Op AND Os ESTIMATION OF ASWAN LAKE REGION, EGYPT

Abstract

The frequency-dependent attenuation characteristics of the body waves viz. P and S waves was estimated for the upper crust of the Aswan Lake region, Egypt using 330 waveforms of 28 local earthquakes within the magnitude range of 1.4-3.8 that occurred in the region mostly along the E-W trending Kalabsha fault during 2007-2009. These were shallow focus earthquakes with 22 km as the maximum depth of focus. The extended coda normalization method was applied to obtain the quality factors of P (Qp) and S (Qs) waves for the central frequencies 1.5, 3, 6, 9 and 12 Hz. The evaluated quality factors show strong frequency dependence with their values increasing with increasing frequency. The average frequency dependent relations (QQ0r) for the Aswan region are Qs= (34.2±2.9)f1023 0107 and Qp (4.8±6.5)f 2244 for S and P waves respectively. The low values of Qp and Qs lie within those observed for seismically active regions of the world. The ratio Qs/Qp is observed to be greater than unity for frequencies less than 3 Hz and becomes less than unity for higher frequencies indicating a predominant S wave attenuation at higher frequencies because shear modulus undergoes greater reduction compared to the bulk modulus. Higher frequency signals come from shallower depths as high frequency signals tend to attenuate more, the reason being earth is a low pass high cut filter. Hence, Qs/Qp values less than unity for higher frequencies indicate presence of fluid filled fractures at shallower depths due to the reservoir. Whereas deeper depths are expected to be more hard dry rocks giving rise to the observed higher Qs/Qp ratio at lower frequencies, so scattering would play more important role there. On comparison of Qs estimated in this study and Qc determined previously Qc > Qs for the entire frequency range indicating the Coda wave enrichment and the significance of scattering attenuation to the attenuation of S waves infested by fractures and faults i.e. heterogeneity present in the area. This gives an indication that it is not the case of complete intrinsic or scattering attenuation that attenuates the body waves but both of them have a significant contribution.