Induced seismicity and changes in water level at Aswan Reservoir, Egypt

A long sequence of earthquakes at Aswan reservoir on the Nile River in southern Egypt has followed a magnitude 5.3 event on November 14, 1981. This earthquake occurred beneath a large westward embayment in the northern end of the reservoir. The reservoir began filling in 1964, but the water did not reach the seismogenic area until 1976. The 1981 mainshock followed four days after the reservoir had reached its seasonal maximum. The earthquakes fall into two depth ranges. The mainshock and most of the immediate aftershocks he between depths of 15 and 25 km, with no activity at shallower depths, along a 10 km segment of the EW Kalabsha, fault, a right-lateral strike-slip structure that extends west of the reservoir. Shallow activity, between depths of 0 and 10 km, occurs on other segments of the Kalabsha fault and nearby conjugate structures. These areas show no deeper activity. From 1981 to 1987, the water level in the reservoir decreased each year due to the severe north African drought. The seismicity has also gradually decayed. In 1982 and 1983, while water remained in the embayment near the active area, short-term increases in seismicity were observed following the annual maxima in water level in the reservoir. From 1984 to 1987, the water receded almost to the original Nile channel and the annual coincidence between seismicity and peak in water level was less obvious. With the strong flood of the Nile in 1988, the water level increased rapidly and the reservoir is again beginning to extend into the embayment where the earlier seismicity occurred. Two intense swarms in seismicity occurred when the water level was not at its annual maximum; one in 1982 just after the annual minimum in water level and one in 1987, following an abrupt increase in the rate at which the water level was decreasing.