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-1c ) 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-1c 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.
Highlights
There are many reasons to study the attenuation of seismic waves
Low or normal attenuation was detected at western desert where there is a stable and simple shelf
The 3D attenuation Qc 1 was calculated for Egypt using 397 local and regional earthquakes recorded by 63 seismic stations belonging to the Egyptian National Seismic Network (1997-2008) in the magnitude (3 - 5.5)
Summary
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]. 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 [2,3,4]. 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 ( Qc 1 ) estimated within the framework of the single-scattering theory [5,6] has proved useful for seismologists because the simplicity of its measurement allows the study of geographical and temporal variations relatively . Knowledge of regional values of Qc 1 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]
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