Three-dimensional seismic attenuation tomography of Egypt

Elsayed Fergany, Mamdouh Abass, Carlos Vargas

Department of Geosciences, Universidad Nacional de Colombia, Bogota, Colombia.
National Research Institute of Astronomy & Geophysics, Helwan, Cairo Egypt.

DOI: 10.4236/ns.2012.428081                

Abstract

The main purpose of the paper is assessing the three-dimensional (3-D) seismic tomography beneath Egyptto reveal the laws of the tectonic activity, dynamic features of the crust and the upper mantle as well as the thermal structure. Thecoda wave attenuation (Q^-1_c ) was obtained using the single scattering theory for the central frequencies of interest laid between 1 and 24 Hz. A regionalization of the estimated Q^-1_c values was performed by means of a generalized inversion technique.The obtained spatial distribution of 3-D attenuation results revealshigh contrasts between East and West Egypt. A remarkable contrast in the attenuation levels was compared with the tectonic structures, geothermal gradient and heat flow features. The highest attenuations are concentrated in the east and north western offshore regionsat central frequency 1.5 Hzthat draw a good matching with the seismic andthermal features of Egypt. Smaller attenuation levels were detected with young sediments of the Nile Valley from South to the northern triangle of Nile Delta basin except seismic active areas. Low or normal attenuation was detected at western desert where there is a stable and simple shelf. We can conclude that the extended highest attenuation joins to the strong seismic sources and geothermal structures at lower frequency and the centralized high attenuation takes place at moderate seismic sources at a higher frequency. The 3D attenuation maps can draw not only tectonic and geothermal structures but also the general geologic structure map.

Keywords

Seismic Coda Waves; Attenuation Tomography; Seismicity; Tectonic; Geothermal; Geologic Epoch

1. INTRODUCTION

There are many reasons to study the attenuation of seismic waves. Variations in regional attenuation (1/Q) can help with structure and tectonic interpretation [1]. Local and regional distance attenuation of seismic phases is important in earthquake hazard prediction. Quantifying seismic wave attenuation and correcting for its effects improves source parameter studies, which will aid in discrimination of small nuclear tests from naturally occurring earthquakes [2-4]. At the same time as developments in theoretical models for relating scattering with coda wave amplitudes were advancing the study of seismic wave attenuation in the Earth’s lithosphere during the last decade, quantitative estimations of the attenuation parameters were carried out in many regions of the world. The decay rate of the coda amplitudes () estimated within the framework of the single-scattering theory [5,6] has also proved useful for seismologists because the simplicity of its measurement allows the study of geographical and temporal variations relatively easily. The physical meaning of has been debated for many years [7]. At present, it is known that whereas some theoretical and experimental model studies have confirmed that it only measures the intrinsic absorption [8-10], the field observations show that in general consists of intrinsic absorption () and total attenuation () [11]. Knowledge of regional values of and its spatial variation is of considerable interest in relation to tectonics and seismicity, being an important subject in seismic risk analysis and engineering seismology [12,13].

Egypt is located on the southeastern part of the Eastern Mediterranean region (northeastern corner of the African continent) and bounded by three active tectonic plate margins: the African-Eurasian plate margin, the Red Sea plate margin, and the Levant transform faultas shown in Figure 1. The Eastern Mediterranean Sea is characterized by high seismicity and complex tectonics. In Egypt no full-scale systematic investigation of tomographic seismic attenuation has been conducted. Only some specific regions have been studied for 1D: Sinai Peninsula [14], North Egypt [15] and Gulf of Suez [16].

In this work, study of the 3D attenuation in the crust and the upper mantle will help us to understand more truly the tectonic activity, dynamic features of the crust and the upper mantle, as well as the thermal structure. Waveform data of 397 local and regional earthquakes (1997-2008) in the moment magnitude (3 - 5.5) recorded by 63 seismic stations belonging to the Egyptian National Seismic Network (ENSN) have been used in this study. We have utilized the single back scattering model developed by [5] and extended by [6,17] for estimation of coda wave attenuation quality factor,. For the inversion, a regionalization of the estimated values was performed by means of a generalized inversion technique. Several synthetic tests were done to evaluate the distribution efficiency of the events and stations. Following [12], one way to regionalize for Q_c is based on the work of [18] who, expanding on the work of [5,6], realized that the first order scatterers responsible for the generation of coda waves at a given lapse time are located on the surface of an ellipsoid having earthquake and station locations as foci. The 3D was calculated for each central frequency ranged from 1.0 to 24 Hz avoiding the unreliable results of the marginal area.

The results clearly reveal a remarkable high contrast in the attenuation levels dependent frequency at different depths in different zones of Egypt. The remarkable high contrasts in the attenuation levels were compared with the tectonic structures geothermal gradient and heat flow data. Generally the high attenuation contrasts are concentrated in the Eastern Egypt and north western offshore region that draw a good matching with the seismic map of Egypt and thermal features. A smaller attenuation levels was detected with young sediments of the Nile Valley from South to the northern triangle of Nile Delta basin except seismic active areas. Low or normal attenuation was detected at western desert where there is a stable and simple shelf.

2. TECTONIC AND GEOTHERMAL FRAMEWORK

The tectonic deformations within Egypt are related to the regional and local tectonic forces. Egypt is located on the southeastern part of the Eastern Mediterranean region (northeastern corner of the African continent) and bounded by three active tectonic plate margins: the African-Eurasian plate margin, the Red Sea plate margin, and the Levant transform fault. The Eastern Mediterranean Sea is characterized by high seismicity and complex tectonics. Several geodynamic models have been adapted to explain the tectonic process in this region [19-21]. Figure 1 shows the tectonic boundaries and compiled tectonic elements of the Eastern Mediterranean Region by [22-24]. Seismicity data (mb ≥ 3) was compiled after [25] from (1900-1964) and ISC (1964-2005).

Generally the major part of tectonic deformation within Egypt is remote and took place along the Red Sea-Gulf of Suez, Gulf of Aqaba and offshore on the Mediterranean Sea to the south of the Hellenic and Cyprean arcs due to the rifting along the Red-Sea-Gulf of Suez, left lateral movement along the Gulf of Aqaba and the subduction of the African plate beneath the Eurasian plate. Recent seismicity data clarify that, all inland seismic activity in Egypt lies conformable over the pre-existing E-W and WNW-ESE or NW-SE faults. There is a clear correlation between the principal areas of current geothermal development [26,27] and the seismically active boundaries of the moving segments of lithosphere defined by the plate tectonic models of the earth [28].

Spatial distribution of earthquake epicenters indicates that Egypt suffered from both interplate and intraplate earthquakes. Most earthquake activity (more than 70%) has been concentrated in northern Egypt along the northern Red Sea and its two branches Suez rift and AqabaDead Sea transform. The crustal thickness for the whole Egypt is between 20 and 34 km. Its thickness thins abruptly towards the Red sea coast 20 km. The crust decreases from 34 km in the western desert to 28 km at the Mediterranean Sea Coast. According to the epicenteral distribution of earthquakes and tectonic setting, seven local seismic zones labeled from 1 to 7 were recognized by [29] in Figure 1 as follows:

1) Gulf of Suez-northern part of the Eastern Desert zone; 2) Gulf of Aqaba zone; 3) East MediterraneanCairo-Fayum Pelusiac zone; 4) Egyptian Mediterranean Coastal Dislocation zone; 5) Southwest Cairo zone; 6) Abu-Dbab zone; 7) Aswan zone.

[31] estimated preliminary heat flow values ranging from 42 to 175 m·Wm^–2 have been estimated for Egypt from numerous geothermal gradient determinations with a reasonably good geographical distribution, and a limited number of thermal conductivity determinations. Figures 2 and 3 show the spatial distribution of the borehole temperature logging sites and water sample locations for the heat flow study. For northern Egypt and the Gulf of Suez, gradients were calculated from oil well bottom hole temperature data; east of the Nile, and at three sites west of the Nile, gradients were calculated from detailed temperature logs in shallow boreholes. Withone exception, the heat flow west of the Nile and in northern Egypt is estimated to be low, 40 - 45 m·Wm^–2, typical of a Precambrian Platform province. A local high, 175 m·Wm^–2, is probably due to local oxidational heating or water movement associated with a phosphate mineralized zone. East of the Nile, however, including the Gulf of Suez, elevated heat flow is indicated at several sites, with a high of 175 m·Wm^–2 measured in a Precambrian granitic gneiss approximately 2 km from the Red Sea coast.

Figure 1. Tectonic boundaries and shallow seismicity pattern (h ≤ 60 km) of the Eastern Mediterranean Region. AEG = Aegean Sea; Al = Alexandria City; CY = Cyprus; ERA = Eratosthenes Seamount; FL = Florence; IB = Ionian Basin; MR = Mediterranean Ridge; LEV = Levantine Basin; LF = Levant Fault (after [30]).

Figure 2. Locations of borehole temperature logging sites in Egypt excluding oil well data sites [31].

Egypt geothermal gradient data and Preliminary heat flow values were summarized in Tables 1 and 2 by [31]. These data indicate potential for development of geothermal resources along the Red Sea and Gulf of Suez coasts. Microearthquake monitoring and gravity data indicate that the high heat flow is associated with the opening of the Red Sea.

3. DATA

Waveform data of 397 local and regional earthquakes (1997-2008) in the moment magnitude (3 - 5.5) recorded by 63 seismic stations belonging to the Egyptian National Seismic Network (ENSN)have been used in this study. The ENSN starts operating in August 1997 and implemented in 2003. Seismological stations are composed byshort period stations (STS1 and L4C seismometer type) with natural frequency 1 Hz and broadband stations (STS2 and Trillium seismometer type) as shown in

Figure 3. Map of water sample locations for the water geothermometry study. Open circles indicate single or multiple sample sites. The ruled areas indicate major oases from which multiple samples were collected [31].

Figure 4. The seismological acquisition at the ENSN is a fully automated and network system dedicated to the digital acquisition and real-time processing of seismological data. All digital recording instruments are equipped with velocity sensors and 24-bit analogue-to-digital converter. The data are digitized at a sampling rate of 100 samples/sec. Figure 5 shows the spatial distribution map of the earthquakes that used in this study.

Conflicts of Interest

The authors declare no conflicts of interest.

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