Earthquakes with local magnitudes from 0.0 to 4.9 recorded by the Tarbela seismic network, in Pakistan, between 1973 and 1982 have been used to study the reservoir-induced seismicity. A comparison between the pre-impounding and the post-impounding seismicity shows a dramatic decrease in the latter. The sharp decrease in seismicity is not related to the reservoir filling since it started two months before the first impounding and affected a very large area extending more than 100 km away from the reservoir. Statistical analyses of the distributions of earthquakes that occurred within the 20-km radial zone centered on the reservoir indicate that earthquakes in the magnitude range 0.0–1.9 are not randomly distributed in time, while larger events (2≤ML <5 ) have a random temporal distribution. Further analysis of the occurrence of larger magnitude earthquakes indicates that there is no simple relationship between their occurrence and the reservoir loading. On the other hand the cross-correlation of the frequency of non-random small-sized events in a 20-km radial zone with the monthly reservoir water level shows that there is a 160-day lag between the two time-series. This time lag, equivalent to a 180° phase shift between the water level curve and the event curve, indicates that the frequency of microearthquakes is reduced when the reservoir level is at high stand and vice-versa. An elastic model consisting of a two-dimensional rectangular load predicts that the effect of reservoir loading alone is to suppress the pre-existing seismicity directly beneath the Tarbela reservoir, while the effect of unloading the reservoir is to lead to a partial recovery of seismicity. The positive correlation between the frequency of earthquakes and the low reservoir water level could be explained mostly by the elastic effects of reservoir unloading. A gradual increase in the seismicity in regions as far as 100 km from the reservoir started in 1979 (five years after the first reservoir filling) and appears not to be a consequence of the slow diffusion of water to hypocentral depths; rather it reflects the long-term behavior of seismicity in the Pakistan Himalayas.

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