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 earthquakes 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 0.5 m/day; and in a number of cases, increased seismicity began soon after the reservoir started to empty by rates as small as 0.2 m/day2. The response in seismicity to decreases in the filling rate is rapid. Increased activity follows abrupt decreases in filling rate with delays as short as 1 to 4 days.
As the reservoir has approached its maximum size, extending 40 km upstream from the dam, the area of induced seismicity has increased as well. The first induced earthquakes in 1971 were located 10 to 15 km southwest of the reservoir. From 1972 to 1978, activity migrated into the immediate reservoir area and followed the growth of the reservoir upstream. The first stage of activity in 1971 to 1972 was characterized by low b values and included the largest earthquakes of M = 4.6 and M = 4.3 in November 1972, when the water level first exceeded 100 m. From 1973 to 1979, activity was confined to the immediate reservoir area and b values were higher. Although the water level has risen to over 250 m, no induced earthquakes larger than M = 4.1 have occurred since November 1972.