Search

In this paper two approaches have been used for delineating a magnitude scale for local and near earthquakes of Aswan High Dam Lake (Aswan, Egypt), depending on either the total duration of oscillation or the maximum S-wave amplitude. In using the first approach, 19 local and near earthquakes in the period from 1982 to 1987 recorded by the Aswan Seismic Network were collected. A relation between F — P 's (duration) recorded at the GMR station and their magnitudes reported by ISC Bulletin has been established, and it yields the following equation: M=2.17 log (F−P)−1.3+0.00075Δ. In the second approach, the maximum ground S-wave amplitude in millimicrons for the micro-earthquakes from three zones were measured and plotted against the mean magnitude (calculated by the previous relation). Applying the method of linear least-squares fitting, a magnitude equation for each station could be obtained. The standard deviations and standard errors of magnitude have been determined. The resulting errors of the magnitude units are within the permissible error in such magnitude determinations.

Seismicity of the Kalabsha area west of the Aswan Lake was examined using microearthquake data recorded by the Aswan Radio Telemetry Network surrounding the northern part of the lake. The seismicity of the Kalabsha area is divided into six seismic zones and composite focal mechanisms for four zones from these six zones were derived using first motions of P-waves. The mechanisms of these zones are characterized by strike-slip faults. According to the seismicity and focal mechanism solutions new linear features could be added to the structural map of Kalabsha area. Each feature corresponds to an active fault. The P (pressure) and T (tension) axes obtained from the focal mechanisms show that the orientation of the compressional stress and the tensional stress prevailing in the Kalabsha area are ESE-WNW and NNE-SSW, respectively. The direction of the compressional stress agrees with the direction of movement in the area reported by Issawi (1968). The direction of this movement is from SE towards NW. The fold axes mapped in the area generally strike NE-SW. This direction of the fold axes is more or less perpendicular to the compression. It is also consistent with the maximum horizontal compression induced from approximately NW and SE due to the north to northwestward drift of the African plate (Schandelmeier et al., 1987). The direction of the tensional stress is consistent with the presence of E-W faults (e.g. Kalabsha, Seiyal faults). The Wadi Kalabsha may be considered the result of down-faulted blocks with E-W normal faults caused by the NNE-SSW tension.

A moderate earthquake occurred in the unpopulated area of Kalabsha in the southeastern part of the Western Desert of Egypt on November 14, 1981. The Kalabsha area lies on a large western embayment of the Aswan Reservoir approximately 60 km south of the Aswan High Dam. The earthquake was the first instrumentally located earthquake in the Aswan region. The earthquake was felt strongly along the Nile River valley. Extensive ground cracking occurred in the desert sands in the epicentral area. Observations of continuing activity with a talemetered network show hypocenters extending to depths of more than 20 km.