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  • Author or Editor: A. Hassoup x
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Shallow earthquake swarms of focal depths <10 km and magnitude <4 occurred beneath Aswan Lake in 1987 and 1993. The 1987 events are located along the E-W Kalabsha fault and their focal mechanism solution is strike-slip. Events of the 1993 sequence are distributed along a fault segment trending in the N-S direction and show a strike slip fault plane solution, too. Source parameters of 29 earthquakes from these two sequences are determined using a software developed by the Institute of Geophysics, Polish Academy of Science. The seismic moment and corner frequency data indicate weakness of the focal region of these swarms. The weakness may be produced by the presence of a fractured zone. Horizontal movements, which were collected by Kalabsha local geodetic network, are analyzed for epochs 1984-February 1987, February 1987-September 1987 and September 1987-November 1988. The results show different spatial patterns and magnitudes of the horizontal displacement. The large magnitude of the horizontal displacements suggests that the crustal deformation measurements are mixture of seismic and aseismic components. Moreover, correlation between the geodetic data and the stress pattern deduced from seismicity exists.

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Suddenly, Aswan Seismic Network detected and observed a number of events beside Aswan city. The magnitude of these events ranged from 0.9 to 2.6. These events were considered very important events because they were located not far from the Aswan High Dam. The main purpose of this work is to investigate and identification of these events using different seismic methods. Small earthquakes and explosions have the same flat and displacement spectrum for short-period P waves and discrimination of small events rests on detection and location. The final result appears that the source of these events is not an artificial type.

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The spatial and temporal distribution of seismicity around the northern part of Lake Nasser in Aswan has been continuously monitored by a short-period seismograph network since 1982. Data of this network, which consists of 13 field stations distributed around the Kalabsha fault, has demonstrated occurrences of three swarm sequences during the past 26 years. The first swarm occurred in June 1987 on a fault segment about 8 km in length beneath the lake water. The largest magnitude of this swarm was 3.7. Its hypocenters were located within the shallow depth’s interval (i.e., 2 to 5 km from the ground surface). The estimated b-value for this swarm was 0.63. The second swarm sequence took place at the intersection of the Seiyal fault with Kurkur fault about 10 km to the north of the June 1987 sequence. This swarm occurred during the period from August to December 2004 with largest magnitude of 4.1. The focal depths of this swarm also ranged from 2 to 5 km. The b -value of this swarm was found to be 0.69. The third swarm occurred recently in April 2007 with largest magnitude of 4.2 at the same location of 1987 sequence but its focal depths ranged from 6 to 8 km and has the b -value of 0.58. The composite focal mechanism study of these three swarm sequences revealed right-lateral strike slip faults with horizontal stress axes pattern. The relationship between the lake water level and the swarm sequences showed that both 1987 swarm and 2007 swarm occurred during the decreasing of water level, whereas the 2004 swarm occurred during the increasing of the water level. The study of these three swarm sequences illustrates an interesting phenomena, that there is a seismic quiescence preceding each swarm sequence. The duration period of the seismic quiescence ranges from 3 to 5 months.

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