Around the beginning of 1970s, there were about 20 known cases, where following the impoundment of an artificial water reservoir, a change in seismicity in the vicinity of the reservoir was observed. In most cases, there was an increase in number and magnitude of the earthquakes occurring near the site of the water reservoir compared to the pre-impoundment stage. Very detailed investigations of the characteristics of the earthquake sequences occurring in the vicinity of the reservoirs indicated that: (1) The b value in the earthquake magnitude–frequency relation (log N = A − b M; where N is the number of earthquakes with magnitude M or larger, A and b are constants) are higher for the aftershocks and foreshocks compared to the regional b values; (2) For RTS main events the decay of the aftershocks is slower compared to the normal earthquake sequences; (3) The magnitude ratio of the largest aftershock to main shock is higher; and (4) The foreshock-aftershock pattern of the RTS sequences belongs to Type 2 of Mogi’s earthquake models, whereas the regional normal earthquake sequences belong to Type 1 of the Mogi’s model. It was realized that these characteristics relate to the mechanical properties of the media hosting the earthquakes. A deviation in these observed properties of the earthquake sequences indicates that impoundment of the reservoir has introduced heterogeneity in the Earth media hosting the earthquakes. In the years to follow all RTS earthquake sequences, globally have exhibited similar characteristics.