Fault orientations in the upper crust beneath an intraplate active zone in northern Egypt
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2012
Received: 2 September 2012
Accepted: 13 December 2012
Published: 23 August 2013
The present study aims to demarcate the active fault orientation in the upper crust of Dahshour Seismic Zone (DSZ) in Northern Egypt, and its relation to the regional tectonics. High-resolution earthquake relocations and focal mechanism solutions were determined using the double-difference location method and the forward modelling of the amplitude spectrum constrained with P-wave polarities, respectively. Up to 244 earthquakes were relocated by measuring differential travel times between events, as obtained from cross-correlation waveform analysis. The relocated hypocenters exhibited a seismogenic crust of 6-km thickness (most hypocenters have a focal depth of 18 to 24 km). Incorporation of hypocentral distributions and focal mechanisms depicts distinct fault planes trending to the NW and ENE to EW, randomly distributed in the whole dislocation zone. The reactivation of these pre-existing faults is attributed to the northern Red Sea-Gulf of Suez rifts and the closure of the Neotethys, respectively; implying a potential impact of the regional tectonic process on the deformation acting in the upper crust of the local seismogenic zones of northern Egypt.
It is generally recognised that the study of intraplate earthquakes is a difficult task because their seismogenic faults may have minor features, strain rates are low, and large size earthquakes are infrequent (Dunn and Chapman, 2006). The accurate estimation of hypocenters and fault plane parameters for small-sized earthquakes recorded by local seismographic networks are essential to understand the ongoing geodynamical processes and the seismotecton-ics of active fault zones (Poupinet et al., 1984; Got et al., 1994; Dodge, 1996; Richards-Dinger and Shearer, 2000). Many techniques are used to determine earthquake locations individually or jointly (Thurber, 1983; Lienert et al., 1988; Pujol, 1988; Lahr, 2001). The double-difference (DD) algorithm (Waldhauser and Ellsworth, 2000; Wald-hauser, 2001) estimates the residuals between the observed and calculated travel-time difference of two closely-spaced events at a single station by assuming similar ray paths propagating from the source zone to the station (Waldhauser and Ellsworth, 2000). This further cancels the common mode errors related to the receiver side structure resulting in no station corrections for the ray path outside the focal volume. The DD algorithm has been extensively used to improve the accuracy of hypocenter parameters along with we can image the trends of blind active faults within the earth crust. The DD technique not only minimizes errors due to an inexact model of the velocity structure, but it also reduces the error between intra-event location by utilizing high-accuracy differential times derived from catalogue picks and/or waveform cross-correlation (Waldhauser and Ellsworth, 2000).
The present study aims to determine the active fault orientation in the DSZ and its relation to the regional tectonics. For this purpose, we examine the spatial distribution of earthquake hypocenters to gain an insight into the orientation of seismogenic structures using the double-difference algorithm for earthquake relocation, developed by Waldhauser and Ellsworth (2000). Besides, the interpretation of the double-difference hypocenter relocations is aided by matching with fault mechanism solutions for ten events, having a good signal-to-noise ratio, recorded by the local seismographic network during the period 2000–2005 in the respective region. The fault mechanisms were obtained using the forward modelling of the amplitude spectrum constrained with P-wave polarities, which was introduced by Zahradník et al. (2001). Earthquake relocations, fault mechanisms, and focal depth determinations, will enhance our understanding of the seismotectonics within the study area. The results will then be used to develop a database for seismic hazard assessment.
2. Geologic and Tectonic Settings
Stress information concerning regions within plate interiors is rare because of the lack of earthquake source mechanisms. Abou Elenean et al. (2000) showed that the stress regime affecting the epicentral area is a dominant exten-sional stress trending to NE-SW; generating normal faulting mechanisms with probabilities of a strike-slip component (Bosworth and Stecker, 1997). The stress field producing earthquakes in the epicentral area is considerably related to the complex relative motions between the African, the Arabian, and the Eurasian plates. Plate-motion slip rates have been accurately determined by recent GPS data (McClusky et al., 2000, 2003); a slip rate of ~6 mm yr−1 is associated with the northwestward motion of the African plate, whereas the northward motion of the Arabian plate has a rate of ~18 mm yr−1. The differential motion of Africa and Arabia relative to Eurasia is accommodated by the sinistral transpressional along the Dead Sea transform fault.
3. Data Analysis and Results
3.1 Data use
The data set of P- and S-wave arrival times from 244 earthquakes having a magnitude between ML 1.0 and ML 4.3 are used in the present analysis. The earthquakes were recorded by the ENSN operated by the National Research Institute of Astronomy and Geophysics (NRIAG). The waveforms recorded by the stations within an epicen-tral distance of less than 100 km were used. The selected stations consisted of ten short-period 1-Hz stations and a three-component broadband station (KOT). The sampling interval of these instruments is 100 samples per second. Arrival times of P- and S-waves were chosen to decrease reading errors.
Velocity model of El-Khrepy (2008).
V p (km/s)
V s (km/s)
4.50 ± 0.20
2.60 ± 0.20
5.10 ± 0.15
3.00 ± 0.15
5.80 ± 0.20
3.40 ± 0.2
6.05 ± 0.15
3.49 ± 0.15
6.45 ± 0.10
3.74 ± 0.10
6.89 ± 0.15
3.98 ± 0.15
7.35 ± 0.15
4.27 ± 0.15
7.88 ± 0.08
4.52 ± 0.08
8.00 ± 0.05
4.60 ± 0.05
8.10 ± 0.05
4.65 ± 0.05
3.2 Hypocenter relocation
P- and S-wave arrivals were picked for earthquakes that occurred between 2000 and 2005. A total of 31607 P-arrivals and 13692 S-arrivals were used to relocate earthquakes in the Dahshour seismic zone. The availability of nearby stations permitted a reliable hypocenter location. The hypocenters were located using the hypo71 software (Lee and Lahr, 1972). The aforementioned one-dimensional velocity models were used. The overall RMS error in the hypocenter estimates of the studied events is 0.16 s. The earthquake locations used in the present study, and the seismic stations, are shown in Fig. 2. The velocity model derived by El-Khrepy (2008) reflects the smallest residuals in comparison with other two velocity models.
To reduce uncertainty, we applied the double-difference earthquake location method (Waldhauser and Ellsworth, 2000). The double-difference residuals for pairs of earthquakes at each station are minimized by weighted least-squares using the methods of conjugate gradients (LSQR) and singular value decomposition (SVD), while linking together all observed event-station pairs. Relocations were accomplished through two steps: firstly, we screened the data to optimize the linkage between events and minimize redundancy in the data set; secondly, we calculated the differential travel-time data using absolute locations. All arrivals were included in the relocation procedure by setting the maximum station-event distance to 200 km. We also test the relocations for inter-event separations less than 5 and 10 km, sequentially. A priori weights assigned for P- and S-waves were 1.0 and 0.5, respectively. Also, accurate P- and S-wave differential arrival times between events were obtained by applying waveform cross-correlation in the time domain to minimize the errors, by an order of magnitude or more, in the case of earthquake pairs having the same rupture mechanism and a similar waveform (Schaff and Waldhauser, 2005).
The differential travel times are directly inverted to relocate earthquakes (e.g., Waldhauser and Ellsworth, 2000) or to improve the absolute arrival times at each station (e.g., Shearer et al., 2005). In the present analysis, travel times were differentially computed using the cross-correlation algorithm described in Schaff et al. (2004) and Schaff and Waldhauser (2005). We computed a total of about 1852, and 864, P- and S-wave differential times, respectively, from pairs of waveforms yielding cross-correlation coefficients of 0.7 or larger. The cross-correlation waveform analysis was uniformly accomplished over the analysed stations on the basis of: (1) seismograms were filtered from 1.5 to 15 Hz (the instrument is reliable in this band); (2) correlations were made for both 1- and 2-s window lengths, for both P- and S-wave windows with lags searched over 1 s; and (3) P-wave windows are initially aligned on phase-pick data, and S-wave travel times are computed as 1.737 times the P-wave travel time as deduced by El-Khrepy (2008) across northern Egypt. 244 earthquakes were relocated by the double-difference relocation algorithm.
Because of the underestimate of the errors reported by LSQR, we checked the final locations by SVD to assess a true errors estimate (Waldhauser and Ellsworth, 2000). As the double-difference method minimizes errors due to an inaccurate velocity structure, the uncertainties of epicenter parameters and focal depth have been reduced by an average error of 0.3 km, 0.4 km, and 0.3 km, for latitude, longitude, and focal depth, respectively. In addition, the average of the root-mean-square value is 0.02 s. Almost all events have a focal depth of 18–23 km, reflecting a thin seismogenic layer of 6-km thickness. In fact, Hy-poDD acts as another earthquake location program based on Geiger’s method (Geiger, 1910) insofar as station geometry is concerned. Therefore, we reduced the effect of error ellipse, the trade-off between focal depth and origin time, by the present distribution of station geometry that reflected the different vertical slowness of the rays. In addition, we used near stations and different arrival phases (e.g., Pg, P*, P n , and Sg) to minimize, and control, the trade-off between depth and origin time.
3.3 Focal mechanism solutions
Focal mechanisms often earthquakes were calculated using a recently-developed method by Zahradník et al. (2001), and Zahradník (2002). This method is based on the amplitude spectra of complete waveform data and the first-motion polarities, hereafter ASPO. The waveforms recorded by velocity sensors at the nearest local stations were employed. The take-off angles were calculated using the ray-method code ANGGRA (Jansky, 2001). In the ASPO method, the amplitude spectra were computed for waveforms of 60 s duration in the frequency range 1.0–2.0 Hz. The spectra were then synthesized by the discrete wave-number method of Bouchon (1981), and Coutant (1989), using the velocity model of El-Khrepy (2008). A grid-search method was applied to three parameters: strike, dip, and rake angles. For each set of these model parameters, both the observed, and synthetic, spectra were normalized in order to put them on a comparable amplitude level. Band-pass filtering from 1.0 to 2.0 Hz was performed for the amplitude spectra. The normalization was performed by dividing the spectra by their average value computed for all components and frequencies at all available stations (taken over all stations, components and frequencies). The misfit at each station was calculated as a weighted L1 norm of the difference between the observed and synthetic amplitude spectra, and then summed over all stations, components and frequencies. The misfit function, introduced by Zahradník et al. (2001), not only reduces the undesired biasing effect of the largest amplitude at the near stations, but also normalizes the misfit values between 0 and 1. Here, amplitude spectra solutions were constrained with first-motion polarities to obtain optimum solutions whose misfits are between ±5%. The procedure was repeated for a set of ten trial source depths. We started with a coarse search with a broad depth range (5–40 km), and then a fine search was focused around the minimum misfit value in a narrow depth range of 2-km intervals. To find the best-fit double-couple solution, the strike, dip, and rake were finely searched at 2-degree intervals. Finally, the scalar moment was retrieved with no grid-search procedure, since the problem is linear with respect to the scalar moment. Instead of grid searching, unit-moment synthetic spectra were calculated for the preferred mechanism and depth. The ratio between the observed and synthetic spectra (averaged over all the data points) yields the seismic moment (Zahradník et al., 2004). The SGRAPH program (Ab-delwahed, 2012), with its implemented ASPO technique, is used to handle the processes of waveform analysis and focal mechanism estimation.
4.2 Focal mechanisms
Location and focal mechanism parameters of the studied events.
Date & Time
Fault plane parameters
5. Discussions and Conclusions
Intraplate seismicity occurs in the vicinity of stress concentrators within pre-existing zones of weakness, where the faults are intersecting. These stress concentrators are structures where plate tectonic stresses can cause a localized buildup of stresses and, ultimately, earthquakes. These include intersecting faults, buried plutons, and rift pillows. The study of intraplate seismicity is difficult because the occurrence of large earthquakes are rare, and the thickness of the seismogenic layer is narrow. Talwani and Rajendran (1991) pointed out that most intraplate earthquakes occur around the intersections of faults, and, in general, not at the intersections themselves or very close to them. Data collected by the local-to-regional network monitoring of small-size earthquakes are vital for understanding intraplate seis-micity (Dunn and Chapman, 2006).
In the present study, two algorithms are used to investigate the fault orientations producing earthquakes in the upper crust beneath the study area. The identification of the NW, ENE to EW trending alignments of seismicity became clearer after relocations. We interpret the concentration of seismicity as being due to the concentration of stress at the intersection of the NW and the ENE to EW fault trends. The current results identify a number of active faults striking NW and ENE to EW. These faults are randomly distributed in space along the dislocation of the seismogenic layer that extends for a few kilometers; revealing that the complex tectonic process of the regional tectonic regime controlled the deformation in the upper crust of the study area. The aforementioned fault trends are consistent with the ruptured faults obtained for small-size earthquakes in northern Egypt as determined by Badawy and Abdel-Fattah (2001), Abdel-Fattah and Badawy (2002), and Abdel-Fattah et al. (2011). The focal mechanism solutions indicated that earthquake activity within the dislocation zone is related to the movement along the NW and ENE to EW planes trending preexisting normal faults of a strike-slip component in response to the prevailing NE-SW tension. The results are consistent with that provided by available magnetic data, and land magnetic and aeromagnetic surveys, in the epi-central area, which reflected the main tectonic trends taking the directions NW and NE to ENE (Brimich et al., 2011). These tectonic trends are attributed to the relative tectonic motions between the African, Eurasian, and Arabian plates, as well as the key role of the Sinai subplate, where the study area is located southeast of the African-Eurasian convergent plate margin and northwest of the Red Sea divergent plate margin. Incorporation of the alignment of seismicity, the focal mechanism solutions, and the geologic and tectonic setting of the study area, the NW trending appears to correspond to the Red Sea rifting plate margin. On the other hand, the ENE to EW trending might be attributed to the African-Eurasian plate margin. The coupling of these tectonic movements plays the major tectonic role in the active deformation in the upper crust of northern Egypt.
The stresses which created the structure in northern Egypt are tension rather than compression as suggested by Shukri (1953) and Said (1962). Tensional and shear tectonic movements have affected the area in the form of basement joints and faults striking NW and EW. In addition, Quaternary sediments are also affected by EW and NE striking faults introducing a significant evidence of neotectonic activity in the epicentral area, as revealed by Kusky et al. (2011). The aforementioned trends are observable everywhere in northern Egypt, including north and central Sinai. The results obtained from the relocation of hypocenters revealed a less-spatial resolution of the 1992 earthquake location due to poor station coverage. The 1992 earthquake is located toward the north, and far from the spatial distribution of recent seismicity by 15 km. After the occurrence of the 1992 earthquake, the seismic stations were installed in the epicentral area, as shown in Fig. 2. Unfortunately, the stations were randomly installed on the basis of the imprecise location of the 1992 earthquake; most global seismic stations which recorded the event were located toward the north. The spatial distribution of seismicity obtained from the present study, with respect to the imprecise location of the 1992 earthquake, recommends an urgent optimization for the current source-station coverage in the epicentral area.
The first author acknowledges the King Saud University for making available time and facilities to accomplish this work. The authors acknowledge the assistance of many people at NRIAG who participated in the ENSN instrument deployment and preliminary data analyses. T. Okada, and anonymous referees, critically read the manuscript and provided us with helpful comments, which have improved the manuscript. Focal mechanism solutions and hypocenter relocation were computed using the ASPO package and HypoDD, respectively. Generic Mapping Tools (GMT) by Wessel and Smith (1995) is used to plot some of the figures in this manuscript. This work was supported by King Saud University, Deanship of Scientific Research, College of Science Research Centre.
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