Dabbab region, showing the major structural features. Circles show the epicenters of earthquakes recorded during August 2004 with the same magnitude color-scale as Figure 1.

Seismological Research Letters Volume 82, Number 1 January/February 2011 83

and the southern group (Shalaby et al. 2005). The first group is known as the Gebel El Umrah complex, while the southern group is referred to as Abu Karahish old granite. Abou Dabbab is characterized by the existence of W. Mubarak–W. Abou Dabbab thrust (Akkad et al. 1996). It appears first in the west-ern part of the area to circumscribe the Abu Karahish old gran-ite from south and east, extending NE to E-W in Abou Dabbab region and attaining a total length of 43 km. The southern part of the thrust strikes NE-SW and dips toward the southeast, while the northern part strikes NE-SW and dips steeply to the SSE direction (Figure 2). These thrusts represent a part of the regional fault and thrust regime subsequent to and partly coeval with the emplacement of older granite (Akkad et al. 1996). The direction of shortening is broadly NNW-SSE. The map also shows that the area is affected by sets of fractures of local importance trending in NW-SE, E-W, and NE-SW directions.

FOCAL MECHANISM

We determined the composite source mechanism for six selected events recorded in the Abou Dabbab region during the period from 1 August to 20 August 2004, based on the P-wave polarities. These events come from the depth range of 8−10 km, i.e., the upper cluster in the depth cross-section. The parameters of these events are listed in Table 1. The polarities of P waves for these events were picked from vertical compo-nents. The focal mechanism solution of the 2 June 1984 Mb 5.1 Abou Dabbab earthquake is also constructed using the P-wave polarity data from the ISC bulletin and the avail-able local seismological stations in Egypt. Table 2 shows the parameters of this event. It also shows the parameters of the

1955 event. The grid search method of Snoke (2003) was then used to determine the best-fitting fault plane solutions, with a grid spacing of 2o. The 1-D velocity model of Marzouk (1988) and the final location of the events were used to determine takeoff angles. A total of 90 polarities were used for construct-ing the composite fault plane solution, of which 88 are consis-tent with the final solution (Figure 3). This solution reflects a thrust fault on a nearly vertical plane (λ = 88). The param-eters of the composite focal mechanism solutions are listed in Table 3. A total of 22 polarities were used for constructing the mechanism of the 2 June 1984 earthquake, of which 21 are consistent with the obtained solution (Figure 4, Table 4). This solution gives a strike-slip faulting mechanism with a normal dip-slip component. It also reflects two types of motion, right-lateral along NE-SW faults and left-lateral along NW-SE faults. The epicentral map of the available earthquake focal mechanisms belonging to the Abou Dabbab area is shown in Figure 5. From this figure, it can be seen that the mechanism of the 2 June 1984 earthquake is nearly consistent with the solution of the 12 November 1955 earthquake. Figure 5 shows the possible solutions of the 12 November 1955 event: 1955A and 1955B (Woodward Clyde Consultants 1982). Table 4 also shows the parameters of the two possible solutions for the 1955 events.

Time-Depth Distribution of the August 2004 Swarm The focal depth plots with time shows the vertical focal depth migrations of hypocenters (Figure 6). The activity began on the Julian day 216. The focal depth of the swarm was around 15 km and then gradually rose to 8 km. The upward migra-

TABLE 1List of Earthquakes Used for Constructing Composite Fault

Plane Solution in Abou Dabbab Area

Date O.T. Long. Lat. Depth ML

20040811 043037.77 34.5142 25.2769 8.90 2.2 20040813 121341.10 34.5172 25.2781 9.04 3.0

20040813 201301.44 34.5141 25.2791 8.96 2.3

20040814 004529.04 34.5126 25.2796 9.67 3.0

20040818 193054.90 34.5298 25.2768 9.29 1.2 20040819 211423.50 34.5017 25.2978 10.0 0.8

TABLE 2List of Moderate Magnitude Earthquakes in Abou Dabbab

No. Date O.T Long. Lat. Depth mb

1 19551112 053214.00 34.5000 25.3000 33 6.1

2 19840702 014659.33 34.5278 25.2542 09 5.1

Figure 3. Composite focal mechanism solution for the August 2004 earthquakes. Lower-hemisphere equal-area projections of the focal sphere. Letter symbols indicate the position of com-pressional (P) and tensional (T ) axes.


tion of earthquakes was repeated on the Julian day 220 in the same depth range. Consequently, the earthquakes start an abrupt upward migration to a depth of about 9–10 km on Julian day 224. This activity stayed at a higher level over a period of eight days.

Frequency-Magnitude RelationsInspection of the frequency-magnitude distribution of earth-quakes for two depth ranges (Figure 7) shows that the b-value increases with depth in the Abou Dabbab region. The average b-value as a function of depth in the region increases from about 0.51 ± 0.08 at a depth of 8–10 km to a maximum 0.82 ± 0.13 at a depth of 11–15 km.

TABLE 3Parameters of the Abou Dabbab Composite Fault Plane

Solution

Plane 1 P axis T axis

Strike Dip Slip Trend Plunge Trend Plunge

237.83 85 87.99 329 39.97 145.62 49.96± 2.39 ± 0.34 ± 1.07 ± 2.05 ± 0.04 ± 2.60 ± 0.07

Figure 4. Lower-hemisphere equal-area projections of the focal sphere for the 1984 Abou Dabbab earthquake. Letter sym-bols indicate the position of compressional (P) and tensional (T ) axes.

TABLE 4Source Mechanism Parameters for the 1955 and 1984 Abou Dabbab Earthquakes; Table 2

No.

Plane 1 P axis T axis

ReferenceStrike Dip Slip Trend Plunge Trend Plunge

1 74104

78 73

–34–53

28127

3214

54167

4820

Woodward Clyde Consultants,1982

2 131.21 ±1.8

71.78±1.76

–29.12±1.8

87.81±2.1

28.88±1.7

181.94±1.5

07.44±0.8

This study

Figure 5. Map showing the focal mechanism solutions from the 1955 and 1984 earthquakes and the composite solution from the August 2004 swarm. The parallel black lines represent the Najd fault system. The “beach balls” 1955A and 1955B represent the pos-sible solutions of 1955 earthquake.


DEPTH ESTIMATES OF THE INTRUDED IGNEOUS BODY

For estimating the depth to the bottom of the magnetic crust (Curie isotherm), which in this case indicates the depth to the intruded high-temperature rocks, we primarily use the azi-muthally averaged Fourier spectra of magnetic anomalies (e.g., Spector and Grant 1970). Analyzing the long wavelength part of the magnetic data can provide information about this depth. This depth represents the depth below which rocks lose their magnetization. We use the slopes of the amplitude spectra to derive the depth to the top (zt) of the magnetic source (e.g., Okubo et al. 1985) and the centroid depths (z0) of the mag-netic source. Then, the basal depth of the magnetic source is zb = 2z0 – zt. Figure 8 shows the amplitude spectrum and the scaled amplitude spectrum of the magnetic data of the cen-

tral Eastern Desert where Abou Dabbab is located using the centroid method of Okubo et al. (1985). It is found that a cell area of around 300 km2 is required to be able to estimate the Curie isotherm depth from spectral estimates using an itera-tive forward modeling approach of the spectral peak method suggested by Ravat et al. (2007). The depth estimates for shal-lower Curie isotherms using the selected cell size are stable, and the shape of the spectral peak did not change with an increase in the block size. The average depth to the intruding source within this area is ~11 km.

Source Parameters of the 2 June 1984 Earthquake Source parameters such as seismic moment (Mo), fault radius (r0), stress drop (∆σ), and the moment magnitude (MW) were determined for the 2 June 1984 Abou Dabbab earthquake using the far-field P-wave displacement spectra. The analyzed data consists of three teleseismic broadband records from the Grafenberg array, Germany, which are characterized by good signal to noise ratio. The time window selected was 40 seconds. However, varying the window length slightly does not affect the shape of the spectra and consequently does not affect our estimation of the low-frequency spectral level and the corner frequency. A cosine taper was applied to the selected signal window. The data within the time window was transformed to the frequency domain using a fast Fourier transform algo-rithm. The Fourier transform was deconvolved with the instru-mental response and converted to the displacement spectrum. Figure 9 shows an example of fit for the P-wave displacement spectrum. Assuming an omega-square Brune’s source model, the low-frequency spectral amplitude Ω0 and corner frequency f0 are estimated using the nonlinear least-square inversion tech-nique. For a circular source model, the values of M0, r0, ∆σ, and MW can be derived from the P-wave displacement spectra fol-

Figure 6. Focal depth plots with time.

Figure 7. Frequency-magnitude distributions for two depth ranges.

Figure 8. Azimuthally averaged amplitude spectra.


lowing the Brune (1970, 1971), Hanks and Wyss (1972), and Kanamori (1977) relations:

M0 =

4 πρα 3 RΩ0

FRϑϕ (1)

r0 =

2.34α2πf0

. (2)

∆σ =

7 M 0

16r03

(3)

MW =

23

log M 0( ) − 10.73 (4)

where α is the P-wave velocity at the source, ρ is the density, R is the epicentral distance, and Ω0 denotes the low-frequency asymptote. The free surface correction F is estimated for indi-vidual stations using the FOCMEC package (Snoke 2003). An average radiation pattern value Rϑϕ of 0.52 was assumed (Boore and Boatwright 1984). For all the calculations we used a value of 6.3 km/sec for the velocity of the P waves and a den-sity of 2.7 g/cm3. The average values were computed for each

parameter (moment, stress drop, fault length, MW) following Archuleta et al. (1982):

x = antilog

1N

log xii=1

N

∑

(5a)

where N is the number of stations used. In the case of simple arithmetic average, the result would

be biased toward large value.The corresponding standard deviation of the logarithm

SD log x and the multiplicative error factor, Ex, were also calculated from the relations of Garcia-Garcia et al. (1996):

SD(log x ) = antilog

1N − 1

(log xi − log x )2

i=1

N

∑

12

(5b)

EX = antilog(SD(log x )) (5c)

Table 5 lists the average values and the multiplicative factor of the scalar seismic moment, the stress drop, and the radius of the fault for the circular model.

DISCUSSION AND CONCLUSIONS

The seismic activity in the Abou Dabbab region is found to be clustered in space and time. Seismic activity in this region occurs in the form of repeating micro-earthquake swarms. The analysis of earthquakes recorded in the period from 1 August to 20 August 2004 shows that the swarm activity was localized at certain spots. This means that there is a localization of stress. Another important characteristic of seismicity is the focal depth migration from ~15 km to ~8–10 km, with the major-ity of the events occurring at a depth of 9–10 km. Rheological studies also show the existence of shallow asthenospheric intru-sion in the crust at a depth of 9 to 10 km (El Hady 1993). This intrusion could be a sufficient factor for localization of tectonic stress. It also increases ductility, as a result of increasing the temperature. The increased ductility would be an incentive for increasing the ductile instability that concentrates stresses in the brittle layer, a process of great importance in the localiza-tion of deformation (Ranilli and Murphy 1987). The presence of intensive activity at a depth range between 9 and 10 km also indicates a more heterogeneous medium with numerous small fractures. Generally, the concentration of earthquakes at a seis-mogenic zone is associated with volcanic features or other frac-tured regions where there is a concentrated application of stress such as intruding magma (Mogi 1963). The estimated depth of

Figure 9. Displacement spectra of the 1984 Abou Dabbab event showing corner frequency (fc) and spectral level (Ω0) to compute source parameters for Brune’s (1970) model.

TABLE 5Spectral Parameters of the Abou Dabbab 2 , July 1984 Earthquake from the Grafenberg Array Stations

Moment (N.m) EM0 MW EMW f0 Ef0 r (km) Er

Stress Drop ∆σ (MPa) E∆σ

1.15 x 1017 0.07 5.3 0.0002 1.35 0.003 1.6 0.003 9.7 0.004


this intrusion using the aeromagnetic data is about 11 km. This depth lies above the Moho depth and could represent a thermal boundary rather than a compositional change. This depth may be identified as the Curie isotherm depth. This depth is nearly the same as the depth to the bottom of the seismogenic layer in which the majority of earthquakes are concentrated.

The b-value is lower for shallower depth earthquakes (0.51 ± 0.08) than for the deeper earthquakes (0.82 ± 0.13). The increase of b-value with depth in Abou Dabbab is con-trary to the usual observation that b-values decrease with depth in the shallow crust (Wiemer and Wyss 1997; Mori and Abercrombie 1997), which is generally interpreted to be a reflection of increased stresses with depth. The higher b-value at depths between 8 and 10 km is interpreted to be caused by an increase of material heterogeneity or thermal gradient.

The composite fault plane solution obtained from the P-wave first motion of the micro-earthquakes located in the shallower depth range during the period from 1 August 2004 to 20 August 2004 corresponds to a reverse faulting mecha-nism with a nearly vertical nodal plane oriented ENE-WSW. The cross-section perpendicular to the fault plane also shows that earthquakes are concentrated in a vertical plane. The obtained mechanism cannot be explained in terms of regional stress field. We suggest that magma intrusion represents the origin of the observed reverse slip in Abou Dabbab. Khodayar and Einarsson (2004) suggest that dike intrusions are one of the kinematic origins for the observed reverse faulting mechanisms. The reverse faulting mechanism can be attributed to a slight local uplift of the hanging wall induced by an underlying prop-agating dike. Rubin and Pollard (1988) show that dike-induced uplift is a common feature in both rift graben and the flanks of rift zones, and that slip occurs on faults that intersect the dike near its top, that is, in the zone of dike-induced tensile stress. Reverse slip motion may also occur due to friction between the dike wall and the host rock during multiple magma injections (Khodayar and Einarsson 2004).

The focal mechanism solutions of the two moderate mag-nitude events that occurred in Abou Dabbab in 1955 and 1984 are fundamentally different from the composite solutions within the same area. The two solutions are characterized by a strike-slip faulting mechanism with a normal dip-slip component. The NW-striking plane coincides closely with the sinistral strike-slip shears of the Najd fault system. These mechanisms are also simi-lar to the mechanism of the 1981 Aswan earthquake, which reflects their relation to the same tectonic stress field. The focal parameters, as reported by the National Research Institute of Astronomy and Geophysics, Egypt, were an epicentral location at 23.55 N–32.50 E, a depth of 10 km, and a moment magni-tude Mb =5.1.

The azimuths of the T-axis mainly trend NNE-WSW to NNW-SSE, while the azimuths of the P-axis trend E-W to ESE-WNW. The T-axis direction changes along the Red Sea to be trending NE-SW, perpendicular to the rift axis. This pic-ture implies a rejuvenation of the preexisting NW-SE striking Najd faults due to a partial transfer of rifting deformation from the Red Sea along these trends. The two moderate-magnitude

earthquakes, however, reflect the average state of regional stress in Abou Dabbab while the composite focal mechanism solu-tion is mainly associated with a local stress field. This result reflects the complex tectonics in the Abou Dabbab region, which is mainly associated with the Red Sea extensional tec-tonics. It is clear that intrusion of the magma changes the stress field within the area of the dike intrusion.

The seismic moment and moment magnitude of the 1984 shock, derived from the displacement spectra of the teleseismic waveform, are 1.15 × 1017 Nm and 5.3, which are larger than the values derived from the Global CMT solution: 0.5 × 1017

Nm and 5.1. The estimated fault radius and stress drop for this event are 1.6 km and 9.7 MPa, respectively. The 1984 Abou Dabbab shock displays a high stress drop but this stress drop is close to the typical value of 10 MPa for intraplate earthquakes. High stress drop usually originates from an area of high stress concentration. This high stress drop occurs at a depth of 9 km, which represents the depth of the brittle-ductile transition zone where the maximum shear stress of the material is associ-ated. Maximum concentration of seismicity is observed in the depth range from 9 to 10 km. This depth is also the depth of the 1984 earthquake. From an engineering point of view, high-stress-drop earthquakes causes more damage compared to low-stress-drop earthquakes with the same magnitude value. The stress drop of this event is five times higher than 1992 Cairo earthquake, which displays a normal faulting mechanism. The focal parameters as reported by the National Research Institute of Astronomy and Geophysics, Egypt, were an epicentral loca-tion at 29.77 N–31.07 E, a depth of 22 km, and a moment mag-nitude Mw =5.8.

This reflects a dependence of the stress drop on the type of the mechanism, where the strike-slip mechanism of earth-quakes shows a three to five times higher stress drop compared to the normal and reverse faulting mechanism (Allmann and Shearer 2009).

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National Research Institute of Astronomy and GeophysicsHelwan 11721 Egypt

[email protected](H. M. H.)

abou dabbab asia

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