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Focal mechanism of earthquakes from the June 1987 swarm in Aswan, Egypt, calculated by the moment tensor inversion

Awad H., Kwiatek G.

Acta Geophysica Polonica

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2005

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Vol. 53, nr 3

275-291

EN

We obtained seismic moment tensor solutions of ten events from the June 1987 earthquake swarm, which occurred along the Kalabsha fault zone in the northern part of the Lake Aswan area in Egypt. In addition, the composite fault plane solution of this sequence was also calculated. The waveform data were obtained from the Aswan seismological network, which consists of 13 field stations with short period seismometers GS-13. The June 1987 swarm was a sequence of microearthquakes (M ? 3.4), shallow (0-10 km) events forming two successive bursts, which took place on June 17 and 19. The moment tensor solutions indicate that the focal mechanisms of events from this swarm sequence are expressed by right-lateral strike slip faults. They represent also an effective east-west compressional stress field acting in the area. Geological and geophysical data demonstrate that the Kalabsha fault zone is a right-lateral strike slip fault that consists of several fault segments trending in the east-west direction, perpendicular to the axis of the main course of the Lake As-wan. Thus, the focal mechanisms of the 1987 events are consistent with the local tectonics of the area.

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Seismicity of the Lake Aswan area, Egypt, between 1991-1999

Hassoup A., Hasseb G., Dahy S.

Acta Geophysica Polonica

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2002

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Vol. 50, nr 3

361-372

EN

The data described here form a part of continuous observations of the earthquake activity and water level variation in Lake Aswan area in Egypt. The earthquake hypocenters from the 1991-1999 period are distributed in four seismic zones (A, B, C and D), trending in the E-W and N-S directions. The hypocenters concentrate within the shallow part of the crust. The number of deeper events, with focal depths between 10 and 30 km, is small. The temporal distribution of seismicity in the 1991-1999 period correlates with water level changes in the lake. This well observed correlation indicates that the water level variation is effective in the Lake Aswam area, and suggests that the 1991-1999 earthquake activity may be classified as reservoir-triggered seismicity. The composite fault plane solution is considered for the earthquakes of magnitude greater than 2. Nine solution were obtained, representing the four epicentral zones. The strike-slip and strike-slip with normal component focal mechanism are dominant in the study area. This result is consistent with the interpretation of tectonics in the Aswan region. The focal mechanism with normal component is found only in zone C. This observation may be related to an effect produced either from the change in water load due to the water level variation or from tectonic characteristics of zone C.

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Microearthquake distribution in the Lake Aswan area, Egypt

Hassoup A.

Acta Geophysica Polonica

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2001

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Vol. 49, nr 3

317-330

EN

The temporal evolution of the Lake Aswan seismicity in Egypt over a 13-year period, from 1982 to 1995, is characterized by two separate sequences which occurred in August 1982 and June 1987 and a nearly continuous low-level activity over the remainder of the period. This study presents details of the space-time analyses of the two sequences. A nonlinear inversion method was used to relocate 200 events. The distribution of August 1982 foci exhibits asperity deformation; its center is located at 23.58°N, 32.57°E and 17 km depth. The foci distribution forms a zone dipping generally to the southwest. The June 1987 foci are distributed in a nearly vertical zone between 5 and 11 km depth, with epicentral coordinate centered at 23.57°N and 32.68°E. The August 1982 sequence is of foreshock-mainshock-aftershock type with a b-value of 0.8 and an aftershock decay rate of 0.78. The June 1987 activity is characterized by a b-value of 0.6 and is made up of two successive swarm bursts, which took place on June 17 and 19. The aftershock decay rates of these bursts are calculated as 1 and 0.99, respectively. The August 1982 cluster is of small amplitude and associated with a high background level. The June 1987 anomaly is of larger amplitude originating in a nearly zero background level. Our results suggest the Lake Aswan seismicity consists of the seismic zones with different characteristics.