.[1] Continuing seismicity for about 30 years near a large western embayment of the Lake Nasser, about 50 km from the Aswan High Dam in Egypt, has led to a debate about the possibility of its relation with the reservoir impoundment. The largest event in the region occurred on 14 November 1981 (M 5.3), 20 km beneath the Wadi Kalabsha embayment, a westward extension of the Lake Aswan. Since then, continuous monitoring of seismic activity has given an excellent opportunity to study the spatiotemporal distribution of seismicity in the area. Most of the immediate aftershocks of the 1981 main shock were located in the Gebel Marawa area at depths between 15 and 30 km. Depths of almost all earthquakes away from this zone were shallower than 12 km. To quantify the effect of the reservoir impoundment on the seismicity of the Aswan area, we calculated changes in stress and pore pressure due to the reservoir impoundment using Green's function approach. The change in Coulomb stress (DS) is calculated on the fault planes responsible for majority of the seismicity of the region. We found that for all the seismogenic faults, DS is negative, i.e., stabilizing, when we consider the effect of the reservoir load only, whereas it is positive, i.e., destabilizing, when we include pore pressure. For example, at the hypocenter of the main earthquake, shear stress, normal stress, and pore pressure due to reservoir operation are estimated as 5.5, 13.2, and 13.5 kPa, respectively, which suggest that DS is À3.1 kPa when we do not consider the effect of pore pressure and 5.7 kPa when contribution from pore pressure is considered. Hence, the seismicity in the Aswan lake region is driven by the pore pressure due to reservoir impoundment.

Egypt is characterized by moderate size seismicity where earthquakes are distributed within several active regions. In the present study, we investigated the source mechanism of earthquakes using the digital waveform data recorded by the Egyptian National Seismic Network (ENSN) during the period from 2004 to 2008. The focal mechanisms are constructed with high reliability based on the polarity of the first motion of P-wave.These solutions are used to examine the mode of tectonic deformation and the present-day stress field pattern affecting on different tectonic provinces in the northern part of Egypt. The results demonstrate mainly a normal faulting mechanism with minor strike slip component generally trending parallel to the northern Red Sea, the Suez rift, Aqaba rift with their connection with the great rift system of the Red Sea and the Gulf of Suez and Cairo-Alexandria trend.The inversion technique scheme is used also in the present study for determining the regional stress field parameters for earthquake focal mechanism solutions based on the grid search method of Gephart and Forsyth (1984). The Results of the stress tensor using focal mechanisms of recent earthquakes show a prevailed tension stress field in N52°E, N41°E and N52°E for the northern Red Sea, Gulf of Suez and Gulf of Aqaba zone respectively.

No abstract

No abstract

In this study, an integrated suite of Remote Sensing (RS) data and Geographic Information System (GIS) techniques supported by fieldwork is used to assess the sand dunes movement hazards at El-Kharga Oasis. Digital Elevation Model (DEM) data obtained from the Shuttle Radar Topography Mission (SRTM) and Advanced Spaceborne Thermal Emission and Reflection (ASTER) were integrated with GIS techniques to model the vulnerable locations and to study the terrain characteristics (slope angles and aspects) in the studied area. The risk assessment model output was verified with the field investigations using multi-temporal satellite images recorded between 1990 and 2019. Monthly wind roses showed that the sand drifts in the southeastern direction differed widely from one direction to another depending on the wind direction and velocity. The most important output of the spatial model’s, results was a geo-hazard map that classified the sand dunes hazard zones into low, slight, moderate, and high-risk zones. It is concluded that, the sand dunes pose a serious hazard because of their fast movement and accumulation near the monumental sites, over roads and invading the agricultural fields. The obtained results can serve as a basis for planners and decision-makers to take the necessary precautions and measures to minimize the sand dune hazard’s impact on the monumental sites (e.g., Hibis, El-Nadura, El-Ghueita and El-Zayyan), roads, and the agricultural fields at El-Kharga Oasis and lead to a sustainable development plan. Keywords Environmental Hazards; Sand Dunes Movement; Remote Sensing; Risk Assessment, Egypt

No abstract

The geology at Kottamiya, Rehab City and Zahraa-Madinat-Nasr to the East of Cairo (Egypt) is composed of low-velocity sediments on top of a rigid rock basement. Such sediments include the loose sands of the Gebel Ahmar formation, marl and shales of Maadi formation, in addition to sparse quaternary soil covers. Due to the contrast of the seismic impedance with the underlying bedrock, these soft sediments have the potential of considerably amplifying the ground motion during an earthquake. For the evaluation of site-specific seismic hazard, we computed the seismic site response in these areas by developing 1-D velocity models and derived average seismic velocities, including Vs 30 . To do that, we applied different active and passive source techniques, including the horizontal to vertical Fourier spectral ratio of ambient vibration recordings and multichannel analysis of artificially generated surface waves. A set of models representing the velocity structure of the site is then obtained by combined inversion of Rayleigh wave dispersion curves and ellipticity functions. While dispersion curves are used to constrain the uppermost low-velocity part of the soil profile, ellipticity helps in resolving the structure at the depth of the sediment-bedrock interface. From the retrieved velocity models, numerical ground-motion amplification is finally derived using 1-D SH-wave transfer function. We account for uncertainty in amplification by using a statistical model that accounts for the misfit of all the inverted velocity profiles. The study reveals that the different sites experience an important frequency-dependent amplification, with largest amplification occurring at the resonance frequencies of the sites. Amplification up to a factor of 5 is found, with some variability depending on the soil type (Vs 30 ranges between 340 and 415 m s −2 ). Moreover, amplification is expected in the frequency range that is important for buildings (0.8-10 Hz), which is additional confirmation for the need of microzonation analysis of the area. The obtained results will be used for the development of a new seismic hazard model.

No abstract