Induced seismicity at Nurek Reservoir, Tadjikistan, USSR

More than 1800 earthquakes (1,4> M> 4.6) have occurred during the first 9 yr of filling of the 300-m deep Nurek Reservoir in Tadjikistan. This is more than four times the average rate of activity in the region prior to the start of filling. The increased seismicity has occurred in a series of bursts, the two most intense of which were related to rapid increases in water level during the first two stages of filling--~to 105 m in 1972 and to 205 m in 1976. All periods of high seismicity take place when the water level is higher than it has been previously or within 10 m of its previous maximum. If the water level drops more than 10 m below its previous maximum, the level of seismicity decreases. All of the largest earth-quakes and most of the bursts of activity are triggered by decreases in the rate of filling of the reservoir. Once the water level is more than 10 m above the previous maximum, the potential for increased seismicity is high. Extremely small changes in filling rate can then trigger the onset of activity. For example, the largest earthquakes all followed decreases in filling rate of approximately

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